According to the IBM PC/XT Technical Referencepage 3-6, pin 2of plug P8 is a "key." (Note that this is the pin specifically; this isseparate from the keying of the shroud, which is independently indicated.)This is also the case on page 2-35 of the original 1981 PC TechnicalReference]pctr, though the connector there is not numbered. Neithermanual mentions the key pin otherwise in the text of that section.
I had thought that this was a filled-in pin-hole in the shroud that wouldprevent the P8 connector from being attached to the header for P9, whichwould connect ±12V from the PSU to the board's ±5V power rails, which wouldobviously be a bad thing.
But looking at images of the PC and XT motherboards, there is nocorresponding missing pin on the motherboard header, so obviously thatpin-hole in the PSU connector shroud can't be filled.
So what exactly is this "key" pin and what was its purpose?
Further, the PC/AT power supply seems to be almost identical, except thatjust that pin has been changed from "key" to "+5V". What was the reason forthis change? Just to add a fourth +5V conductor to handle the increasedamperage on the 5V rail as compared to the XT? Since the key pin on themotherboard appears unconnected in the schematics, does this mean that anAT power supply will work fine for a PC or PC/XT motherboard? How aboutvice versa, where an AT motherboard would presumably be connecting the"key" pin on the PSU connector to +5V (assuming that the AT motherboard isnot otherwise drawing more current than the PC or XT PSU can supply)?
