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