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We obtained a mysterious box of 1960s electronics. I reverse-engineered it, and with much effort, we got it running. It turns out to be a test unit for testing NASA's Up-Data Link, a system from the Apollo moon landing to control the spacecraft from the ground. Let's take a look inside... 1/n

This box has thirteen orange digits at the top, which look like Nixie tubes. But they are a different technology called edge-lit lightguide display. Each digit has ten plastic sheets and ten lightbulbs. Each sheet has dots etched in the shape of a number. Lighting a sheet lights up that number.

The box uses an old style of electronics that predates integrated circuits. Each thumb-sized encapsulated module implements a few logic gates or other simple circuit. A cryptic label such as "2/2G&2/1G" hints at the function. 3/n

The modules were mounted on circuit boards in rows with a module-sized gap in between. Two boards could be fitted together with modules sliding into gaps to form a tight sandwich. These dense sandwiches slid into the top of the box to create its circuits. 4/n

This test box sent test messages to the Up-Data Link box (below). The UDL (below) on the Apollo spacecraft received encoded digital messages and controlled the Apollo Guidance Computer, clock, and other circuits. The UDL (yellow) was one of many electronics boxes inside the spacecraft. 6/n

The test box displayed success or errors on its lights, showing status for the Apollo Guidance Computer (AGC), clock (Central Timing Equipment, CTE), and other Apollo systems.

For lots more information on Apollo's Up-Data Link and this test box, see my blog post: www.righto.com/2025/07/reve... I worked on this with @curiousmarc.bsky.social, @tubetime.bsky.social, Mike Stewart, and others. Thanks to Marcel for supplying the box.