The Space Shuttle contained a bulky printer so the astronauts could receive procedures, mission plans, weather reports, crew activity plans, and other documents. Needed for the first Shuttle launch in 1981, this printer was designed in just 7 months, built around an Army communications terminal. Unlike modern printers, the Shuttle's printer contains a spinning metal drum with raised characters, allowing it to rapidly print a line at a time.

This printer is known as the Space Shuttle Interim Teleprinter System.1 As the name "Interim" suggests, this printer was intended as a stop-gap measure, operating for a few flights until a better printer was operational. However, the teleprinter proved to be more reliable than its replacement, so it remained in use as a backup for over 50 flights, often printing thousands of lines per flight. This didn't come cheap: with a Shuttle flight costing $27,000 per pound, putting the 59-pound teleprinter in space cost over $1.5 million per flight.

We obtained access to a Shuttle teleprinter (probably a development system that remained on the ground) and wanted to put it into operation. I had to reverse engineer three of the boards inside the printer to determine the data format the printer accepted: serial data encoded into audio. But after analyzing the printer and performing a lot of maintenance, we succeeded in getting the printer to print. In this article, I'll describe the Shuttle's Interim Teleprinter, explain its circuitry and drum-based printing mechanism, and show it in operation.

History of the Shuttle's Interim Teleprinter

The motivation for the teleprinter goes back to the Apollo program. During Apollo missions, the only way to send information to the astronauts was by talking to them over the radio and having the astronauts write down the data. NASA decided that the Space Shuttle should include a mechanism to send text and images to the astronauts, a 78-pound, high-tech fax machine called the Uplink Text & Graphics System (TAGS). A high-resolution grayscale image was sent to the Shuttle as a digital data stream. Onboard the Shuttle, a squat CRT displayed the image one line at a time and a fiber-optic faceplate transferred each line to light-sensitive silver emulsion paper. The paper was developed by passing it over a hot roller at 260ºF for 25 seconds, creating a permanent image.

The one flaw in this plan was that sending the digital image to the Shuttle required the Tracking and Data Relay Satellite System (TDRS), which due to delays wouldn't be ready until the sixth Shuttle flight. (The TDRS was a space-based replacement for the worldwide network of ground stations that was used during Apollo.) As a result, NASA decided just seven months before the first Shuttle launch that they needed an interim system "for transmission of real-time, flight-plan changes and other operational data to the crew."2

The Shuttle teleprinter is the result of this rushed effort to create a printer that could work over the existing audio channel rather than the digital TDRS satellite. Due to the time pressure, the Shuttle teleprinter needed to be based on an off-the-shelf printer. Thermal and electrostatic printers were rejected due to toxicity and flammability problems. (The Shuttle teleprinter used a roll of yellowish paper, which required a NASA waiver due to its flammability, a concern ever since the Apollo-1 disaster).

The decision was made to use a military communications terminal, the the AN/UGC-743 "Tactical Teletype". The terminal's interfacing was very flexible, supporting serial data in either ASCII or Baudot format, with multiple configurations and baud rates (up to 1200 baud), using either a current-loop or voltage signals. The military terminal supported two-way communication, so it had a keyboard. Remarkably, the terminal also implemented a word processor, controlled by a Motorola 6800 microprocessor (ancestor of the famous MOS 6502). The word processor allowed messages to be composed offline, minimizing the radio transmission time, which was important in a hostile environment. As will be seen, this 100-pound military system required many large changes to be usable on the Space Shuttle, most visibly removing the keyboard.

The printing mechanism

The teleprinter uses a spinning drum with raised characters, shown below.4 To print a character, the printer fires a hammer, forcing the inked ribbon and paper against the raised character on the drum. The drum is 80 characters wide, matching the line length, and there are 80 corresponding hammers, one for each print position. The drum has 64 printable characters, wrapped around each position of the drum.

The printer prints a line at a time, not instantaneously, but during each revolution of the drum. When the drum makes one complete revolution, each of the 64 characters passes by each print position once. Printing requires precise timing of the hammers to strike the right character on the drum as it whizzes by. The printer control circuitry triggers each hammer at the proper time, when the desired character on the drum is lined up with the hammer, producing the desired text.5

The character set is slightly different between the military printer and the Shuttle printer. The military drum had 64 ASCII characters (upper-case letters only, numbers, and special characters). The drum doesn't contain an explicit space character, since nothing is printed for a space. In its place, the drum has a diamond "◊", used as a special character to indicate a parity error or other error. The drum for the Shuttle teleprinter replaces 10 ASCII special characters with symbols that are more useful to the Shuttle, such as Greek letters for angles. Specifically, the characters ;@[\]^!"#$ are replaced by θ✓‾↑↓~αβΔϕ.

The video below shows a closeup of the hammers as they strike the paper to print text. The text is the teleprinter's built-in test message: "THE LAZY YELLOW DOG WAS CAUGHT BY THE SLOW RED FOX AS HE LAY SLEEPING IN THE SUN". This test message is based on the traditional quick brown fox..., which is a pangram, containing all 26 letters, but the teleprinter's test sentence is missing J, K, M, Q, and V. However, the test message is exactly 80 characters long and replaces spaces with the diamond "◊", so it is effective for verifying that all 80 columns work.

The electronics

The photo below shows the circuitry inside the teleprinter, looking down from above. At the left are the three interface boards, custom boards that demodulate the incoming audio signal. In front of the interface boards are large inductors to filter the incoming power. Hidden beneath them, a solid-state relay controls the power to the rest of the printer, implementing the low-power standby mode. In the middle, the blue board is the surprisingly complex switching power supply, mounted on a thick metal plate for cooling. Normally, the large roll of paper is mounted above the power supply board. At the right, four large circuit boards implement the main logic of the printer: a printer driver board, a communications board, a memory board, and the processor board. The rotating drum is protected by the perforated black metal grill at the front.

The demodulator boards

The original military teleprinter received data as a serial bitstream. However, on the Space Shuttle, data was encoded as frequencies on the audio link. Three custom boards were constructed to demodulate the audio data so the rest of the printer could handle it. These boards also performed Shuttle-specific tasks such as powering up the printer when a message comes in, and then returning the printer to standby mode. I reverse-engineered these boards to determine how they work and to determine the data encoding. (Schematics are in the footnotes.7) In this section, I'll discuss these three boards, which are on the left side of the printer.

To summarize, the serial bitstream is encoded with Frequency Shift Keying, with a 1 represented by 3600 Hz and a 0 represented by 7200 Hz.6 The serial data is transmitted at 600 baud, even parity, one stop bit. The demodulation process first converts the input audio to a digital signal by thresholding it. (That is, the input sine wave is converted to a square wave.) The digital signal is autocorrelated to distinguish the 3600 Hz and 7200 Hz signals, recovering the underlying serial data. This signal is passed to the printer's logic boards (part of the original military teleprinter), which convert the serial signal to ASCII bytes and prints them.

Signal processing starts with the "FSK input" board, shown below. First, it amplifies the input audio signal. (The two large resistors provide a 600 Ω load for the audio input.) Next, a 900 Hz high-pass filter eliminates low-frequency noise. (The filter is implemented by a two-stage Sallen-Key topology.)

The signal bounces from board to board, going to the "output FSK demod" board next. This board has a carrier-detect circuit that turns on the rest of the printer if it detects an input signal. This allows the printer to sit idle until it receives a signal from Earth. This board also applies the threshold to the signal to turn it into a digital waveform, which goes to the "control" board.

The output board also holds the 5-volt and 12-volt linear regulators that power the three boards; these are the metal-can ICs at the bottom of the board. To reduce the load on the regulators, two large resistors drop the input voltage (28 volts) to a lower level before it is regulated.

The control board holds the FSK decoder, an interesting circuit that converts the two FSK frequencies to binary by implementing a digital auto-correlator. It uses a 64-bit shift register to delay the digital input by 139 µs. The input and the delayed input are XOR'd together, generating a result that depends on the frequency. A 7200 Hz signal repeats every 139 µs, so the input and the delayed input match, yielding 0 from the XOR. However, a 3600 Hz square wave switches state every 139 µs, so the two XOR inputs will always differ, resulting in a 1 output. Thus, the circuit cleanly distinguishes between a 3600 Hz input and a 7200 Hz input.

The digital demodulator avoids some of the problems of an analog FSK demodulator. It is not sensitive to signal levels, since the signal is converted to digital. The digital demodulator is also not sensitive to harmonics, which can cause problems with analog demodulators. Finally, it doesn't require the carefully-tuned filters of an analog circuit.

The demodulated signal passes from the control board back to the output board. This board applies a 400 Hz low-pass filter and then a threshold to convert the signal back to binary. If the input frequencies are not exact, the demodulator will produce the correct 0 or 1 value over most of the waveform, but there will be glitches at the edges. The low-pass filter removes these glitches. (You might be concerned that a 600-baud signal would be wiped out by a 400 Hz low-pass filter. However, the worst case signal (alternating 0's and 1's) would be 300 Hz because it takes two bits to make one cycle, so the filter has plenty of margin.) Next, the board blocks the signal unless a carrier is detected. This ensures that random noise isn't demodulated and printed. Finally, the serial binary signal leaves the custom Shuttle boards and goes to the teleprinter's communication board, part of the standard teleprinter.

I noticed two unusual things about these boards. First, they have some modifications: "bodge" wires and added components. Second, the boards are not conformal coated, which is unusual for aerospace boards. (The four logic cards, in comparison, are protected with conformal coating.) My hypothesis is that these boards were development boards, early in the design process of the Shuttle teleprinter, so they were modified as the design changed. The teleprinter is also marked "Not for flight", which supports this theory.

The logic cards

The military teleprinter contained four logic circuit cards: a CPU card, a memory card, a communications card, and a print control card, mounted at the right rear of the teleprinter. These cards are used unchanged in the Shuttle teleprinter.

The circuitry is more complex than you might expect, with four large cards full of ICs. There are several reasons for this. First, the cards use 1970s microprocessor technology, so it takes a lot of circuitry to do anything. In particular, many simple 7400-series logic chips perform "glue" functions: decoding addresses, buffering data, latching signals, and so forth. Moreover, a drum printer is inherently complicated, since 80 hammers must be driven at the right time based on the desired characters. Third, the teleprinter is very flexible, supporting multiple signal levels and two character formats (ASCII and Baudot). Most surprisingly, the teleprinter implements a word processor, allowing messages to be composed and edited offline. Of course, since the Shuttle's teleprinter is only used to receive data, and doesn't even have a keyboard, the word processor feature is entirely useless.

The CPU card

The CPU card holds the microprocessor that controls the teleprinter. Its most important function is to convert a line of ASCII characters into print drum codes. These codes are stored in memory for use by the print control card. The CPU also implements configuration and self-test functions.

The diagram below shows some of the main components. The CPU card contains a Motorola 6800 CPU, 4 kilobytes of memory, and a ROM that holds its program code.8 Inconveniently, all the IC part numbers are military numbers so it takes some investigation to determine what a part really is. The MC6822 is a Peripheral Interface Adapter, a Motorola chip that provides two parallel I/O ports. This chip is used on three of the cards to support a variety of I/O tasks. On the CPU card, the I/O ports drive eight status lamps (most of which were removed for the Shuttle teleprinter) as well as internal status signals such as "paper low" or "keyboard present" and the baud rate setting input.

The print control card

In a sense, the print control card is the heart of the printer, since it causes characters to be printed by firing hammers against the rotating drum. As the drum goes through one revolution, all 64 characters will spin past each of the 80 print positions. By firing hammers at the exact time, the card prints a line of text.9 In more detail, for each row on the drum, the printer card scans through the 80-character memory buffer using Direct Memory Access (DMA). If the value in memory matches the current drum row number, the hammer is fired. Note that the hammers don't fire simultaneously, but in sequence as memory is scanned.

The diagram above shows the interaction between the drum, the print control card, and the 80 hammers. The hammers are implemented on 20 print hammer cards, each with 4 hammers. Electrically, the hammers are arranged in a matrix. One wire out of 20 (S1-S20) selects the hammer board, the group of four. Another wire selects one of four hammers (Col 1-4). This approach simplifies the electronics, since 20 + 4 driver circuits and wires are used, rather than 80 (one for each column). The print control card is synchronized to the drum by two photo-transistor sensors that detect the drum's position. One sensor is triggered on each row, while the other sensor triggers once per revolution.

The print control card is shown below, with the main functional blocks labeled. The large purple-and-gold chip is the PIA, the same I/O chip that appeared on the CPU card. It handles a variety of signals such as the self-test request, paper out, and the drum stop signal. The mode control logic generates timing signals depending on the printer's mode. The data compare logic increments the row counter on each drum pulse, and compares the row counter to the value read from memory.10 The hammer driver circuitry on the left selects one of the 20 hammer cards, while the hammer driver circuitry on the right selects one of four hammers. The ribbon circuitry raises and lowers the ribbon so the ribbon doesn't block the text when the printer is idle. The line feed circuitry advances the paper for a line feed operation.

The photo below shows one of the hammer cards, with four hammers. Each hammer has an electromagnet that pulls a lever, rotating the hammer wheel, and causing the hammer to strike the paper. (The hammers themselves are in the upper right of the photo.) A screw adjustment controls the distance between each hammer and the paper, allowing precise adjustment of the timing. (Marc had to carefully adjust all the hammers to make the print quality readable.)

The communication card

The communication card handles the teleprinter's serial data input. The key chip is the 8251A, a USART (Universal Synchronous/Asynchronous Receiver/Transmitter). This complex chip performs the conversion between the serial data stream and the bytes that the processor uses. (Note that the military teleprinter both sent and received serial data, while the Shuttle teleprinter only receives data.) The chip has a few support chips, labeled "UART" in the diagram below. The board has another Peripheral Interface Adapter chip, providing two I/O ports. These ports have functions such as reading the serial line settings (ASCII vs. Baudot, odd or even parity, number of stop bits, and current loop levels).

