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).

In the raw telemetries of the landing we can see some strange values for "verb" and "noun", at first 1202 alarm:

Transcripted and embellished:

Some sources say that "V90N50E" sequence is an "Aldrin input to stop the alarm", others say that sequence was "V50N90E", others "V05N09E" and others "V5N9E".

The recorded sequence visible in telemetries, V50N90E is not a valid sequence.

What really (probably) happened is:

"Verb = 50, Noun = 90" i ntelemetries record actually reads "Verb = 5, Noun = 9".

i.e., Aldrin probably typed V5N9E; but computer expected two digits per number, so it actually did "see" V - 5 - blank - N - 9 - blank" sequence.

But, to save precious memory locations (each single bit required a complex wiring of core memories, and it increased weight), a very peculiar hardware/software algorithm (link for real nerds: link) was invented to represent the 18 digits of the 3 registers (5 digits plus sign), which eventually resulted in this truth table:

• BLANK: 00000
• 0: 10101
• 1: 00011
• 2: 11001
• 3: 11011
• 4: 01111
• 5: 11110
• 6: 11100
• 7: 10011
• 8: 11101
• 9: 11111

Notice the first two lines!

A "blank" digit (all segments turned off) corresponds to value 00000 sent to ground; instead, a "0" digit (6 segments turned on) is represented by octal number 10101.

Hence, 5+blank, 9+blank resulted in 11110+00000, 11111+00000, which was erroneously decoded into 50, 90.

Typing V5N9E is perfectly legal, given that there is the "N" which separates verb from noun and prevents amibiguity. Usually the astronauts typed single-digit verbs and nouns as two digits (0 followed by digit); in this specific case, probably due to hurry caused by alarm, they just typed the "short version", which resulted in the telemetry bug.

This is better explained here: https://github.com/virtualagc/virtualagc/discussions/1166

Why did it happend?

As Aldrin supposed at that times ("It appears to come up when we have a 1668" in transcript), the alarm occurred every time he typed sequence V16N68E, which showed on the display the difference between calculated and measured altitude above ground, which caused an overload on the computer.

Why did Aldrin type the sequence?

Because the landing radar was supposed to start providing valid data between 35000 and 40000 ft altitude, and the difference between calculated and measured data was supposed to decrease until they almost match, as it actually happened at around 102:39:30, when Ground confirmed "Delta H is looking good to us" (it was 76.7 ft difference).

There were no graphical display available on the Lunar Module, but if there would one, it would have shown something like this:

(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:

Telemetries:

The sources

These are excerpts from LMA790-3-LM - APOLLO OPERATIONS HANDBOOK:

It's not exactly for LM-5, hence alarm codes are slightly different, 31201 and 31202 rather than 01201 and 01202, but it explains well the V05N09E sequence.

Another possible confirmation that V50N90 is an user input rather than LGC output comes from timing and flashing; in the below transcription of raw telemetries we can see that:

• 1201/1202 codes are displayed only after 50/90 (V5N9E) appears
• 50/90 (5/9) do not flash, although verb/noun flashing is recorded by LGC in bit 6 of channel 11, and as far as I can understand from documentation available around, verb+noun flashing means "expecting user input"
• R1 is filled with 1201/1202 only 2 to 6 seconds after the alarm code is loaded in FAIL REGISTER, and after couple 50-90 appeared in verb/noun telemetry.

Hence the sequence is:

• Astronaut types V16N68E
• LGC overflow
• 1201/1202 appears in FAILREG+0
• astronaut types V5N9E (V50N90 is sent to ground)
• FAILREG+0 content is displayed in R1

Full log of the alarms:

Legenda:

Full image of telemetries: https://github.com/jumpjack/Apollo11LEMdata/blob/master/registers-verb-noun-prog.png

Data file for telemetries: CSV, TSV