Apollo Guidance Computer, Part 29—Coupling Data Unit

The long story of restoring of a genuine (ground test) Apollo Guidance Computer (AGC) to working condition has been chronicled in a series of Curious Marc videos featured on ancestors of this site. You can view all of them at the Apollo Guidance Computer Restoration playlist.

Now, obtained at an auction of surplus Apollo hardware fortuitously saved from the scrap heap, the team lays its hands on a Coupling Data Unit (CDU), the component that interfaced the AGC to the optical alignment telescope, inertial measuring unit, and, on the Lunar Module (LM), its rendezvous radar. Unlike the AGC, which was absolutely identical in the Command Module and LM, the CDU differed since the LM had a simpler alignment telescope and needed to interface the rendezvous radar, which the Command Module lacked. The LM CDU was the piece of hardware that generated the flood of signals that caused the famous “1202 program alarm” events during the landing of Apollo 11.

The restorers had hoped to obtain an LM CDU, which would allow re-creating the circumstances of the program alarms, but the unit at auction was marked as being a Command Module configuration—oh well, close enough. Imagine their surprise when they opened it up and discovered that, for whatever reasons lost in the mists of time, this ground test unit contained the components for both the Command Module and LM interfaces.

The CDU is essentially a five channel 16-bit analogue to digital and digital to analogue converter (ADC/DAC). Today, such functions are integrated on a single chip, but in the 1960s required a hefty box filled with potted discrete electronics modules almost as big as the AGC itself.

The adventure will continue as the details of this unit are puzzled out and work begins trying to get it working again after all these years.

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