The board also has circuitry to generate the clock pulses for the selected baud rate. The mode circuitry handles various phases of transmit/receive. The filter/demod circuitry handles different input types, digitally filtering and demodulating as necessary.11

The memory card

The memory card supports the word-processing feature. It provides additional RAM to hold the text buffer as well as the ROM holding the software for editing. The 16 DRAM chips on the left (MK4027) provide 8 KB of RAM while the two ROM chips on the right provide 8K of ROM. The chips in the middle to the right of the resistors split the 12 address bits into row and column addresses as required by the RAM chips. The address signals go through the numerous 24 Ω resistors in the middle; I don't know why. According to the manual, the printer operates fine without this card, except without the word processor. Since the word processor was irrelevant to the Shuttle, I wonder why this card wasn't removed to reduce weight.

The power supply

The power supply board (shown earlier) implements separate power supplies for different parts of the printer.12 The supplies are implemented as switching power supplies, which were not as common at the time as now. The microprocessor supply provides +5V, +12V, and -5V, voltages required by memory chips in the 1970s. A separate switching power supply provides +5V, -8.6V, and +8.6V for the keyboard, dustcover, and interface module, components that were removed for the Shuttle teleprinter. Another supply powers the printer's status lamps.

The drum motor supply is important because its voltage is regulated to control the rotational speed of the drum. A sensor on the drum provides a feedback pulse for each row on the drum. (I think the drum speed is 868 RPM.) These pulses control the drum motor's switching supply. If the drum spins too slowly, the voltage is increased, and similarly if it spins too fast.

The hammers have an unusual constant-current power supply. When the printer is active, this power supply generates +18 V. However, the power supply is designed to use a constant current of 600 mA regardless of the hammer activity. A capacitor provides a reservoir of power that is filled by the constant current. If the hammers are using less current, the excess current is bled off through a resistor. The purpose of this is "to mask printing intelligence during periods of message traffic." In other words, if you used a teleprinter in the embassy in Moscow, for instance, spies could monitor power transients to see when hammers are firing, and perhaps figure out what is being printed. By keeping the current constant, this source of intelligence is blocked. Of course, this feature is useless on the Space Shuttle and only wastes power.

The military teleprinter accepted multiple input voltages: 22-30 VDC, 115 VAC, or 230 VAC, along with a 12 VDC battery backup. The transformers and diodes to support these voltages were part of the interface module that was removed for the Shuttle teleprinter. Instead, the Shuttle teleprinter is powered by 28 VDC.

Mechanical changes

The military teleprinter underwent significant mechanical changes to make it suitable for the Shuttle. These changes reduced its weight from 100 pounds to 59 pounds. The most visible change to the printer is the removal of the keyboard. The entire front section of the printer was replaced, removing the controls that were not needed in the Shuttle.13 The rugged frame of the original printer was replaced with a lighter-weight (but still substantial) frame. Horizontal rails were added to the frame to support the printer in the Shuttle locker.

The photo below shows the front of the Shuttle teleprinter. While the military teleprinter had numerous lights and switches on the front, the Shuttle teleprinter has just two lights and four switches.

NASA was concerned that the temperature of the teleprinter could become hazardous to the astronauts. To mitigate this danger, the teleprinter had a large heat-sensitive warning sticker. The yellow sticker on the left of the teleprinter changes color and displays an image if it heats up: it shows a bandaged hand and the word "HOT". Above it is an "Omegalabel" temperature monitoring sticker that shows the highest temperature the device reached. There are more of these stickers inside the teleprinter on various motors.

The Interim Teleprinter inside the Space Shuttle

The teleprinter was too large to be mounted on the flight deck, so it was mounted in a storage locker on the middeck, one level lower. The photo below shows the location of the locker that held the teleprinter (although the teleprinter was not present in this photo), looking backward (aft) toward the airlock. The locker is denoted MA9F, indicating Mid-deck Aft, position 9F (details), in the back on the right side of the Shuttle.

The teleprinter was noisy because of its impact printing; even with it in a locker, the sound outside was 69.5 dB. The solution was to soundproof the locker with acoustic insulation. Various insulating materials were tested until one was found that passed the toxicity requirements. Another flammability waiver was required for the insulation.

Putting the teleprinter in an insulated locker without cooling caused another problem: overheating. The military teleprinter used 34 watts even while idle, which would cause the printer to become dangerously hot after just 6 orbits. The printer was redesigned to support a standby mode that used just 1 watt. When a signal from Earth was detected, the printer would power up while in use, and then return to standby mode. A circuit was added to send a tone back to Earth when the printer was activated, reassuring Mission Control that the printer had switched out of standby mode. These circuits were on the three custom Shuttle boards described earlier.

Putting the teleprinter in a locker made cabling difficult. The solution was a panel on the locker door with connectors for power and audio. The panel has a power switch and light as well as a light to indicate that a message has been received.

The photo below shows the teleprinter locker with the connection panel on the far left. Note the cables attached to the connectors. These cables went across the back of the Shuttle to the left side, where they went up to the flight deck; the cable routing was performed before launch.14 For this flight, the neighboring locker MA16F held 3300 honeybees for a student experiment.

The teleprinter cables connect to the shuttle at panel A15 on the aft bulkhead of the flight deck on the left side of the Shuttle. In other words, if you sat in the Shuttle Commander's seat in the cockpit and turned around, this is what you would see.

The audio cable from the teleprinter went to the Payload Specialist communication connection on panel A15, while the power cable went to the DC power connection right below. During launch, this audio connection was needed for crew communication, so the teleprinter was plugged in after launch and the audio settings were reconfigured on panel L9. A cue card was placed above panel L9 with instructions on the teleprinter.

The teleprinter's replacements

The Shuttle teleprinter was supposed to be used for a short time until the Uplink Text and Graphics System (TAGS) entered service, but things didn't work out that way. TAGS, described earlier, was the fax-like system that could receive grayscale images, but it depended on the TDRS satellites with their support for digital data. The first TDRS satellite was launched by the sixth shuttle flight, STS-6 (1983). This allowed the use of TAGS on STS-7, but the printer promptly jammed.15 TAGS had constant problems with jamming; on STS-35, the printer jammed and then the unjamming tool broke. Due to the unreliability of the TAGS, the Interim Teleprinter was kept in service as a backup device. TAGS was mounted on a dual cold plate in avionics bay 3 of the crew compartment middeck (details), on the other side of the airlock from the teleprinter.

After a decade, another printer, the Thermal Impulse Printer System (TIPS) was put into service, probably on flight STS-56 in 1993. Once TIPS proved its reliability, it replaced both the teleprinter and the Text and Graphics System (TAGS). The TIPS printer was installed in mid-deck locker MF28E; the F indicates the locker was on the forward wall, not the aft wall that held the Interim Teleprinter. As a backup for the TIPS, the Shuttle flew with a second TIPS.

One motivation behind the TIPS thermal printer was NASA's desire to use more commercial-off-the-shelf (COTS) equipment instead of expensive custom equipment. The TIPS printer is the Raytheon TDU-850 printer (below), a commercial product that sold for $4950. A custom communication interface board inside the printer provided the interface between the printer and the Shuttle's S-Band and Ku-Band communications systems. This interface also allowed astronauts to use the TIPS as a printer for an onboard personal computer.

The photo below shows the TIPS printer in use, printing a long stream of output that Eileen Collins is reading. Collins was the first woman to pilot the Space Shuttle; she flew on the Shuttle four times, twice as pilot and twice as commander.

The teleprinter, operational

We succeeded in making the Shuttle teleprinter operational. The printer had many mechanical problems, mainly because the rubber rollers had turned to liquid and gummed up the mechanism. Marc disassembled the printer, carefully cleaned the mechanism, and realigned everything. I won't discuss the restoration process here since there will be a video on CuriousMarc's channel. We were able to send FSK-modulated data to the printer and it was printed successfully, as shown below.

Conclusions

At first, I thought that the Shuttle's Interim Teleprinter was a terrible design. It's absurdly heavy and was in danger of overheating. Although the design started with an existing product, much of it required redesign: the front section, the new drum, the interface, and even the frame. The design inherited features it couldn't use, such as the built-in word processor. And the constant-current feature was pointless for the Shuttle and just wasted power.

When I learned that the design had to be completed in just seven months, my opinion of the teleprinter improved. Moreover, the design had many constraints, such as toxicity and flammability restrictions, that limited the potential approaches.

In the end, the teleprinter was used on over 50 flights, acting as a reliable backup to the somewhat flaky Text and Graphics System (TAGS).16 Despite its name, the Interim Teleprinter turned out to be a long-lasting solution, not interim at all. So I have to conclude that the teleprinter was a good design, working much better and much longer than intended.17

In any case, the Interim Teleprinter is an interesting piece of hardware and I hope you enjoyed this article. Follow me on Mastodon as @kenshirriff@oldbytes.space or RSS. Thanks to Marcel for providing the printer. Restoration performed with CuriousMarc, Eric Schlapefer, and Mike Stewart.

Notes and references

The Space Shuttle contained a bulky printer so the astronauts could receive procedures, mission plans, weather reports, crew activity plans, and other documents. Needed for the first Shuttle launch in 1981, this printer was designed in just 7 months, built around an Army communications terminal. Unlike modern printers, the Shuttle's printer contains a spinning metal drum with raised characters, allowing it to rapidly print a line at a time.

This printer is known as the Space Shuttle Interim Teleprinter System.1 As the name "Interim" suggests, this printer was intended as a stop-gap measure, operating for a few flights until a better printer was operational. However, the teleprinter proved to be more reliable than its replacement, so it remained in use as a backup for over 50 flights, often printing thousands of lines per flight. This didn't come cheap: with a Shuttle flight costing $27,000 per pound, putting the 59-pound teleprinter in space cost over $1.5 million per flight.

We obtained access to a Shuttle teleprinter (probably a development system that remained on the ground) and wanted to put it into operation. I had to reverse engineer three of the boards inside the printer to determine the data format the printer accepted: serial data encoded into audio. But after analyzing the printer and performing a lot of maintenance, we succeeded in getting the printer to print. In this article, I'll describe the Shuttle's Interim Teleprinter, explain its circuitry and drum-based printing mechanism, and show it in operation.

History of the Shuttle's Interim Teleprinter

The motivation for the teleprinter goes back to the Apollo program. During Apollo missions, the only way to send information to the astronauts was by talking to them over the radio and having the astronauts write down the data. NASA decided that the Space Shuttle should include a mechanism to send text and images to the astronauts, a 78-pound, high-tech fax machine called the Uplink Text & Graphics System (TAGS). A high-resolution grayscale image was sent to the Shuttle as a digital data stream. Onboard the Shuttle, a squat CRT displayed the image one line at a time and a fiber-optic faceplate transferred each line to light-sensitive silver emulsion paper. The paper was developed by passing it over a hot roller at 260ºF for 25 seconds, creating a permanent image.

The one flaw in this plan was that sending the digital image to the Shuttle required the Tracking and Data Relay Satellite System (TDRS), which due to delays wouldn't be ready until the sixth Shuttle flight. (The TDRS was a space-based replacement for the worldwide network of ground stations that was used during Apollo.) As a result, NASA decided just seven months before the first Shuttle launch that they needed an interim system "for transmission of real-time, flight-plan changes and other operational data to the crew."2

The Shuttle teleprinter is the result of this rushed effort to create a printer that could work over the existing audio channel rather than the digital TDRS satellite. Due to the time pressure, the Shuttle teleprinter needed to be based on an off-the-shelf printer. Thermal and electrostatic printers were rejected due to toxicity and flammability problems. (The Shuttle teleprinter used a roll of yellowish paper, which required a NASA waiver due to its flammability, a concern ever since the Apollo-1 disaster).

The decision was made to use a military communications terminal, the the AN/UCG-743 "Tactical Teletype". The terminal's interfacing was very flexible, supporting serial data in either ASCII or Baudot format, with multiple configurations and baud rates (up to 1200 baud), using either a current-loop or voltage signals. The military terminal supported two-way communication, so it had a keyboard. Remarkably, the terminal also implemented a word processor, controlled by a Motorola 6800 microprocessor (ancestor of the famous MOS 6502). The word processor allowed messages to be composed offline, minimizing the radio transmission time, which was important in a hostile environment. As will be seen, this 100-pound military system required many large changes to be usable on the Space Shuttle, most visibly removing the keyboard.

The printing mechanism

The teleprinter uses a spinning drum with raised characters, shown below.4 To print a character, the printer fires a hammer, forcing the inked ribbon and paper against the raised character on the drum. The drum is 80 characters wide, matching the line length, and there are 80 corresponding hammers, one for each print position. The drum has 64 printable characters, wrapped around each position of the drum.

The printer prints a line at a time, not instantaneously, but during each revolution of the drum. When the drum makes one complete revolution, each of the 64 characters passes by each print position once. Printing requires precise timing of the hammers to strike the right character on the drum as it whizzes by. The printer control circuitry triggers each hammer at the proper time, when the desired character on the drum is lined up with the hammer, producing the desired text.5

The character set is slightly different between the military printer and the Shuttle printer. The military drum had 64 ASCII characters (upper-case letters only, numbers, and special characters). The drum doesn't contain an explicit space character, since nothing is printed for a space. In its place, the drum has a diamond "◊", used as a special character to indicate a parity error or other error. The drum for the Shuttle teleprinter replaces 10 ASCII special characters with symbols that are more useful to the Shuttle, such as Greek letters for angles. Specifically, the characters ;@[\]^!"#$ are replaced by θ✓‾↑↓~αβΔϕ.

