Can some please explain "What exactly is Derating?"
Operating the part at lower values than rated specifications
How it is related to positive and negative temperature coefficient.
Running a part at a lower voltage or current means less heat is generated. Powering a 16v max capacitor, at 16v, is stressing it. Running a 20mA led at 20mA will only provide x number of hours of life, while running it at 10mA will provide y hours, where y is greatly larger than x.
Derating is limited to only resistors and capacitors or it applies to ASICs also.
Derating can apply to almost anything. Resistors, LEDs, Other Diodes, Capacitors, ICs, CPUs. Another common word for derating, in the context of computers and CPUs, is underclocking. As opposed to overclocking, where you run a cpu at a higher than rated speed, underclocking (aka derating) runs it at a lower speed, allowing for less heat and longer life. Mainly used when modders want to remove noisy fans.
As a note, some devices demand derating in certain situations. Resistors listed as x wattage, expect the resistor to be in free air at ambient temperatures with air movement over it. If you put it in a sealed case, or in heat shrink, or in a hot environment, you NEED to derate it down. Same with Solar Panels. The given rating is for IDEAL sunlight. Average sunlight will derate the output current and voltage.
The trick here is to look at the spacing. It's subtle, but when you know what to look for it's obvious.
The right-hand band of the top resistor has a slightly wider space infront of it. That's the % band.
So you have Brown Black Black Silver, at Brown %, which is
100×10-2Ω at 1%. (1Ω±1%)
The bottom one, the left hand band has a slightly wider space. That's the % band. As I say, subtle differences. The top one is more obvious than the bottom one.
So you have Brown Black Black Brown at Brown %, which is
100×101Ω at 1%. (1kΩ±1%)
Best Answer
These were hand drawn and you usually have to check around the first few sheets to understand the syntax.
Check out NASA Drawing 2005904.
NOM means NOMINAL.
As for the NOM values. These refer to the actual 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.
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
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.
NASA's use of nominal is clearly normal operation or expectations or tolerances or alternatives.