Delete comment from: Ken Shirriff's blog
Ken: The Apollo 11 1201/1202 alarms stemmed from poor integration. The rendezvous radar used resolvers to track it's orientation. The resolvers were referenced to a 800 Hz clock. When the radar was moved, pulses were sent to the Lunar Module Guidance Computer (LGC) to increment/decrement position.
The problem was the Lunar Module (LM) was built by Grumman, while the LGC was built at MIT. Each had their own reference. If the radar was in LGC mode, the 800 Hz reference came from the LGC and all is well. In AUTO or SLEW mode, the reference came from the Grumman system. The two references were not synchronized, and the LGC was always connected to the resolvers. Therefore, in AUTO or SLEW mode the LGC got a huge barrage of meaningless pulses; each pulse triggering an interrupt. Normally the LGC could handle the extra load, but in the computationally intensive landing phase it was enough to overload the system.
This was only noticed twice in about 100 pre-flight integration tests. It all depended on what part of the cycle the Grumman signal was in when the LGC powered on. Apollo 11 was just lucky enough for the two signals to be almost completely out of phase. MIT's George Silver had discovered the problem the year before, and wrote up a hardware fix to synchronize the signals. However, the fix was declined on grounds of budget and schedule.
Mar 26, 2020, 1:40:08 PM
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