The video below shows a closeup of the hammers as they strike the paper to print text. The text is the teleprinter's built-in test message: "THE LAZY YELLOW DOG WAS CAUGHT BY THE SLOW RED FOX AS HE LAY SLEEPING IN THE SUN". This test message is based on the traditional quick brown fox..., which is a pangram, containing all 26 letters, but the teleprinter's test sentence is missing J, K, M, Q, and V. However, the test message is exactly 80 characters long and replaces spaces with the diamond "◊", so it is effective for verifying that all 80 columns work.

The electronics

The photo below shows the circuitry inside the teleprinter, looking down from above. At the left are the three interface boards, custom boards that demodulate the incoming audio signal. In front of the interface boards are large inductors to filter the incoming power. Hidden beneath them, a solid-state relay controls the power to the rest of the printer, implementing the low-power standby mode. In the middle, the blue board is the surprisingly complex switching power supply, mounted on a thick metal plate for cooling. Normally, the large roll of paper is mounted above the power supply board. At the right, four large circuit boards implement the main logic of the printer: a printer driver board, a communications board, a memory board, and the processor board. The rotating drum is protected by the perforated black metal grill at the front.

The demodulator boards

The original military teleprinter received data as a serial bitstream. However, on the Space Shuttle, data was encoded as frequencies on the audio link. Three custom boards were constructed to demodulate the audio data so the rest of the printer could handle it. These boards also performed Shuttle-specific tasks such as powering up the printer when a message comes in, and then returning the printer to standby mode. I reverse-engineered these boards to determine how they work and to determine the data encoding. (Schematics are in the footnotes.7) In this section, I'll discuss these three boards, which are on the left side of the printer.

To summarize, the serial bitstream is encoded with Frequency Shift Keying, with a 0 represented by 3600 Hz and a 1 represented by 7200 Hz.6 The serial data is transmitted at 600 baud, even parity, one stop bit. The demodulation process first converts the input audio to a digital signal by thresholding it. (That is, the input sine wave is converted to a square wave.) The digital signal is autocorrelated to distinguish the 3600 Hz and 7200 Hz signals, recovering the underlying serial data. This signal is passed to the printer's logic boards (part of the original military teleprinter), which convert the serial signal to ASCII bytes and prints them.

Signal processing starts with the "FSK input" board, shown below. First, it amplifies the input audio signal. (The two large resistors provide a 600 Ω load for the audio input.) Next, a 900 Hz high-pass filter eliminates low-frequency noise. (The filter is implemented by a two-stage Sallen-Key topology.)

The signal bounces from board to board, going to the "output FSK demod" board next. This board has a carrier-detect circuit that turns on the rest of the printer if it detects an input signal. This allows the printer to sit idle until it receives a signal from Earth. This board also applies the threshold to the signal to turn it into a digital waveform, which goes to the "control" board.

The output board also holds the 5-volt and 12-volt linear regulators that power the three boards; these are the metal-can ICs at the bottom of the board. To reduce the load on the regulators, two large resistors drop the input voltage (28 volts) to a lower level before it is regulated.

The control board holds the FSK decoder, an interesting circuit that converts the two FSK frequencies to binary by implementing a digital auto-correlator. It uses a 64-bit shift register to delay the digital input by 139 µs. The input and the delayed input are XOR'd together, generating a result that depends on the frequency. A 7200 Hz signal repeats every 139 µs, so the input and the delayed input match, yielding 0 from the XOR. However, a 3600 Hz square wave switches state every 139 µs, so the two XOR inputs will always differ, resulting in a 1 output. Thus, the circuit cleanly distinguishes between a 3600 Hz input and a 7200 Hz input. (The XOR output is opposite from the final value since it gets inverted later.)

The digital demodulator avoids some of the problems of an analog FSK demodulator. It is not sensitive to signal levels, since the signal is converted to digital. The digital demodulator is also not sensitive to harmonics, which can cause problems with analog demodulators. Finally, it doesn't require the carefully-tuned filters of an analog circuit.

The demodulated signal passes from the control board back to the output board. This board applies a 400 Hz low-pass filter and then a threshold to convert the signal back to binary. If the input frequencies are not exact, the demodulator will produce the correct 0 or 1 value over most of the waveform, but there will be glitches at the edges. The low-pass filter removes these glitches. (You might be concerned that a 600-baud signal would be wiped out by a 400 Hz low-pass filter. However, the worst case signal (alternating 0's and 1's) would be 300 Hz because it takes two bits to make one cycle, so the filter has plenty of margin.) Next, the board blocks the signal unless a carrier is detected. This ensures that random noise isn't demodulated and printed. Finally, the serial binary signal leaves the custom Shuttle boards and goes to the teleprinter's communication board, part of the standard teleprinter.

I noticed two unusual things about these boards. First, they have some modifications: "bodge" wires and added components. Second, the boards are not conformal coated, which is unusual for aerospace boards. (The four logic cards, in comparison, are protected with conformal coating.) My hypothesis is that these boards were development boards, early in the design process of the Shuttle teleprinter, so they were modified as the design changed. The teleprinter is also marked "Not for flight", which supports this theory.

The logic cards

The military teleprinter contained four logic circuit cards: a CPU card, a memory card, a communications card, and a print control card, mounted at the right rear of the teleprinter. These cards are used unchanged in the Shuttle teleprinter.

The circuitry is more complex than you might expect, with four large cards full of ICs. There are several reasons for this. First, the cards use 1970s microprocessor technology, so it takes a lot of circuitry to do anything. In particular, many simple 7400-series logic chips perform "glue" functions: decoding addresses, buffering data, latching signals, and so forth. Moreover, a drum printer is inherently complicated, since 80 hammers must be driven at the right time based on the desired characters. Third, the teleprinter is very flexible, supporting multiple signal levels and two character formats (ASCII and Baudot). Most surprisingly, the teleprinter implements a word processor, allowing messages to be composed and edited offline. Of course, since the Shuttle's teleprinter is only used to receive data, and doesn't even have a keyboard, the word processor feature is entirely useless.

The CPU card

The CPU card holds the microprocessor that controls the teleprinter. Its most important function is to convert a line of ASCII characters into print drum codes. These codes are stored in memory for use by the print control card. The CPU also implements configuration and self-test functions.

The diagram below shows some of the main components. The CPU card contains a Motorola 6800 CPU, 4 kilobytes of memory, and a ROM that holds its program code.8 Inconveniently, all the IC part numbers are military numbers so it takes some investigation to determine what a part really is. The MC6822 is a Peripheral Interface Adapter, a Motorola chip that provides two parallel I/O ports. This chip is used on three of the cards to support a variety of I/O tasks. On the CPU card, the I/O ports drive eight status lamps (most of which were removed for the Shuttle teleprinter) as well as internal status signals such as "paper low" or "keyboard present" and the baud rate setting input.

The print control card

In a sense, the print control card is the heart of the printer, since it causes characters to be printed by firing hammers against the rotating drum. As the drum goes through one revolution, all 64 characters will spin past each of the 80 print positions. By firing hammers at the exact time, the card prints a line of text.9 In more detail, for each row on the drum, the printer card scans through the 80-character memory buffer using Direct Memory Access (DMA). If the value in memory matches the current drum row number, the hammer is fired. Note that the hammers don't fire simultaneously, but in sequence as memory is scanned.

The diagram above shows the interaction between the drum, the print control card, and the 80 hammers. The hammers are implemented on 20 print hammer cards, each with 4 hammers. Electrically, the hammers are arranged in a matrix. One wire out of 20 (S1-S20) selects the hammer board, the group of four. Another wire selects one of four hammers (Col 1-4). This approach simplifies the electronics, since 20 + 4 driver circuits and wires are used, rather than 80 (one for each column). The print control card is synchronized to the drum by two photo-transistor sensors that detect the drum's position. One sensor is triggered on each row, while the other sensor triggers once per revolution.

The print control card is shown below, with the main functional blocks labeled. The large purple-and-gold chip is the PIA, the same I/O chip that appeared on the CPU card. It handles a variety of signals such as the self-test request, paper out, and the drum stop signal. The mode control logic generates timing signals depending on the printer's mode. The data compare logic increments the row counter on each drum pulse, and compares the row counter to the value read from memory.10 The hammer driver circuitry on the left selects one of the 20 hammer cards, while the hammer driver circuitry on the right selects one of four hammers. The ribbon circuitry raises and lowers the ribbon so the ribbon doesn't block the text when the printer is idle. The line feed circuitry advances the paper for a line feed operation.

The photo below shows one of the hammer cards, with four hammers. Each hammer has an electromagnet that pulls a lever, rotating the hammer wheel, and causing the hammer to strike the paper. (The hammers themselves are in the upper right of the photo.) A screw adjustment controls the distance between each hammer and the paper, allowing precise adjustment of the timing. (Marc had to carefully adjust all the hammers to make the print quality readable.)

The communication card

The communication card handles the teleprinter's serial data input. The key chip is the 8251A, a USART (Universal Synchronous/Asynchronous Receiver/Transmitter). This complex chip performs the conversion between the serial data stream and the bytes that the processor uses. (Note that the military teleprinter both sent and received serial data, while the Shuttle teleprinter only receives data.) The chip has a few support chips, labeled "UART" in the diagram below. The board has another Peripheral Interface Adapter chip, providing two I/O ports. These ports have functions such as reading the serial line settings (ASCII vs. Baudot, odd or even parity, number of stop bits, and current loop levels).

The board also has circuitry to generate the clock pulses for the selected baud rate. The mode circuitry handles various phases of transmit/receive. The filter/demod circuitry handles different input types, digitally filtering and demodulating as necessary.11

The memory card

The memory card supports the word-processing feature. It provides additional RAM to hold the text buffer as well as the ROM holding the software for editing. The 16 DRAM chips on the left (MK4027) provide 8 KB of RAM while the two ROM chips on the right provide 8K of ROM. The chips in the middle to the right of the resistors split the 12 address bits into row and column addresses as required by the RAM chips. The address signals go through the numerous 24 Ω resistors in the middle; I don't know why. According to the manual, the printer operates fine without this card, except without the word processor. Since the word processor was irrelevant to the Shuttle, I wonder why this card wasn't removed to reduce weight.

The power supply

The power supply board (shown earlier) implements separate power supplies for different parts of the printer.12 The supplies are implemented as switching power supplies, which were not as common at the time as now. The microprocessor supply provides +5V, +12V, and -5V, voltages required by memory chips in the 1970s. A separate switching power supply provides +5V, -8.6V, and +8.6V for the keyboard, dustcover, and interface module, components that were removed for the Shuttle teleprinter. Another supply powers the printer's status lamps.

The drum motor supply is important because its voltage is regulated to control the rotational speed of the drum. A sensor on the drum provides a feedback pulse for each row on the drum. (I think the drum speed is 868 RPM.) These pulses control the drum motor's switching supply. If the drum spins too slowly, the voltage is increased, and similarly if it spins too fast.

The hammers have an unusual constant-current power supply. When the printer is active, this power supply generates +18 V. However, the power supply is designed to use a constant current of 600 mA regardless of the hammer activity. A capacitor provides a reservoir of power that is filled by the constant current. If the hammers are using less current, the excess current is bled off through a resistor. The purpose of this is "to mask printing intelligence during periods of message traffic." In other words, if you used a teleprinter in the embassy in Moscow, for instance, spies could monitor power transients to see when hammers are firing, and perhaps figure out what is being printed. By keeping the current constant, this source of intelligence is blocked. Of course, this feature is useless on the Space Shuttle and only wastes power.

The military teleprinter accepted multiple input voltages: 22-30 VDC, 115 VAC, or 230 VAC, along with a 12 VDC battery backup. The transformers and diodes to support these voltages were part of the interface module that was removed for the Shuttle teleprinter. Instead, the Shuttle teleprinter is powered by 28 VDC.

Mechanical changes

The military teleprinter underwent significant mechanical changes to make it suitable for the Shuttle. These changes reduced its weight from 100 pounds to 59 pounds. The most visible change to the printer is the removal of the keyboard. The entire front section of the printer was replaced, removing the controls that were not needed in the Shuttle.13 The rugged frame of the original printer was replaced with a lighter-weight (but still substantial) frame. Horizontal rails were added to the frame to support the printer in the Shuttle locker.

The photo below shows the front of the Shuttle teleprinter. While the military teleprinter had numerous lights and switches on the front, the Shuttle teleprinter has just two lights and four switches.

NASA was concerned that the temperature of the teleprinter could become hazardous to the astronauts. To mitigate this danger, the teleprinter had a large heat-sensitive warning sticker. The yellow sticker on the left of the teleprinter changes color and displays an image if it heats up: it shows a bandaged hand and the word "HOT". Above it is an "Omegalabel" temperature monitoring sticker that shows the highest temperature the device reached. There are more of these stickers inside the teleprinter on various motors.

The Interim Teleprinter inside the Space Shuttle

The teleprinter was too large to be mounted on the flight deck, so it was mounted in a storage locker on the middeck, one level lower. The photo below shows the location of the locker that held the teleprinter (although the teleprinter was not present in this photo), looking backward (aft) toward the airlock. The locker is denoted MA9F, indicating Mid-deck Aft, position 9F (details), in the back on the right side of the Shuttle.

The teleprinter was noisy because of its impact printing; even with it in a locker, the sound outside was 69.5 dB. The solution was to soundproof the locker with acoustic insulation. Various insulating materials were tested until one was found that passed the toxicity requirements. Another flammability waiver was required for the insulation.

Putting the teleprinter in an insulated locker without cooling caused another problem: overheating. The military teleprinter used 34 watts even while idle, which would cause the printer to become dangerously hot after just 6 orbits. The printer was redesigned to support a standby mode that used just 1 watt. When a signal from Earth was detected, the printer would power up while in use, and then return to standby mode. A circuit was added to send a tone back to Earth when the printer was activated, reassuring Mission Control that the printer had switched out of standby mode. These circuits were on the three custom Shuttle boards described earlier.

