By Maxpm

2019-02-05 22:49:46 8 Comments

On this schematic of part of the Apollo Guidance Computer (found here), some resistors have a value of "NOM."

NOM resistors

According to this question, "NOM" stands for "nominal" when it's found in a datasheet, but this isn't a datasheet and I don't see how "nominal" would make sense here.


@StainlessSteelRat 2019-02-06 01:40:14

These were hand drawn and you usually have to check around the first few sheets to understand the syntax.

Check out NASA Drawing 2005904.



As for the NOM values. These refer to the actual part numbers.

Part Numbers

R3 and R4 are 22kΩ, 1/2W resistors, Part No 1006760-64.

R1 and R2 2%, 1/4W resistors that are referenced in the table, Part Number 1006750.

Looks like 1006760 is the generic part number for 1/2W resistors and 1006750 are 1/4W resistors. These resistors are in bins and these boards were probably assembled by hand.

R1 & R2

With Part No 1006750-XXX referring to different actual resistors for different part lists. Different versions for different iterations of the design. As in 126 is bin 126 in the parts inventory.

Part No 1006750-126 to 1006750-129 are (probably) 1% resistors towards the end of the design period.

So R2 is Part No 1006750-25, which is a 510Ω resistor. Similarly R3 on the sheet you reference is the same part number.


My initial answer is sort of right. The parts in the Table do not show the design evolution but are possible alternatives (nominals) to be verified by testing.

From Apollo Block II and LEM - Computer Design Review

8.3.1 Selection of Nominals (A)

In the build-up of several of the computer modules certain nominal values are selected in order to make the circuit work. This technique requires that a group of parts be supplied to the person building up the unit. Questions that arise in this area are concerned with the criteria being used for the selection of the nominals and the control of the piece parts which are not used. The circuits are most often tested at room temperature, presumably using criteria which will guarantee that the design is adequate at the temperature extremes. The review has not been complete enough in several of these areas to indicate whether this is a satisfactory procedure or not. In the case of one resistor value in the A circuit of the interface module, it has been determined that the range of nominals is inadequate to account for the range of parameters of associated components of the circuit which are presumably not selected.

These are engineers who are sending a computer made from NOR gates into space to land men on the moon. They are going to test the crap out of it to minimize failure of key components. No simulations, but physical variance of key components to ensure mission success.

R3 is in module A. If you look at image for R3, the first 18 lines (390Ω to 2000Ω) appear to be darker than the rest (270Ω to 360Ω), which confirms the quote. These are all the 10% resistors from 270Ω to 2000Ω. The list was modified to add more alternatives for testing.

The last 4 entries (285Ω to 375Ω) may represent the refinement of nominal values as tests were conducted.

The envelope dimensions called out for the computer are not specified as maximum. The dimension called out is 6.00 inch. The nominal computer dimension is 5.996 inch, and if there is any internal pressurization at all it will exceed the 6-inch value.

NASA's use of nominal is clearly normal operation or expectations or tolerances or alternatives.

@Maxpm 2019-02-06 03:46:42

I suppose they used "nominal" to mean "you should look up this component by name." It's a usage I haven't seen before, but I guess this is the definitive answer.

@alex.forencich 2019-02-06 07:38:22

"So R2 is Part No 1006750-25, which is a 510Ω resistor" why is it this value specifically?

@StainlessSteelRat 2019-02-06 12:08:54

Odds are that bin 25 has that resistor

@Lightness Races in Orbit 2019-02-06 21:11:54

@Maxpm It's actually the fully correct use of the word "nominal", it's just that more modern usage (at least in some industries) has skewed towards a corruption which means something closer to "normal/expected" (though the corruption itself certainly already existed at the time).

@Stilez 2019-02-06 06:39:08

Nominal is basically used to mean "roughly" or "as stated", or "trade description". But there's a second related meaning which is something like "some small or user-decided (but unspecified) amount/type", and is probably what's meant here. And based on that, is a third meaning, of "some acceptable value", specifically used in space technology.

Meaning "roughly" or "as stated":

You'll see the same word used for many components which aren't sized precisely. For example if you bought some wood or steel for commercial purposes its length might be described as "3m nominal", meaning its about 3m and will do the job, but could be 3m, or 3.02m or something.

Meaning "some small or user-decided (but unspecified) amount/type"

This is a less common usage of the term, as in "leave a nominal air gap" for ventilation. Sorry for poor quality links, I've seen this used in manufacturer documents but can't find examples right now.

Dictionary definitions related specifically to space technology:

"Informal (chiefly in the context of space travel): functioning normally or acceptably."

Its not quite the same usage, but perhaps as its in the same field, this was a usage adopted in NASA design, beyond being used to describe the status of a system in operation?

@Richard the Spacecat 2019-02-06 06:50:19

I can't see this being very helpful in context of this question, especially with the StainlessSteelRat's answer showing it has little to do with "roughness". Although one could appreciate getting a wider picture of what it could mean elsewhere.

@Stilez 2019-02-06 07:02:23

Hopefully expanded and fixed now

@StainlessSteelRat 2019-02-06 13:14:41

I believe it does have some bearing because the design is evolving.

@zebonaut 2019-02-05 22:57:25

The notes in the bottom left of the schematic tell you to "select R3, R7 and R14 per applicable PS from appropriate chart", so "NOM" indicates to look in the tables.

As of yet, I have no idea what to do exactly with the numbers in the appropriate tables, and neither do I have any idea what "PS" means.

I'm a bit in the mist about the exact etymology of "NOM", but it cold be "nomogram", despite some tables giving the values instead of graphs. Maybe it's "Notice of Modification"?

@winny 2019-02-05 23:23:42

That “interpret schematic in accordance standard prescribed by MIL D-70327” is probably where the answer lies.

@jochen 2019-02-06 15:14:49

"PS" could be "parts"?

@StainlessSteelRat 2019-02-06 16:15:37

as in per applicable parts from appropriate chart.

@Dan W 2019-02-06 16:47:06

PS usually means Post Script – ie the table of values is an addendum to the schematic. Given the words 'applicable' and 'appropriate', I'd guess that different tables might be applicable for different scenarios, e.g. different revisions, or calibrations?

@supercat 2019-02-06 17:10:54

I would guess that the people drawing the schematic knew about what the right value should be, but expected that it might need to be refined through experimentation. If the original draftsman expected that 1K should work, but it turned out that 1.5K was required, it would be necessary to ensure that all copies of the schematic that showed a 1K value were corrected or destroyed. Having to reprint schematics for such a change would have been much more expensive than saying "look at the latest version of XX table for the latest value".

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