Ken ShirriffThe Space Shuttle had a large teleprinter onboard so the astronauts could receive text documents. We want to get it working, but first some reverse engineering... 1/13
The teleprinter was based on the military's AN/UGC-74, heavily modified to receive data over the Shuttle's audio system. It's more like a line printer than a Teletype, using a spinning drum and a row of hammers to print each line. 2/13
The teleprinter is absurdly heavy (~60 pounds) considering that every pound launched on the Shuttle cost $30,000. Moreover, it will overheat if left on for more than 20 to 30 minutes. I find it hard to imagine this was the best printer option available. 3/13
The right side of the teleprinter has four complex circuit boards: a CPU board with a Motorola 6800 (photo), a memory board, a communications board, and a printer driver board. Fortunately, these boards are somewhat documented: https://radionerds.com/index.php/AN~UGC-74 4/13
The left side has three circuit boards, specific to the Space Shuttle. They interface to the Space Shuttle's systems so it can receive messages through the Shuttle's audio system. I reverse-engineered these boards. 5/13
This board receives audio, amplifies it, and applies a 900 Hz high-pass filter. It also has some control logic. 6/13
The bits are encoded with two frequencies (Frequency-Shift Keying, FSK). This board uses a 64-bit shift register as a delay line to convert the 3600 Hz or 7200 Hz tones to bits. It also generates timing and control signals from the crystal. 7/13
The final board applies a 400 Hz low-pass filter to the output and sends the serial data stream to the communication board. It also has carrier detection, control logic, and regulated +5 and +12 volt power supplies. 8/13
There is a lot of control logic on these boards to power up the teleprinter when a message comes in and then power it off so it doesn't overheat. 9/13
The boards use standard ICs, but they mostly have weird military part numbers, making reverse engineering annoying. 10/13
The military teleprinter took all sorts of protocols: ASCII, Baudot, 45.5 to 1200 baud, but the Shuttle takes just one (unspecified) After reverse-engineering the boards, I think I know what type of signal it needs (ASCII, even parity, 1 stop bit, 600 baud, 3600/7200 Hz). 11/13
DrScriptt@kenshirriff does ASCII imply 7 data bits?
spmatich vk3spm 
@kenshirriff ah you answered my question here
Christian Berger DECT 2763@kenshirriff Didn't those get replaced by a system that could also reproduce graphics?
@kenshirriff are those military grade cmos ics?
YetAnotherGeekGuy 
@spmatich Those chip are CMOS, but I don't know if they are military grade. I suspect that this board was a prototype so they may have used regular grade chips.
Brad Bell@kenshirriff I wonder if this shares any code with my 1980ish 6800-based portable terminal and integrated spark printer 😄
Joe Cooper 🇺🇦 🍉@kenshirriff I would have sent one of these and saved a few hundred thousand dollars.
Infinity@kenshirriff This post is amazing.
penguin42@kenshirriff Yeh it's a weird choice isn't it; impact dot matrix machines had been around for a while at that point; a DECwriter or an Epson MX would surely have worked.
simonbp@penguin42 @kenshirriff Depends on when you define "then". Shuttle's contract was let in 1972 and the design was finalized in circa 1976, with few changes after the start of the free flight tests. Computing changed quite a bit between 1972 and the first Shuttle spaceflight in 1981.
danimrich@kenshirriff One wonders why they didn't use a thermal printer, for example. (Yes, they were commercially available back then.)
vxo@danimrich @kenshirriff I wonder if there was a fire safety concern
@vxo
Possibly. But then I'd also be concerned about a machine that would overheat if it were left on for 20 minutes. Maybe this is a machine that was already tested and certified for use in aviation.
@vxo @kenshirriff
By the way, if this was present on all 135 Space Shuttle flights, NASA would have spent roughly 243 million dollars just to launch this printer.
By the way, if this was present on all 135 Space Shuttle flights, NASA would have spent roughly 243 million dollars just to launch this printer.
@danimrich @kenshirriff ah! so that's where HP's business model came from. (Fnord.)
Ψ*Ψ
jnbhlr
Penguin@kenshirriff Worked for US DoD in late 1960’s on teletype communication for field offices in Vietnam. Printer tech hadn’t gone too far past that used for law enforcement & newspapers in the 1940’s, there was little incentive for improvement since no-one had a computer at home. And military use meant much heavier components & associated parts.
Joe Lion@kenshirriff they put that 60lb monstrosity on the Shuttle, but the Apollo astronauts had to use a modified hand held camera with no view-finder, to save weight, in the Saturn rockets? 🤨
The photography behind Earthrise
Peter Evans@kenshirriff I imagine the strongest constraint was to avoid Drops In Space floating all around the middeck. #microgravity
Urwumpe@kenshirriff not so absurd if you remember that this printer was selected at some point between 1973 and 1975, and thus had to be on the market before and had to survive intense vibrations and accellerations. NASA could have designed a lighter printer instead, but who'd paid for this?
Later, this "interim" printer was replaced by lighter and cheaper gear. But it stayed as backup until 1998.
@kenshirriff
space folks: “low SWaP pls”
you: “ok i made smol”
also space folks: “no you have to use bigger resistors and caps, oh and also this particular chip because it has flight heritage”
you: 😭 “v sad not smol anymore”
space folks: “low SWaP pls”
you: “ok i made smol”
also space folks: “no you have to use bigger resistors and caps, oh and also this particular chip because it has flight heritage”
you: 😭 “v sad not smol anymore”
MrEmerson@kenshirriff makes me think how many rolls of paper and ink must be stocked for each launch.
Kevin Karhan 
@kenshirriff So basically a more rugged and simplified mechanical typewriter...