Putting the teleprinter in a locker made cabling difficult. The solution was a panel on the locker door with connectors for power and audio. The panel has a power switch and light as well as a light to indicate that a message has been received.

The photo below shows the teleprinter locker with the connection panel on the far left. Note the cables attached to the connectors. These cables went across the back of the Shuttle to the left side, where they went up to the flight deck; the cable routing was performed before launch.14 For this flight, the neighboring locker MA16F held 3300 honeybees for a student experiment.

The teleprinter cables connect to the shuttle at panel A15 on the aft bulkhead of the flight deck on the left side of the Shuttle. In other words, if you sat in the Shuttle Commander's seat in the cockpit and turned around, this is what you would see.

The audio cable from the teleprinter went to the Payload Specialist communication connection on panel A15, while the power cable went to the DC power connection right below. During launch, this audio connection was needed for crew communication, so the teleprinter was plugged in after launch and the audio settings were reconfigured on panel L9. A cue card was placed above panel L9 with instructions on the teleprinter.

The teleprinter's replacements

The Shuttle teleprinter was supposed to be used for a short time until the Uplink Text and Graphics System (TAGS) entered service, but things didn't work out that way. TAGS, described earlier, was the fax-like system that could receive grayscale images, but it depended on the TDRS satellites with their support for digital data. The first TDRS satellite was launched by the sixth shuttle flight, STS-6 (1983). This allowed the use of TAGS on STS-7, but the printer promptly jammed.15 TAGS had constant problems with jamming; on STS-35, the printer jammed and then the unjamming tool broke. Due to the unreliability of the TAGS, the Interim Teleprinter was kept in service as a backup device. TAGS was mounted on a dual cold plate in avionics bay 3 of the crew compartment middeck (details), on the other side of the airlock from the teleprinter.

After a decade, another printer, the Thermal Impulse Printer System (TIPS) was put into service, probably on flight STS-56 in 1993. Once TIPS proved its reliability, it replaced both the teleprinter and the Text and Graphics System (TAGS). The TIPS printer was installed in mid-deck locker MF28E; the F indicates the locker was on the forward wall, not the aft wall that held the Interim Teleprinter. As a backup for the TIPS, the Shuttle flew with a second TIPS.

One motivation behind the TIPS thermal printer was NASA's desire to use more commercial-off-the-shelf (COTS) equipment instead of expensive custom equipment. The TIPS printer is the Raytheon TDU-850 printer (below), a commercial product that sold for $4950. A custom communication interface board inside the printer provided the interface between the printer and the Shuttle's S-Band and Ku-Band communications systems. This interface also allowed astronauts to use the TIPS as a printer for an onboard personal computer.

The photo below shows the TIPS printer in use, printing a long stream of output that Eileen Collins is reading. Collins was the first woman to pilot the Space Shuttle; she flew on the Shuttle four times, twice as pilot and twice as commander.

The teleprinter, operational

We succeeded in making the Shuttle teleprinter operational. The printer had many mechanical problems, mainly because the rubber rollers had turned to liquid and gummed up the mechanism. Marc disassembled the printer, carefully cleaned the mechanism, and realigned everything. I won't discuss the restoration process here since there will be a video on CuriousMarc's channel. We were able to send FSK-modulated data to the printer and it was printed successfully, as shown below.

Conclusions

At first, I thought that the Shuttle's Interim Teleprinter was a terrible design. It's absurdly heavy and was in danger of overheating. Although the design started with an existing product, much of it required redesign: the front section, the new drum, the interface, and even the frame. The design inherited features it couldn't use, such as the built-in word processor. And the constant-current feature was pointless for the Shuttle and just wasted power.

When I learned that the design had to be completed in just seven months, my opinion of the teleprinter improved. Moreover, the design had many constraints, such as toxicity and flammability restrictions, that limited the potential approaches.

In the end, the teleprinter was used on over 50 flights, acting as a reliable backup to the somewhat flaky Text and Graphics System (TAGS).16 Despite its name, the Interim Teleprinter turned out to be a long-lasting solution, not interim at all. So I have to conclude that the teleprinter was a good design, working much better and much longer than intended.17

In any case, the Interim Teleprinter is an interesting piece of hardware and I hope you enjoyed this article. Follow me on Mastodon as @kenshirriff@oldbytes.space or RSS. Thanks to Marcel for providing the printer. Restoration performed with CuriousMarc, Eric Schlapefer, and Mike Stewart.

Notes and references

I recently obtained an aerospace computer from the early 1970s, apparently part of a navigation system. Aerospace computers are an interesting but mostly neglected area of computer hardware, so I'm always delighted to examine one up close. In an era when most computers were large mainframes, aerospace computers packed dense electronics into a small package, using technologies such as surface-mounted components and multi-layer printed circuit boards, technologies that wouldn't reach the mainstream for another decade. This blog post examines the circuitry and components inside this computer, including an unusual electromechanical display. Although I was unable to determine who manufactured this system or even its exact function, this system illustrates how hundreds of integrated circuits and a core memory stack can be crammed into a compact package.

The keyboard

The device has a simple numeric keyboard with a few unexpected features. The numeric keypad can also be used for direction entry, as four of the keys have N, S, E, and W on them. The keys are large, roughly the size of the Apollo spacecraft's DSKY buttons. My theory is that these buttons are designed for operation with gloves, perhaps in a fighter plane where the pilot wears a pressure suit. The buttons are hinged at the top, so they don't push straight in, but pivot when pressed.

Numeric keypads typically use one of two layouts: a telephone-style keypad has the digits 123 at the top, while a calculator-style keypad has the digits 789 at the top. Interestingly, this device uses a calculator layout, while most aviation devices have a telephone layout. The Apollo DSKY also used a calculator layout, which could be a hint at a NASA connection for this device.

Above the keyboard are four codes for self-test: N4576, E9384, S9021, and W4830. Entering these codes on the keyboard presumably triggered the appropriate test of the system when the switch is in test mode.

The display

The computer's display is simple, showing a latitude and longitude. Each value has one decimal position, providing 0.1° of accuracy. The latitude and longitude are prefixed with a compass direction: North/South for latitude and East/West for longitude.

The display is constructed from an unusual type of electromechanical indicator, with an indicator module for each digit. Each digit position has a rotating wheel with 11 positions (ten digits and a blank). When the indicator module for a position is energized, the wheel spins to the specified position, showing the selected digit. The two leftmost indicators are slightly different as they show a compass direction instead of a digit: N, S, E, or W. Moreover, the direction indicators can also show the compass direction with a diagonal slash through it, as seen above. Perhaps the slashed direction indicates a problem with the value.

The diagram below shows how a digit indicator operates. Each digit position has an electromagnet with a wire to energize it. The dial wheel has an attached permanent magnet (indicated by N and S). Energizing one of the electromagnets causes the dial to spin to that position, aligning the permanent magnet on the dial with the electromagnet. This mechanism forms a reliable indicator with just one moving part. The displayed digit is clearer than a seven-segment display since the digit uses a real font rather than being created from segments.

Looking at the back of the keyboard/display unit shows the wiring of the display indicators. Each indicator has a common connection and ten wires to energize one of the electromagnets.1 The electromagnets are connected in a matrix, with all the "1" wires connected, the "2" wires connected, and so forth. To rotate an indicator to a particular digit, a common wire and an electromagnet wire are energized. For instance, powering the common wire of the second indicator and the "5" electromagnetic wire causes the second indicator to rotate to the "5" position. The wiring has a three-dimensional structure with ten bare wires running between the boards, one for each digit value. A yellow wire hangs off each bare wire, linking it to the connector on the left. Each indicator has ten diodes on a circuit board to block "sneak" paths that would energize unselected electromagnets.

This matrix circuit reduces the amount of wiring required: although there are 100 electromagnets in total, just 20 wires are sufficient to control them. The driver circuitry, however, is a bit more complex as it must scan through the ten digit positions, activating the right pair of driver wires at the right time. Some of the logic circuitry described below must implement this scanning, as well as the driver circuitry to energize the indicators.

The display and keyboard have many similarities to the Delco Carousel Inertial Navigation System (INS) shown below. (The Delco Carousel was used in many military and civilian aircraft, from the C-141 cargo plane to the Boeing 747 passenger plane.) Both devices have two digital displays, one for latitude North/South and one for longitude East/West. Also note the numeric keypads with four keys assigned to the four compass directions. The controls of the Carousel INS system are considerably more complicated, though. The Carousel has a knob position "TK/GS" (track/ground speed), which may correspond to the "T/G" position on my device.

Note that the display on my unit has just four digits of accuracy, with one digit after the decimal point. A tenth of a degree would provide an accuracy of about ±7 miles, which is low for a navigation device. In comparison, the Delco Carousel has six digits of accuracy (± 100 feet perhaps). This suggests that the device does not provide INS navigation, but some other guidance with lower accuracy.

Packaging the electronics

The unit contains 14 circuit boards, crammed with TTL integrated circuits, along with a core memory stack. The photo below shows how circuit boards surround the core memory stack. The mechanical design of the unit is advanced, allowing the boards to be opened up like a book. This provides compact packaging while allowing access to the boards.

The circuit boards are four-layer printed circuit boards, more advanced than the common two-layer boards of the time. The boards use a mixture of surface-mounted and through-hole components. The flat-pack ICs and the tiny round transistors are surface mounted, which was rare at the time. On the other hand, the resistors, capacitors, diodes, and larger transistors use standard through-hole components. At the time, most electronics used through-hole components, although aerospace systems often used surface-mounted components for higher density. It wasn't until the late 1980s that surface-mount technology became commonplace.

The boards are mounted in solid metal frames, providing both structural integrity and heat conduction for cooling. Most of the frames hold two boards, mounted back-to-back for higher density.

The logic boards

Four of the circuit boards are logic boards, packed with flat-pack integrated circuits. The board below holds 55 integrated circuits, showing the high density that is possible with flat packs.

The logic ICs are Signetics 400-series chips, an early type of TTL (Transistor-Transistor Logic) chip. Just three types of these ICs are used: SE440J "Dual exclusive OR" (really AND-OR-INVERT but XOR if provided with particular inputs), SE455J "Dual 4-input buffer/driver" (4-input NAND or NOR gates depending on polarity), and SE480J "Quad 2-input NAND/NOR". These integrated circuits cost $15.45 each in 1966 (about $150 each in current dollars).2

The schematic below shows the circuit that implements AND-OR-INVERT (or exclusive or) in the SE440J. The multiple-emitter transistors on the inputs may appear unusual, but this is the standard way to implement TTL gates. It is important to note that this chip only contains 12 transistors, so the density is low. (Since the chip contains two of these gates, this circuit is duplicated.) In the mid-1960s, integrated circuits only contained a few transistors—the Apollo Guidance Computer's ICs had just 6 transistors—but by the time this unit was built in the early 1970s, some chips had thousands of transistors, tracking Moore's Law. Thus, this unit both illustrates how aviation computers could be built from simple integrated circuits and how the dramatic improvements in IC technology rapidly obsoleted these computers.

The Signetics 400-series seems to have been obscure and short-lived, probably killed off by the wild success of 7400-series TTL chips. I was able to find only a few announcements and datasheets for these chips. The only users of these chips that I could find were NASA projects from the late 1960s.3 Signetics 400-series chips were used in the Mariner Mars and Venus probes, in the Data Automation Subsystem (DAS) (link, link). The Voyager Mars probes also used them. The SE455J gates were also used to interface the Apollo Guidance Computer to a core-rope simulator. JPL used the SE455J in a core memory system. NASA used the SE455J, SE480J, and other Signetics chips in its design for the MICROMIN computer. None of these systems appear to be related to the navigation system, but they illustrate that NASA was using these specific Signetics chips at the time in multiple designs.

The chips are labeled "CDC", raising the possibility that these chips were built by Control Data Corporation (CDC) under license from Signetics. The Aerospace Division of CDC was active at the time, building various compact computer systems. For instance, the CDC 480 computer (1976) was a 16-bit computer based on the Am2900 bit-slice chip. Also known as the AN/AYK-14, this system was used on numerous aircraft including the F-18. An earlier CDC aerospace computer is the AN/AWG-9 Airborne Missile Control System (1965), a 24-bit computer in a compact 1.1 cubic foot package. Used on the F-14 fighter plane, this computer guided the Phoenix air-to-air missile. Based on CDC's activity in aerospace computers at the time, the mystery computer could be a CDC system, although this hypothesis is based solely on integrated circuits labeled "CDC".

The photo below shows another logic board. This one has numerous red and white wires attached, linking it to the rest of the system. Curiously, this board has a single transistor, with two associated resistors, in the middle of the board.

Analog boards

The computer contains not only logic boards but also boards full of analog circuitry to interface with the core memory, keyboard, and display. The board below contains 17 of the logic ICs seen earlier. However, it also uses many resistors, capacitors (red cylinders), transistors (white circles), inductors (white banded cylinders), and glass diodes. The board also has some analog integrated circuits. In particular, it has three TI SN52709 op-amps, the smaller 10-pin packages. The board also contains some integrated circuits that I couldn't identify: UT1000, UT1027, UD4001, and D245F. The SM 60 ICs in white packages have a logo that I don't recognize. The op-amps could function as sense amplifiers for the core memory, or this board could provide other analog interfacing.

The board has multiple gray four-pin packages labeled "926D". Based on the + and - markings, these packages are probably bridge rectifiers, maybe providing power for the circuits. Many of the other boards have these rectifiers. The analog boards also contain a few Halex flat-pack devices labeled "HALEX 101205 727". Hanlex manufactured thin-film resistors in flat packs, so these are probably resistor networks. NASA used Halex resistor networks in some devices (link).4

The analog board shown below sits next to the core memory stack. It uses a different set of flat-pack components: Signetics C8930G and PL 98321. Unfortunately, I could not identify these ICs. This board, unlike the previous boards, has a copper ground plane in the second layer of the circuit board; this layer is visible in the photo as the copper-colored background occupying most of the board.

