During powered descent a 1202 alarm occurred when LM Pilot Buzz Aldrin asked the Apollo Guidance Computer (AGC) for the difference between computed and measured altitude above ground by typing V16N68 (this is well explained here).
I am analyzing the raw telemetries of the landing; the part showing registers status is not clear to me at 102:38:27, at first 1202 alarm:
The above source states that "V90N50E" is an "Aldrin input to stop the alarm", but there are some problems here:
- The recorded squence is V50N90E, not V90N50E
- Same author says in another site that the sequence was V05N09E
- I read that verb 50 means "Please perform", i.e. it's an AGC request to the operator.
- I also read that verb 50 means user request to AGC
- Noun 90 does not even exist
So it's a mess...
Possibily 50 and 90 have a different meaning w.r.t. the "cheatsheets", being them associated to the alarm, just like R2 and R3 have no meaning during the alarm, but where to find some documentation about this topic?
As a side note, here it is what astronauts were trying to see "by numbers", not being available any graphical monitor:
(data taken data from Register 3 (R3) of Apollo 11 LM-5 AGC (=LGC), recorded here, pages 1-10, and from "APOLLO MISSION 11 TRAJECTORY RECONSTRUCTION AND POSTFLIGHT ANALYSIS - VOLUME 1", data transcripted here). Register 3 contents becomes 00000 upon alarms 1201/1202 and DeltaH when V16N68E is working. Data table: https://github.com/jumpjack/Apollo11LEMdata/blob/master/registers-verb-noun-prog-altitude.tsv )
Per Apollo GN&CS guide:
Per Timeline book: