Ken Shirriff and Friends Puzzle-Out an Apollo Test Set — and Get It Singing Once Again

Noted reverse engineer Ken Shirriff has once again turned his attention to a piece of spacefaring history, uncovering the purpose behind an odd device from the Apollo program dubbed the Up-Data Link Confidence Test Set.

"Back in 2021, a collector friend of ours was visiting a dusty warehouse in search of Apollo-era communications equipment," Shirriff explains. "A box with NASA-style lights caught his eye — the 'AGC Confirm' light suggested a connection with the Apollo Guidance Computer. Disappointingly, the box was just an empty chassis and the circuit boards were all missing. He continued to poke around the warehouse when, to his surprise, he found a bag on the other side of the warehouse that contained the missing boards!"

With the chassis and its boards reunited, it was possible to figure out what the device did — aided in no small part by the presence of a label at the rear, identifying the machine as the "Up-Data Link Confidence Test Set" from Motorola. This, Shirriff and friends surmised, was designed to be used on the ground to test the Up-Data Link (UDL) system — designed to send digitized commands from the ground to an orbiting spacecraft.

Documentation, however, proved sparse. "Mike [Stewart] found one NASA document that mentioned the Test Set," Shirriff recalls, "but the document was remarkably uninformative. Unfortunately, this was not very helpful since the diagram merely shows the Test Set as a rectangle with one wire in and one wire out."

A lack of documentation is one thing, but the machine as-recovered was also missing key components — leaving Shirriff with more of a puzzle than usual. Inspecting each of the gadget's 25 individual circuit boards, a block diagram was produced — with more puzzling required to figure out exactly what was going on with Motorola-branded modules, built from discrete components but fully encapsulated like a more modern integrated circuit. This, Shirriff explains, required an X-ray machine to peer at the parts within.

"Once I figured out the circuitry on each board, the next problem was determining how the boards were connected," Shirriff writes. "I soon realized that manually tracing the wiring was impractically slow: with 25 boards and 47 connections per board, brute-force testing of every pair of connections would require hundreds of thousands of checks. To automate the beeping-out of connections, I built a system that I call Beep-o-matic. The idea behind Beep-o-matic is to automatically find all the connections between two motherboard slots by plugging two special boards into the slots."

With that, Shirriff and friends were able to complete the reverse engineering and restore the test box to functional status — not only being able to use its built-in paper-tape reader but actively use it to generate signals for use with other recovered Apollo hardware, including an S-band communications system. "We haven't transmitted these signals to the Moon," Shirriff admits, "but we have transmitted signals between antennas a few feet apart, receiving them with a box called the S-band Transponder."

The full write-up is available on Shirriff's website; design files and source code for the Beep-o-matic have been released on GitHub under an unspecified license, though Shirriff warns "this is unlikely to be useful to anyone else."