Core memory

The unit is built around a core memory stack, as was common in the era before semiconductor memory took over. Magnetic core memory consists of a grid of tiny ferrite cores with wires threaded through them, forming a core plane. Typically, a core memory unit consists of multiple planes, one for each bit in the word, stacked to form a three-dimensional block of memory.

The photo below shows a closeup of the stack. It appears to have 20 planes, suggesting a 20-bit processor. Soldered wires connect the planes together to provide continuous wiring through the stack. The soldering on these wires looks somewhat haphazard, suggesting that this was not a production unit.

The photo below shows the other side of the core memory stack, with similar wiring between the planes. At the right are a few layers of a different type, connected with 26 wires. The tape measure shows that the core memory stack is compact, about 6 cm on a side (2¼").

Some of the boards are drivers for the core memory stack. The board below has 48 small round transistors, colored either blue or red. Note the green, white, and yellow wires in the lower right, mostly hidden under the brown ground ribbon. These wires are connected to the core memory stack.

The board below also has numerous wires to the core stack, underneath the brown ground ribbon, so it is presumably another driver board. This board has some round driver transistors with yellow dots. Curiously, in the upper left there are a few circuit board pads where transistors could be mounted but are missing. Perhaps with the additional components the board would support a system with more of something: a larger keyboard? more memory?

Looking at the back of the unit, you can see the display indicator wiring at the top and a circuit board at the bottom. This board contains 20 transistors in metal cans, specifically Motorola 2N3736 NPN transistors. The core memory stack has 20 planes, matching the 20 transistors on this board, so the board probably implements the core memory "inhibit drivers", controlling the bit written to each plane. The board also has numerous tiny surface-mount transistors in white, red, and black packages. Close examination shows a few thin green "bodge" wires on this board, indicating that rework was performed on the board to fix a circuit problem, another piece of evidence that this unit is a prototype.

The core memory stack is enclosed by two sheet metal boxes, which I removed for the photos. The stack also has two flexible ground planes attached to it. The designers clearly wanted to ensure that the memory was well shielded, to a degree that I haven't seen in other systems.

Conclusions

Despite my research, this aerospace computer remains a mystery. I was unable to identify who manufactured it or even its exact function. One hypothesis is a NASA connection since NASA was extensively using these Signetics chips at the time. Moreover, this computer was obtained in the Houston area. Another hypothesis, based on the "CDC" label on the chips, is that this computer was built by Control Data's Aerospace Division. If you have any leads on this mysterious aviation computer, please contact me.

This system may have been a prototype. It has no part numbers, manufacturer name, or identifying plate.5 Moreover, the soldering on the core memory stack doesn't seem to be flight quality. Finally, the boards don't have conformal coating, which is typically used for spaceflight systems. However, the mechanical design looks advanced for a prototype, with dense boards that fold together like a book.

This unit clearly has a navigation role, but seems to be too inaccurate for an inertial navigation system (INS). It contains many integrated circuits, but not enough to form a full computer. I hypothesize that this unit contains the circuitry to drive the core memory and the display, and handle keyboard input. Looking at the underside of the unit (below), there are three connectors. I suspect these connectors were plugged into a larger box that held the computer itself.

The date codes on the integrated circuits range from 1966 to 1973, so the computer was probably manufactured in 1973. The seven-year range for date codes is a bit surprising, since integrated circuit technology changed a lot during these years. I suspect that the Signetics 400-series ICs had older date codes because this line didn't catch on so there was a lot of old stock rather than newly-manufactured parts. I also suspect that this system was designed around 1969, based on the multiple NASA systems using these chips then, suggesting that the design and manufacturing of this unit was a multi-year project.

Despite the lingering mysteries of this device, it provides an interesting example of aerospace computers at the beginning of the 1970s. Even though integrated circuits were primitive at the time, with just a few transistors per chip, aerospace computers used these chips and high-density packaging to build computers that were compact, reliable, and low power. These miniature computers controlled aircraft, missiles, and spacecraft, worlds away from the room-filling mainframes that attracted most of the attention.

Thanks to Usagi Electric for providing the aerospace computer. Eric Schlaepfer and Marc Verdiell helped with the analysis. Thanks to Don Straney for his research and comments. Various commenters on Reddit and Twitter provided suggestions. Follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon as oldbytes.space@kenshirriff.

Notes and references

What would you say is the first microcomputer?1 The Apple I from 1976? The Altair 8800 from 1974? Perhaps the lesser-known Micral N (1973) or Q1 (1972)? How about the Arma Micro Computer from way back in 1962. The Arma Micro Computer was a compact 20-pound transistorized computer, designed for applications in space such as inertial or celestial navigation, steering, radar, or engine control.

Obviously, the Arma Micro Computer is not a microcomputer according to modern definitions, since its processor was made from discrete components. But it's an interesting computer in many ways. First, it is an example of the aerospace computers of the 1960s, advanced systems that are now almost entirely forgotten. People think of 1960s computers as room-filling mainframes, but there was a whole separate world of cutting-edge miniaturized aerospace computers. (Taking up just 0.4 cubic feet, the Arma Micro Computer was smaller than an Apple II.) Second, the Arma Micro Computer used strange components such as transfluxors and had an unusual 22-bit serial architecture. Finally, the Arma Micro Computer evolved into a series of computers used on Navy ships and submarines, the E-2C Hawkeye airborne early warning plane, the Concorde, and even Air Force One.

The Arma Micro Computer

The Micro Computer used 22-bit words, which may seem like a strange size from the modern perspective. But there's no inherent need for a word size to be a power of 2. In particular, the Micro Computer was designed for mathematical calculations, not dealing with 8-bit characters. The word size was selected to provide enough accuracy for its navigational tasks.

Another strange aspect of the Micro Computer is that it was a serial machine, sequentially operating on one bit of a word at a time.2 This approach was often used in early machines because it substantially reduced the amount of hardware required: it only needs a 1-bit data bus and a 1-bit ALU. The downside is that a serial machine is much slower because each 22-bit word takes 22 clock cycles (plus 5 cycles of overhead). As a result, the Micro Computer executed just 36000 operations per second, despite its 1 megahertz clock speed.

The Micro Computer had a small instruction set of 19 instructions.3 It included multiply, divide, and square root, instructions that weren't implemented in early microprocessors. This illustrates how early microprocessors were a significant step backward in functionality. Moreover, the multiply, divide, and square root instructions used a separate arithmetic unit, so they could execute in parallel with other arithmetic instructions. Because the Micro Computer needed to interact with spacecraft systems, it had a focus on I/O, with 120 digital inputs or outputs, configured as needed for a particular mission.

Circuits

The Micro Computer was built from silicon transistors and diodes, using diode-transistor logic. The construction technique was somewhat unusual. The basic circuits were the flip-flop, the complementary buffer (i.e. an inverter), and the diode gate. Each basic circuit was constructed on a small wafer, .77 inches on a side.5 The photo below shows wafers for a two-transistor flip-flop and two diode gates. Each wafer had up to 16 connection tabs on the edges. These wafers are analogous to integrated circuits, but constructed from discrete components.

The wafers were mounted on printed circuit boards, with up to 22 wafers on a board. Pairs of boards were mounted back to back with polyurethane foam between the boards to form a "sandwich", which was conformally coated. The result was a module that was protected against the harsh environment of a missile or spacecraft. The computer could handle a shock of 100 g's and temperatures of 0°C to 85°C as well as 100% humidity or a vacuum.

Because the Micro Computer was a serial machine, its bits were constantly moving. For register storage such as the accumulator, it used six magnetostrictive torsional delay lines, storing a sequence of bits as physical twists that formed pulses racing through a long coil of wire.

The photo below shows the Arma Micro Computer with the case removed. If you look closely, you can see the 22 small circuit wafers mounted on each printed circuit board. The memory driver boards and delay lines are towards the back, spaced more widely than the other printed circuit boards. The cable harness underneath the boards provides the connections between boards.4

Transfluxors

One of the most unusual parts of the Micro Computer was its storage. Computers at the time typically used magnetic core memory, with each bit stored in a tiny ferrite ring, magnetized either clockwise or counterclockwise to store a 0 or 1. One drawback of standard core memory was that the process of reading a core also cleared the core, requiring data to be written back after a read.

The Micro Computer used ferrite cores, but these were "two-aperture" cores, with a larger hole and a smaller hole, as shown above. Data is written to the "major aperture" and read from the "minor aperture". Although the minor aperture switches state and is erased during a read, the major aperture retains the bit, allowing the minor aperture to be switched back to its original state. Thus, unlike regular core memory, transfluxors don't lose their data when reading.

The resulting system is called non-destructive readout (NDRO), compared to the destructive readout (DRO) of regular core memory.6 The Micro Computer used non-destructive readout memory to ensure that the program memory remained uncorrupted. In contrast, if a program is stored in regular core memory, each instruction must be written back as it is executed, creating the possibility that a transient could corrupt the software. By using transfluxors, this possibility of error is eliminated. (In either case, core memory has the convenient property that data is preserved when power is removed, since data is stored magnetically. With modern semiconductor memory, you lose data when the power goes off.)

The photo below shows a compact transfluxor-based storage module used in the Micro Computer, holding 512 words. In total, the computer could hold up to 7808 words of program memory and 256 words of data memory. It appears that transfluxors didn't live up to their promise, since most computers used regular core memory until semiconductor memory took over in the early 1970s.

Arma's history and the path to the Micro Computer

The Arma Engineering Company was founded in 1918 and built advanced military equipment.7 Its first product was a searchlight for the Navy, followed by a gyroscopic compass and analog computers for naval gun targeting. In 1939, Arma produced the Torpedo Data Computer, a remarkable electromechanical analog computer. US submarines used this computer to track target ships and automatically aim torpedos. The Torpedo Data Computer performed complex trigonometric calculations and integration to account for the motion of the target ship and the submarine. While the Torpedo Data Computer performed well, the Navy's Mark 14 torpedo had many problems—running too deep, exploding too soon, or failing to explode—making torpedoes often ineffectual even with a perfect hit.

Arma underwent major corporate changes due to World War II. Before the war, the German-owned Bosch Company built vehicle starters and aircraft magnetos in the United States. When the US entered World War II in 1941, the government was concerned that a German-controlled company was manufacturing key military hardware so the Office of Alien Property Custodian took over the Bosch plant. In 1948, the banking group that controlled Arma bought Bosch from the Office of the Alien Property Custodian, merging them into the American Bosch Arma Corporation (AMBAC).8 (Arma had earlier received the rights to gyrocompass technology from the German Anschutz company, seized by the Navy after World War I, so Arma benefitted twice from wartime government seizures.)

In the mid-1950s, Arma moved into digital computers, building an inertial guidance computer for the Atlas nuclear missile program. America's first ICBM was the Atlas missile, which became operational in 1959. The first Atlas missiles used radio guidance from the launch site to direct the missile. Since radio signals could be jammed by the enemy, this wasn't a robust solution.

The solution to missile guidance was an inertial navigation system. By using sensitive gyroscopes and accelerometers, a missile could continuously track its position and velocity without any external input, making it unjammable. A key developer of this system was Arma's Wen Tsing Chow, one of the driving forces behind digital aviation computers. He faced extreme skepticism in the 1950s for the idea of putting a computer in a missile. One general mocked him, asking "Where are you going to put the five Harvard professors you'll need to keep it running?" But computerized navigation was successful and in 1961, the Atlas missile was updated to use the Arma inertial guidance computer. It was said to be the first production airborne digital computer.9 Wen Tsing Chow also invented the programmable read-only memory (PROM), allowing missile targeting information to be programmed into a computer outside the factory.

The photo below shows the Atlas ICBM's guidance system. The Arma W-107A computer is at the top and the gyroscopes are in the middle. This computer was an 18-bit serial machine running at 143.36 kHz. It ran a hard-wired program that integrated the accelerometer information and solved equations for the crossrange error function, range error function, and gravity, making these computations every half second.10 The computer weighed 240 pounds and consumed 1000 watts. The computer contained about 36,000 components: discrete transistors, diodes, resistors, and capacitors mounted on 9.5" × 6.5" printed-circuit boards. On the ground, the computer was air-cooled to 55 °F, but there was no cooling after launch as the computer only operated for five minutes of powered flight and wouldn't overheat during that time.

The Atlas wasn't originally designed for a computerized guidance system so there wasn't room inside the missile for the computer. To get around this, a large pod was stuck on the side of the missile to hold the computer and gyroscopes, as indicated in the photo below. This doesn't look aerodynamic, but I guess it worked.

The Atlas guidance computer (left, below) consisted of three aluminum sections called "decks". The top deck held two replaceable target constant units, each providing 54 navigation constants that specified a target. The constants were stored in a stack of printed circuit boards 16" × 8" × 1.5", covered in over a thousand diodes, Wen Tsing Chow's PROM memory. A target was programmed into the stack by a rack of equipment that would selectively burn out diodes, changing the corresponding bit to a 1. (This is why programming a PROM is referred to as "burning the PROM".11) The diode matrix was later replaced with a transfluxor memory array, which had the advantage that it could be reprogrammed as necessary. The top deck also had connectors for the accelerometer inputs, the outputs, and connections for ground support equipment. The bottom deck had power connectors for 28 volts DC and 115V 400 Hz 3-phase AC. In the bottom deck, quartz delay lines were used for storage, representing bits as acoustic waves. Twelve circuit cards, each with a faceted quartz block four inches in diameter, provided a total of 32 words of storage.

Arma considered the Micro Computer the third generation of its airborne computers. The first generation was the Atlas guidance computer, constructed from germanium transistors and diodes (in the pre-silicon era). The second-generation computer moved to silicon transistors and diodes. The third-generation computers still used discrete components, but mounted on the small square wafers. The third generation also had a general-purpose architecture and programmable transfluxor memory instead of a hard-wired program.

After the Micro Computer

Arma continued to develop computers, improving the Arma Micro Computer. The Micro C computer (1965) was developed for Navy ships and submarines. Much like the original Micro, the Micro C used transfluxor storage, but increased the clock frequency to 972 kHz. The computer was much larger: 3.87 cubic feet and 150 pounds. This description states that "the machine is an outgrowth of the ARMA product line of micro computers and is logically and electrically similar to micro-computers designed for missile environments."

In mid-1966, Arma introduced the Micro D computer, built from TTL integrated circuits. Like the original Micro, this computer was serial, but the Micro D had a word length of 18 bits and ran at 1.5 MHz. It weighed 5.25 pounds and was very compact, just 0.09 ft³. Instead of transfluxors, the Micro D used regular magnetic core memory, 4K to 31K words.

The widely-used Litton LTN-51 inertial navigation system was built around the Arma Micro-D computer.12 This navigation system was designed for commercial aircraft, but was also used for military applications, ships, and NASA aircraft. Aircraft from early Concordes to Air Force One used the LTN-51 for navigation. The photo below shows a navigation unit with the Arma Micro-D computer in the lower left and the gyroscope unit on the right.

In early 1968, the Arma Portable Micro D was introduced, a 14-pound battery-powered computer also called the Celestial Data Processor. This handheld computer was designed for navigation in crewed earth orbital flight, determining orbital parameters from stadimeter and sextant measurements performed by astronauts. As far as I can tell, this computer never made it beyond the prototype stage.

Conclusions

The Arma Micro Computer is just one of the dozens of compact aerospace computers of the 1960s, a category that is mostly forgotten and ignored. Another example is the Delco MAGIC I (1961), said to be the "first complete airborne computer to have its logic functions mechanized exclusively with integrated circuits". IBM's 4 Pi series started in 1966 and was used in many systems from the F-15 to the Space Shuttle. By 1968, denser MOS/LSI chips were used in general-purpose aerospace computers such as the Rockwell MOS GP and the Texas Instruments Model 2502 LSI Computer. 13

Arma also illustrates that a company can be on the cutting edge of technology for decades and then suddenly go out of business and be forgotten. After some struggles, Arma was acquired by United Technologies in 1978 for $210 million and was then shut down in 1982. (The German Bosch corporation remains, now a large multinational known for products such as dishwashers, auto parts, and power tools.) Looking at a list of aerospace computers shows many innovative but vanished companies: Univac, Burroughs, Sperry (now all Unisys), AC Electronics (now part of Raytheon), Autonetics (acquired by Boeing), RCA (bought by GE), and TRW (acquired by Northrop Grumman).

Finally, the Micro Computer illustrates that terms such as "microcomputer" are not objective categories but are social constructs. At first, it seems obvious that the Arma Micro Computer is not a real microcomputer. If you consider a microcomputer to be a computer built around a microprocessor, that's true. (Although "microprocessor" is also not as clear as you might think.) But a microcomputer can also be defined as "A small computer that includes one or more input/output units and sufficient memory to execute instructions" (according to the IBM Dictionary of Computing, 1994)14 and the Arma Micro Computer meets that definition. The "microcomputer" is a shifting concept, changing from the 1960s to the 1990s to today.

For more, follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon as @kenshirriff@oldbytes.space. Thanks to Daniel Plotnick for providing a great deal of information and photos. Thanks to John Hartman for obtaining an obscure conference proceedings for me.

Notes and references

The Bendix Central Air Data Computer (CADC) is an electromechanical analog computer that uses gears and cams for its mathematics. It was a key part of military planes such as the F-101 and the F-111 fighters, computing airspeed, Mach number, and other "air data". The rotating gears are powered by six small servomotors, so these motors are in a sense the fundamental component of the CADC. In the photo below, you can see one of the cylindrical motors near the center, about 1/3 of the way down.

The servomotors in the CADC are unlike standard motors. Their name—"Motor-Tachometer Generator" or "Motor and Rate Generator"1—indicates that each unit contains both a motor and a speed sensor. Because the motor and generator use two-phase signals, there are a total of eight colorful wires coming out, many more than a typical motor. Moreover, the direction of the motor can be controlled, unlike typical AC motors. I couldn't find a satisfactory explanation of how these units worked, so I bought one and disassembled it. This article (part 5 of my series on the CADC2) provides a complete teardown of the motor/generator and explain how it works.

The image below shows a closeup of two motors powering one of the pressure signal outputs. Note the bundles of colorful wires to each motor, entering in two locations. At the top, the motors drive complex gear trains. The high-speed motors are geared down by the gear trains to provide much slower rotations with sufficient torque to power the rest of the CADC's mechanisms.

The motor/tachometer that we disassembled is shorter than the ones in the CADC (despite having the same part number), but the principles are the same. We started by removing a small C-clip on the end of the motor and and unscrewing the end plate. The unit is pretty simple mechanically. It has bearings at each end for the rotor shaft. There are four wires for the motor and four wires for the tachometer.3

The rotor (below) has two parts on the shaft. the left part is for the motor and the right drum is for the tachometer. The left part is a squirrel-cage rotor4 for the motor. It consists of conducting bars (light-colored) on an iron core. The conductors are all connected at both ends by the conductive rings at either end. The metal drum on the right is used by the tachometer. Note that there are no electrical connections between the rotor components and the rest of the motor: there are no brushes or slip rings. The interaction between the rotor and the windings in the body of the motor is purely magnetic, as will be explained.

The motor/tachometer contains two cylindrical stators that create the magnetic fields, one for the motor and one for the tachometer. The photo below shows the motor stator inside the unit after removing the tachometer stator. The stators are encased in hard green plastic and tightly pressed inside the unit. In the center, eight metal poles are visible. They direct the magnetic field onto the rotor.

The photo below shows the stator for the tachometer, similar to the stator for the motor. Note the shallow notches that look like black lines in the body on the lower left. These are probably adjustments to the tachometer during manufacturing to compensate for imperfections. The adjustments ensure that the magnetic fields are nulled out so the tachometer returns zero voltage when stationary. The metal plate on top shields the tachometer from the motor's magnetic fields.

The poles and the metal case of the stator look solid, but they are not. Instead, they are formed from a stack of thin laminations. The reason to use laminations instead of solid metal is to reduce eddy currents in the metal. Each lamination is varnished, so it is insulated from its neighbors, preventing the flow of eddy currents.

In the photo below, I removed some of the plastic to show the wire windings underneath. The wires look like bare copper, but they have a very thin layer of varnish to insulate them. There are two sets of windings (orange and blue, or red and black) around alternating metal poles. Note that the wires run along the pole, parallel to the rotor, and then wrap around the pole at the top and bottom, forming oblong coils around each pole.5 This generates a magnetic field through each pole.

The motor

The motor part of the unit is a two-phase induction motor with a squirrel-cage rotor.6 There are no brushes or electrical connections to the rotor, and there are no magnets, so it isn't obvious what makes the rotor rotate. The trick is the "squirrel-cage" rotor, shown below. It consists of metal bars that are connected at the top and bottom by rings. Assume (for now) that the fixed part of the motor, the stator, creates a rotating magnetic field. The important principle is that a changing magnetic field will produce a current in a wire loop.7 As a result, each loop in the squirrel-cage rotor will have an induced current: current will flow up9 the bars facing the north magnetic field and down the south-facing bars, with the rings on the end closing the circuits.

But how does the stator produce a rotating magnetic field? And how do you control the direction of rotation? The next important principle is that current flowing through a wire produces a magnetic field.8 As a result, the currents in the squirrel cage rotor produce a magnetic field perpendicular to the cage. This magnetic field causes the rotor to turn in the same direction as the stator's magnetic field, driving the motor. Because the rotor is powered by the induced currents, the motor is called an induction motor.

The diagram below shows how the motor is wired, with a control winding and a reference winding. Both windings are powered with AC, but the control voltage either lags the reference winding by 90° or leads the reference winding by 90°, due to the capacitor. Suppose the current through the control winding lags by 90°. First, the reference voltage's sine wave will have a peak, producing the magnetic field's north pole at A. Next (90° later), the control voltage will peak, producing the north pole at B. The reference voltage will go negative, producing a south pole at A and thus a north pole at C. The control voltage will go negative, producing a south pole at B and a north pole at D. This cycle will repeat, with the magnetic field rotating counter-clockwise from A to D. Conversely, if the control voltage leads the reference voltage, the magnetic field will rotate clockwise. This causes the motor to spin in one direction or the other, with the direction controlled by the control voltage. (The motor has four poles for each winding, rather than the one shown below; this increases the torque and reduces the speed.)

The purpose of the capacitor is to provide the 90° phase shift so the reference voltage and the control voltage can be driven from the same single-phase AC supply (in this case, 26 volts, 400 hertz). Switching the polarity of the control voltage reverses the direction of the motor.

There are a few interesting things about induction motors. You might expect that the motor would spin at the same rate as the rotating magnetic field. However, this is not the case. Remember that a changing magnetic field induces the current in the squirrel-cage rotor. If the rotor is spinning at the same rate as the magnetic field, the rotor will encounter an unchanging magnetic field and there will be no current in the bars of the rotor. As a result, the rotor will not generate a magnetic field and there will be no torque to rotate it. The consequence is that the rotor must spin somewhat slower than the magnetic field. This is called "slippage" and is typically a few percent of the full speed, with more slippage as more torque is required.

Many household appliances use induction motors, but how do they generate a rotating magnetic field from a single-phase AC winding? The problem is that the magnetic field in a single AC winding will just flip back and forth, so the motor will not turn in either direction. One solution is a shaded-pole motor, which puts a copper bar around part of each pole to break the symmetry and produce a weakly rotating magnetic field. More powerful induction motors use a startup winding with a capacitor (analogous to the control winding). This winding can either be switched out of the circuit once the motor starts spinning,10 or used continuously, called a permanent-split capacitor (PSC) motor. The best solution is three-phase power (if available); a three-phase winding automatically produces a rotating magnetic field.

Tachometer/generator

The second part of the unit is the tachometer generator, sometimes called the rate unit.11 The purpose of the generator is to produce a voltage proportional to the speed of the shaft. The unusual thing about this generator is that it produces a 400-hertz output that is either in phase with the input or 180° out of phase. This is important because the phase indicates which direction the shaft is turning. Note that a "normal" generator is different: the output frequency is proportional to the speed.

The diagram below shows the principle behind the generator. It has two stator windings: the reference coil that is powered at 400 Hz, and the output coil that produces the output signal. When the rotor is stationary (A), the magnetic flux is perpendicular to the output coil, so no output voltage is produced. But when the rotor turns (B), eddy currents in the rotor distort the magnetic field. It now couples with the output coil, producing a voltage. As the rotor turns faster, the magnetic field is distorted more, increasing the coupling and thus the output voltage. If the rotor turns in the opposite direction (C), the magnetic field couples with the output coil in the opposite direction, inverting the output phase. (This diagram is more conceptual than realistic, with the coils and flux 90° from their real orientation, so don't take it too seriously. As shown earlier, the coils are perpendicular to the rotor so the real flux lines are completely different.)

But why does the rotating drum change the magnetic field? It's easier to understand by considering a tachometer that uses a squirrel-cage rotor instead of a drum. When the rotor rotates, currents will be induced in the squirrel cage, as described earlier with the motor. These currents, in turn, generate a perpendicular magnetic field, as before. This magnetic field, perpendicular to the orginal field, will be aligned with the output coil and will be picked up. The strength of the induced field (and thus the output voltage) is proportional to the speed, while the direction of the field depends on the direction of rotation. Because the primary coil is excited at 400 hertz, the currents in the squirrel cage and the resulting magnetic field also oscillate at 400 hertz. Thus, the output is at 400 hertz, regardless of the input speed.

Using a drum instead of a squirrel cage provides higher accuracy because there are no fluctuations due to the discrete bars. The operation is essentially the same, except that the currents pass through the metal of the drum continuously instead of through individual bars. The result is eddy currents in the drum, producing the second magnetic field. The diagram below shows the eddy currents (red lines) from a metal plate moving through a magnetic field (green), producing a second magnetic field (blue arrows). For the rotating drum, the situation is similar except the metal surface is curved, so both field arrows will have a component pointing to the left. This creates the directed magnetic field that produces the output.

The servo loop

The motor/generator is called a servomotor because it is used in a servo loop, a control system that uses feedback to obtain precise positioning. In particular, the CADC uses the rotational position of shafts to represent various values. The servo loops convert the CADC's inputs (static pressure, dynamic pressure, temperature, and pressure correction) into shaft positions. The rotations of these shafts power the gears, cams, and differentials that perform the computations.

The diagram below shows a typical servo loop in the CADC. The goal is to rotate the output shaft to a position that exactly matches the input voltage. To accomplish this, the output position is converted into a feedback voltage by a potentiometer that rotates as the output shaft rotates.12 The error amplifier compares the input voltage to the feedback voltage and generates an error signal, rotating the servomotor in the appropriate direction. Once the output shaft is in the proper position, the error signal drops to zero and the motor stops. To improve the dynamic response of the servo loop, the tachometer signal is used as a negative feedback voltage. This ensures that the motor slows as the system gets closer to the right position, so the motor doesn't overshoot the position and oscillate. (This is sort of like a PID controller.)

The error amplifier and motor drive circuit for a pressure transducer are shown below. Because of the state of electronics at the time, it took three circuit boards to implement a single servo loop. The amplifier was implemented with germanium transistors (since silicon transistors were later). The transistors weren't powerful enough to drive the motors directly. Instead, magnetic amplifiers (the yellow transformer-like modules at the front) powered the servomotors. The large rectangular capacitors on the right provided the phase shift required for the control voltage.

Conclusions

The Bendix CADC used a variety of electromechanical devices including synchros, control transformers, servo motors, and tachometer generators. These were expensive military-grade components driven by complex electronics. Nowadays, you can get a PWM servo motor for a few dollars with the gearing, feedback, and control circuitry inside the motor housing. These motors are widely used for hobbyist robotics, drones, and other applications. It's amazing that servo motors have gone from specialized avionics hardware to an easy-to-use, inexpensive commodity.

Follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon as @oldbytes.space@kenshirriff. Thanks to Joe for providing the CADC. Thanks to Marc Verdiell for disassembling the motor.

Notes and references

The Bendix Central Air Data Computer (CADC) is an electromechanical analog computer that uses gears and cams for its mathematics. It was a key part of military planes such as the F-101 and the F-111 fighters, computing airspeed, Mach number, and other "air data". This article reverse-engineers the two pressure transducers, on the right in the photo below. It is part 3 of my series on the CADC.1

Aircraft have determined airspeed from air pressure for over a century. A port in the side of the plane provides the static air pressure,2 the air pressure outside the aircraft. A pitot tube points forward and receives the "total" air pressure, a higher pressure due to the speed of the airplane forcing air into the tube. The airspeed can be determined from the ratio of these two pressures, while the altitude can be determined from the static pressure.

But as you approach the speed of sound, the fluid dynamics of air change and the calculations become very complicated. With the development of supersonic fighter planes in the 1950s, simple mechanical instruments were no longer sufficient. Instead, an analog computer calculated the "air data" (airspeed, air density, Mach number, and so forth) from the pressure measurements. This computer then transmitted the air data electrically to the systems that needed it: instruments, weapons targeting, engine control, and so forth. Since the computer was centralized, such a system was called a Central Air Data Computer or CADC, manufactured by Bendix and other companies.

Each value in the Bendix CADC is indicated by the rotational position of a shaft. Compact electric motors rotated the shafts, controlled by magnetic amplifier servo loops. Gears, cams, and differentials performed computations, with the results indicated by more rotations. Devices called synchros converted the rotations to electrical outputs that controlled other aircraft systems. The CADC is said to contain 46 synchros, 511 gears, 820 ball bearings, and a total of 2,781 major parts (but I haven't counted). These components are crammed into a compact cylinder: 15 inches long and weighing 28.7 pounds.

The equations computed by the CADC are impressively complicated. For instance, one equation computes the Mach number $M$ from the total pressure \( P_t \) and the static pressure \( P_s \):3

\[~~~\frac{P_t}{P_s} = \frac{166.9215M^7}{( 7M^2-1)^{2.5}}\]

It seems incredible that these functions could be computed mechanically, but three techniques make this possible. The fundamental mechanism is the differential gear, which adds or subtracts values. Second, logarithms are used extensively, so multiplications and divisions become additions and subtractions performed by a differential, while square roots are calculated by gearing down by a factor of 2. Finally, specially-shaped cams implement functions: logarithm, exponential, and other one-variable functions.4 By combining these mechanisms, complicated functions can be computed mechanically.

The pressure transducers

In this article, I'm focusing on the pressure transducers and how they turn pressures into shaft rotations. The CADC receives two pressure inputs: the total pressure \( P_t \) from the pitot tube, and the static pressure \( P_s \) from the static pressure port.5 The CADC has two independent pressure transducer subsystems, one for total pressure and one for static pressure. The two pressure transducers make up the right half of the CADC. The copper pressure tube for the static pressure is visible on top of the CADC below. This tube feeds into the black-domed pressure sensor at the right. The gears, motors, and other mechanisms to the left of the pressure sensor domes generate shaft rotations that are fed into the remainder of the CADC for calculations.

The pressure transducer has a tricky job: it must measure tiny pressure changes, but it must also provide a rotational signal that has enough torque to rotate all the gears in the CADC. To accomplish this, the pressure transducer uses a servo loop that amplifies small pressure changes into accurate rotations. The diagram below provides an overview of the process. The pressure input causes a small movement in the bellows diaphragm. This produces a small shaft rotation that is detected by a sensitive inductive pickup. This signal is amplified and drives a motor with enough power to drive the output shaft. The motor is also geared to counteract the movement of the bellows. The result is a feedback loop so the motor's rotation tracks the air pressure, but provides much more torque. An adjustable cam corrects for any error produced by irregularities in the diaphragm response. This complete mechanism is implemented twice, once for each pressure input.

To summarize, as the pressure moves the diaphragm, the induction pick-up produces an error signal. The motor is driven in the appropriate direction until the error signal becomes zero. At this point, the output shaft rotation exactly matches the input pressure. The advantage of the servo loop is that the diaphragm only needs to move the sensitive inductive pickup, rather than driving the gears of the CADC, so the pressure reading is more accurate.

In more detail, the process starts with connections from the aircraft's pitot tube and static pressure port to the CADC. The front of the CADC (below) has connections for the total pressure and the static pressure. The CADC also has five round military connectors for electrical connections between the CADC and the rest of the aircraft. (The outputs from the CADC are electrical, with synchros converting the shaft rotations into electrical representations.) Finally, a tiny time clock at the upper right keeps track of how many hours the CADC has been in operation, so it can be maintained according to schedule.

The photo below shows the main components of the pressure transducer system. At the upper left, the pressure line from the CADC's front panel goes to the pressure sensor, airtight under a black dome. The error signal from the sensor goes to the amplifier, which consists of three boards. The amplifier's power transformer and magnetic amplifiers are the most visible components. The amplifier drives the motors to the left. There are two motors controlled by the amplifier: one for coarse adjustments and one for fine adjustments. By using two motors, the CADC can respond rapidly to large pressure changes, while also accurately tracking small pressure changes. Finally, the output from the motor goes through the adjustable cam in the middle before providing the feedback signal to the pressure sensor. The output from the transducer to the rest of the CADC is a shaft on the left, but it is in the middle of the CADC and isn't visible in the photo.

The pressure sensor

Each pressure sensor is packaged in a black airtight dome and is fed from its associated pressure line. Inside the sensor, two sealed metal bellows (below) expand or contract as the pressure changes. The bellows are connected to opposite sides of a metal shaft, which rotates as the bellows expand or contract. This shaft rotates an inductive pickup, providing the error signal. The servo loop rotates a second shaft that counteracts the rotation of the first shaft; this shaft and gears are also visible below.

The end view of the sensor below shows the inductive pickup at the bottom, with colorful wires for the input (400 Hz AC) and the output error signal. The coil visible on the inductive pickup is an anti-backlash spring to ensure that the pickup doesn't wobble back and forth. The electrical pickup coil is inside the inductive pickup and isn't visible.

The amplifier

Each transducer feedback signal is amplified by three circuit boards centered around magnetic amplifiers, transformer-like amplifiers that were popular before high-power transistors came along. The photo below shows how the amplifier boards are packed next to the transducers. The boards are complex, filled with resistors, capacitors, germanium transistors, diodes, relays, and other components.

I reverse-engineered the boards and created the schematic below. I'll discuss the schematic at a high level; click it for a larger version if you want to see the full circuitry. The process starts with the inductive sensor (yellow), which provides the error input signal to the amplifier. The first stage of the amplifier (blue) is a two-transistor amplifier and filter. From there, the signal goes to two separate output amplifiers to drive the two motors: fine (purple) and coarse (cyan).

The inductive sensor provides its error signal as a 400 Hz sine wave, with a larger signal indicating more error. The phase of the signal is 0° or 180°, depending on the direction of the error. In other words, the error signal is proportional to the driving AC signal in one direction and flipped when the error is in the other direction. This is important since it indicates which direction the motors should turn. When the error is eliminated, the signal is zero.

Each output amplifier consists of a transistor circuit driving two magnetic amplifiers. Magnetic amplifiers are an old technology that can amplify AC signals, allowing the relatively weak transistor output to control a larger AC output. The basic idea of a magnetic amplifier is a controllable inductor. Normally, the inductor blocks alternating current. But applying a relatively small DC signal to a control winding causes the inductor to saturate, permitting the flow of AC. Since the magnetic amplifier uses a small signal to control a much larger signal, it provides amplification.

In the early 1900s, magnetic amplifiers were used in applications such as dimming lights. Germany improved the technology in World War II, using magnetic amplifiers in ships, rockets, and trains. The magnetic amplifier had a resurgence in the 1950s; the Univac Solid State computer used magnetic amplifiers (rather than vacuum tubes or transistors) as its logic elements. However, improvements in transistors made the magnetic amplifier obsolete except for specialized applications. (See my IEEE Spectrum article on magnetic amplifiers for more history of magnetic amplifiers.)

In the CADC, magnetic amplifiers control the AC power to the motors. Two magnetic amplifiers are visible on top of the amplifier board stack, while two more are on the underside; they are the yellow devices that look like transformers. (Behind the magnetic amplifiers, the power transformer is labeled "A".)

The transistor circuit generates the control signal to the magnetic amplifiers, and the output of the magnetic amplifiers is the AC signal to the motors. Specifically, the CADC uses two magnetic amplifiers for each motor. One magnetic amplifier powers the motor to spin clockwise, while the other makes the motor spin counterclockwise. The transistor circuit will pull one magnetic amplifier winding low; the phase of the input signal controls which magnetic amplifier, and thus the motor direction. (If the error input signal is zero, neither winding is pulled low, both magnetic amplifiers block AC, and the motor doesn't turn.)6 The result of this is that the motor will spin in the correct direction based on the error input signal, rotating the mechanism until the mechanical output position matches the input pressure. The motors are "Motor / Tachometer Generator" units that also generate a voltage based on their speed. This speed signal is fed into the transistor amplifier to provide negative feedback, limiting the motor speed as the error becomes smaller and ensuring that the feedback loop doesn't overshoot.

The other servo loops in the CADC (temperature and position error correction) have one motor driver constructed from transistors and two magnetic amplifiers. However, each pressure transducer has two motor drivers (and thus four magnetic amplifiers), one for fine adjustment and one for coarse adjustment. This allows the servo loop to track the input pressure very closely, while also adjusting rapidly to larger changes in pressure. The coarse amplifier uses back-to-back diodes to block small changes; only input voltages larger than a diode drop will pass through and energize the coarse amplifier.

The CADC is powered by standard avionics power of 115 volts AC, 400 hertz. Each pressure transducer amplifier has a simple power supply running off this AC, using a multi-winding power transformer. A center-tapped winding and full wave rectifier produces DC for the transistor amplifiers. Other windings supply AC (controlled by the magnetic amplifiers) to power the motors, AC for the magnetic amplifier control signals, and AC for the sensor. The transformer ensures that the transducer circuitry is electrically isolated from other parts of the CADC and the aircraft. The power supply is indicated in red in the schematic above.

The schematic also shows test circuitry (blue). One of the features of the CADC is that it can be set to two test configurations before flight to ensure that the system is operating properly and is correctly calibrated.7 Two relays allow the pressure transducer to switch to one of two test inputs. This allows the CADC to be checked for proper operation and calibration. The test inputs are provided from an external board and a helical feedback potentiometer (Helipot) that provides simulated sensor input.

Getting the amplifiers to work was a challenge. Many of the capacitors in the CADC had deteriorated and failed, as shown below. Marc went through the CADC boards and replaced the bad capacitors. However, one of the pressure transducer boards still failed to work. After much debugging, we discovered that one of the new capacitors had also failed. Finally, after replacing that capacitor a second time, the CADC was operational.

The mechanical feedback loop

The amplifier boards energize two motors that rotate the output shaft,8 the coarse and fine motors. The outputs from the coarse and fine motors are combined through a differential gear assembly that sums its two input rotations.9 While the differential functions like the differential in a car, it is constructed differently, with a spur-gear design. This compact arrangement of gears is about 1 cm thick and 3 cm in diameter. The differential is mounted on a shaft along with three co-axial gears: two gears provide the inputs to the differential and the third provides the output. In the photo, the gears above and below the differential are the input gears. The entire differential body rotates with the sum, connected to the output gear at the top through a concentric shaft. The two thick gears inside the differential body are part of its mechanism.

(Differential gear assemblies are also used as the mathematical component of the CADC, as it performs addition or subtraction. Since most values in the CADC are expressed logarithmically, the differential computes multiplication and division when it adds or subtracts its inputs.)

The CADC uses cams to correct for nonlinearities in the pressure sensors. The cam consists of a warped metal plate. As the gear rotates, a spring-loaded vertical follower moves according to the shape of the plate. The differential gear assembly under the plate adds this value to the original input to obtain a corrected value. (This differential implementation is different from the one described above.) The output from the cam is fed into the pressure sensor, closing the feedback loop.

At the top, 20 screws can be rotated to adjust the shape of the cam plate and thus the correction factor. These cams allow the CADC to be fine-tuned to maximize accuracy. According to the spec, the required accuracy for pressure was "40 feet or 0.15 percent of attained altitude, whichever is greater."

Conclusions

The Bendix CADC was built at an interesting point in time, when computations could be done digitally or analog, mechanically or electrically. Because the inputs were analog and the desired outputs were analog, the decision was made to use an analog computer for the CADC. Moreover, transistors were available but their performance was limited. Thus, the servo amplifiers are built from a combination of transistors and magnetic amplifiers.

Modern air data computers are digital but they are still larger than you might expect because they need to handle physical pressure inputs. While a drone can use a tiny 5mm MEMS pressure sensor, air data computers for aircraft have higher requirements and typically use larger vibrating cylinder pressure sensors. Even so, at 45 mm long, the modern pressure sensor is dramatically smaller than the CADC's pressure transducer with its metal-domed bellows sensor, three-board amplifier, motors, cam, and gear train. Although the mechanical Bendix CADC seems primitive, this CADC was used by the Air Force until the 1980s. I guess if the system worked, there was no reason to update it.

I plan to continue reverse-engineering the Bendix CADC,10 so follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon as @oldbytes.space@kenshirriff. Thanks to Joe for providing the CADC. Thanks to Nancy Chen for obtaining a hard-to-find document for me. Marc Verdiell and Eric Schlaepfer are working on the CADC with me.

Notes and references

How did fighter planes in the 1950s perform calculations before compact digital computers were available? The Bendix Central Air Data Computer (CADC) is an electromechanical analog computer that used gears and cams for its mathematics. It was used in military planes such as the F-101 and the F-111 fighters, and the B-58 bomber to compute airspeed, Mach number, and other "air data".

Aircraft have determined airspeed from air pressure for over a century. A port in the side of the plane provides the static air pressure,1 the air pressure outside the aircraft. A pitot tube points forward and receives the "total" air pressure, a higher pressure due to the speed of the airplane forcing air into the tube. The airspeed can be determined from the ratio of these two pressures, while the altitude can be determined from the static pressure.

But as you approach the speed of sound, the fluid dynamics of air changes and the calculations become very complicated. With the development of supersonic fighter planes in the 1950s, simple mechanical instruments were no longer sufficient. Instead, an analog computer calculated the "air data" (airspeed, air density, Mach number, and so forth) from the pressure measurements. This computer then transmitted the air data electrically to the systems that needed it: instruments, weapons targeting, engine control, and so forth. Since the computer was centralized, the system was called a Central Air Data Computer or CADC, manufactured by Bendix and other companies.

Each value in the CADC is indicated by the rotational position of a shaft. Compact electric motors rotated the shafts, controlled by magnetic amplifier servos. Gears, cams, and differentials performed computations, with the results indicated by more rotations. Devices called synchros converted the rotations to electrical outputs that controlled other aircraft systems. The CADC is said to contain 46 synchros, 511 gears, 820 ball bearings, and a total of 2,781 major parts (but I haven't counted). These components are crammed into a compact cylinder: 15 inches long and weighing 28.7 pounds.

The equations computed by the CADC are impressively complicated. For instance, one equation is:2

\[~~~\frac{P_t}{P_s} = \frac{166.9215M^7}{( 7M^2-1)^{2.5}}\]

It seems incredible that these functions could be computed mechanically, but three techniques make this possible. The fundamental mechanism is the differential gear, which adds or subtracts values. Second, logarithms are used extensively, so multiplications and divisions become additions and subtractions performed by a differential, while square roots are calculated by gearing down by a factor of 2. Finally, specially-shaped cams implement functions: logarithm, exponential, and functions specific to the application. By combining these mechanisms, complicated functions can be computed mechanically, as I will explain below.

The differential

The differential gear assembly is the mathematical component of the CADC, as it performs addition or subtraction. The differential takes two input rotations and produces an output rotation that is the sum or difference of these rotations.3 Since most values in the CADC are expressed logarithmically, the differential computes multiplication and division when it adds or subtracts its inputs.

While the differential functions like the differential in a car, it is constructed differently, with a spur-gear design. This compact arrangement of gears is about 1 cm thick and 3 cm in diameter. The differential is mounted on a shaft along with three co-axial gears: two gears provide the inputs to the differential and the third provides the output. In the photo, the gears above and below the differential are the input gears. The entire differential body rotates with the sum, connected to the output gear at the top through a concentric shaft. (In practice, any of the three gears can be used as the output.) The two thick gears inside the differential body are part of the mechanism.

Note that multiplying a rotation by a constant factor doesn't require a differential; it can be done simply with the ratio between two gears. (If a large gear rotates a small gear, the small gear rotates faster according to the size ratio.) Adding a constant to a rotation is even easier, just a matter of defining what shaft position indicates 0. For this reason, I will ignore constants in the equations.

The cams

The CADC uses cams to implement various functions. Most importantly, cams compute logarithms and exponentials. Cams also implement complicated functions of one variable such as ${M}/{\sqrt{1 + .2 M^2}}$. The function is encoded into the cam's shape during manufacturing, so a hard-to-compute nonlinear function isn't a problem for the CADC. The photo below shows a cam with the follower arm in front. As the cam rotates, the follower moves in and out according to the cam's radius.

However, the shape of the cam doesn't provide the function directly, as you might expect. The main problem with the straightforward approach is the discontinuity when the cam wraps around, which could catch the follower. For example, if the cam implemented an exponential directly, its radius would spiral exponentially and there would be a jump back to the starting value when it wraps around.

Instead, the CADC uses a clever patented method: the cam encodes the difference between the desired function and a straight line. For example, an exponential curve is shown below (blue), with a line (red) between the endpoints. The height of the gray segment, the difference, specifies the radius of the cam (added to the cam's fixed minimum radius). The point is that this difference goes to 0 at the extremes, so the cam will no longer have a discontinuity when it wraps around. Moreover, this technique significantly reduces the size of the value (i.e. the height of the gray region is smaller than the height of the blue line), increasing the cam's accuracy.5

To make this work, the cam position must be added to the linear value to yield the result. This is implemented by combining each cam with a differential gear that performs the addition or subtraction.4 As the diagram below shows, the input (23) drives the cam (30) and the differential (25, 37-41). The follower (32) tracks the cam and provides a second input (35) to the differential. The sum from the differential produces the desired function (26).

Pressure inputs

The CADC receives two pressure inputs from the pitot tube.6 Inside the CADC, two pressure transducers convert the pressures into rotational positions. Each pressure transducer contains a pair of bellows that expand and contract as the applied pressure changes. The pressure transducer has a tricky job: it must measure tiny pressure changes, but it must also provide a rotational signal that has enough torque to rotate all the gears in the CADC. To accomplish this, each pressure transducer uses a servo loop that drives a motor, controlled by a feedback loop. Cams and differentials convert the rotation into logarithmic values, providing the static pressure as \( log \; P_s \) and the pressure ratio as \( log \; ({P_t}/{P_s}) \) to the rest of the CADC.

The synchro outputs

A synchro is an interesting device that can transmit a rotational position electrically over three wires. In appearance, a synchro is similar to an electric motor, but its internal construction is different, as shown below. Before digital systems, synchros were very popular for transmitting signals electrically through an aircraft. For instance, a synchro could transmit an altitude reading to a cockpit display or a targeting system. Two synchros at different locations have their stator windings connected together, while the rotor windings are driven with AC. Rotating the shaft of one synchro causes the other to rotate to the same position.7

For the CADC, most of the outputs are synchro signals, using compact synchros that are about 3 cm in length. For improved resolution, some of the CADC outputs use two synchros: a coarse synchro and a fine synchro. The two synchros are typically geared in an 11:1 ratio, so the fine synchro rotates 11 times as fast as the coarse synchro. Over the output range, the coarse synchro may turn 180°, providing the approximate output, while the fine synchro spins multiple times to provide more accuracy.

Examining the left section of the CADC

The Bendix CADC is constructed from modular sections. The right section has the pressure transducers (the black domes), along with the servo mechanisms that control them. The middle section is the "Mach section". In this blog post, I'm focusing on the left section of the CADC, which computes true airspeed, air density, total temperature, log true free air temperature, and air density × speed of sound. I had feared that any attempt at disassembly would result in tiny gears flying in every direction, but the CADC was designed to be taken apart for maintenance. Thus, I could remove the left section of the CADC for analysis.

The diagram below shows the side that connects to the aircraft.8 The various synchros generate the outputs. Some of the synchros have spiral anti-backlash springs installed. These springs prevent wobble in the synchro and gear train as the gears change direction. Three of the exponential cams are visible. The differentials and gears are between the two metal plates, so they are not visible from this angle.

Attached to the right side is the temperature transducer, a modular wedge that implements a motorized servo loop to convert the temperature input to a rotation. The servo amplifier consists of three boards of electronic components, including transistors and magnetic amplifiers to drive the motor. The large red potentiometer provides feedback for the servo loop. A flexible cam with 20 adjustment screws allows the transducer to be tuned to eliminate nonlinearities or other sources of error. I'll describe this module in more detail in another post.9

The photo below shows the other side of the section. This communicates with the rest of the CADC through the electrical connector and three gears that mesh with gears in the other section. Two gears receive the pressure signals \( P_t / P_s \) and \(P_s\) from the pressure transducer subsystem. The third gear sends the log total temperature to the rest of the CADC. The electrical connector (a standard 37-pin D-sub) supplies 120 V 400 Hz power to the rest of the CADC and passes synchro signals from the rest of the CADC to the output connectors.

The equations

Although the CADC looks like an inscrutable conglomeration of tiny gears, it is possible to trace out the gearing and see exactly how it computes the air data functions. With considerable effort, I have reverse-engineered the mechanisms to create the diagram below, showing how each computation is broken down into mechanical steps. Each line indicates a particular value, specified by a shaft rotation. The ⊕ symbol indicates a differential gear, adding or subtracting its inputs to produce another value. The cam symbol indicates a cam coupled to a differential gear. Each cam computes either a specific function or an exponential, providing the value as a rotation. At the right, the rotations are converted to outputs, either by synchros or a potentiometer. This diagram abstracts out the physical details of the gears. In particular, scaling by constants or reversing the rotation (subtraction versus addition) are not shown.

I'll go through each calculation briefly.

Total temperature

The external temperature is an important input to the CADC since it affects the air density. A platinum temperature probe provides a resistance that varies with temperature. The resistance is converted to rotation by the temperature transducer, described earlier. The definition of temperature is a bit complicated, though. The temperature outside the aircraft is called the true free air temperature, T. However, the temperature probe measures a higher temperature, called the indicated total air temperature, T_i. The reason for this discrepancy is that when the aircraft is moving at high speed, the air transfers kinetic energy to the temperature probe, heating it up.

The temperature transducer provides the log of the total temperature as a rotation. At the top of the equation diagram, cam and differential D15 simply take the exponential of this value to determine the total temperature. This rotates the shaft of a synchro to produce the total temperature as an electrical output. As shown above, the D15 cam is attached to the differential by a shaft passing through the metal plate. The follower rotates according to the cam radius, turning the follower gear which meshes with the differential input. The result from the differential is the total temperature.

log free air temperature

A more complicated task of the CADC is to compute the true free air temperature from the measured total temperature. Free air temperature, T, is defined by the formula below, which compensates for the additional heating due to the aircraft's speed. \(T_i\) is the indicated total temperature, M is the Mach number and K is a temperature probe constant.10

\[ T = \frac {T_i} {1 + .2 K M^2 } \]

The diagram below shows the cams, differentials, gear trains, and synchro that compute \(log \; T\). First, cam D11 computes \( log \; (1 + .2 K M^2 ) \). Although that expression is complicated, the key is that it is a function of one variable (M). Thus, it can be computed by cam D11, carefully shaped for this function and attached to differential D11. Differential D10 adds the log total temperature (from the temperature transducer) to produce the desired result. The indicated servo outputs this value to other aircraft systems. (Note that the output is a logarithm; it is not converted to a linear value.11 This value is also fed (via gears) into the calculations of three more equations, below.

Air density

Air density is computed from the static pressure and true temperature:

\[ \rho = C_1 \frac{P_s} {T} \]

It is calculated using logarithms. D16 subtracts the log temperature from the log pressure and cam D20 takes the exponential.

True airspeed

True airspeed is computed from the Mach number and the total temperature according to the following formula:

\[V = 38.94 M \frac{\sqrt{T_i}}{\sqrt{1+.2KM^2}}\]

Substituting the true free air temperature simplifies the formula to the equation implemented in the CADC:

\[V = 38.94 M \sqrt{T} \]

This is computed logarithmically. First, cam and differential D12 compute \(log \; M\) from the pressure ratio.13 Next differential D19 adds half the log temperature to multiply by the square root. Exponential cam D13 removes the logarithms, producing the final result. (The constant 38.94 is an important part of the equation, but is easily implemented with gear ratios.) The output goes to two synchros, geared to provide coarse and fine outputs.12

Air density × speed of sound

Air density × speed of sound14 is given by the formula

\[ \rho \cdot a = C_2 \frac {P_s} {\sqrt{T}} \]

The calculation is almost the same as the air density calculation. Differential D18 subtracts half the log temperature from the log pressure and then cam D14 computes the exponential. Unlike the other values, this output rotates the shaft of a 1 KΩ potentiometer (above), changing its resistance. I don't know why this particular value is output as a resistance rather than a synchro angle.

Conclusions

The CADC performs nonlinear calculations that seem way too complicated to solve with mechanical gearing. But reverse-engineering the mechanism shows how the equations are broken down into steps that can be performed with cams and differentials, using logarithms for multiplication, division, and square roots. I'll point out that reverse engineering the CADC is not as easy as you might expect. It is difficult to see which gears are in contact, especially when gears are buried in the middle of the CADC and are hard to see. I did much of the reverse engineering by rotating one differential to see which other gears turn, but usually most of the gears turned due to the circuitous interconnections.15

By the late 1960s, as fighter planes became more advanced and computer technology improved, digital processors replaced the gears in air data computers. Garrett AiResearch's ILAAS air data computer (1967) was the first all-digital unit. Other digital systems were Bendix's ADC-1000 Digital Air Data Computer (1967) which was "designed to solve all air data computations at a rate of 75 times per second", Conrac's 3-pound solid-state air data computer (1967), Honeywell's Digital Air Data System (1968), and the LSI-based Garrett AiResearch F-14 CADC (1970). Nonetheless, the gear-based Bendix CADC provides an interesting reverse-engineering challenge as well as a look at the forgotten era of analog computing.

For more background on the CADC, see my overview article on the CADC. I plan to continue reverse-engineering the Bendix CADC and get it operational,16 so follow me on Twitter @kenshirriff or RSS for updates. I've also started experimenting with Mastodon as @oldbytes.space@kenshirriff. Thanks to Joe for providing the CADC. Thanks to Nancy Chen for obtaining a hard-to-find document for me. Marc Verdiell and Eric Schlaepfer are working on the CADC with me.

Notes and references