According to the IBM PC/XT Technical Reference page 3-6, pin 2 of plug P8 is a "key." (Note that this is the pin specifically; this is separate from the keying of the shroud, which is independently indicated.) This is also the case on page 2-35 of the original 1981 PC Technical Reference]pctr, though the connector there is not numbered. Neither manual mentions the key pin otherwise in the text of that section.

IBM PC/XT PSU/motherboard connector

I had thought that this was a filled-in pin-hole in the shroud that would prevent the P8 connector from being attached to the header for P9, which would connect ±12V from the PSU to the board's ±5V power rails, which would obviously be a bad thing.

But looking at images of the PC and XT motherboards, there is no corresponding missing pin on the motherboard header, so obviously that pin-hole in the PSU connector shroud can't be filled.

IBM PC motherboard power header

So what exactly is this "key" pin and what was its purpose?

Further, the PC/AT power supply seems to be almost identical, except that just that pin has been changed from "key" to "+5V". What was the reason for this change? Just to add a fourth +5V conductor to handle the increased amperage on the 5V rail as compared to the XT? Since the key pin on the motherboard appears unconnected in the schematics, does this mean that an AT power supply will work fine for a PC or PC/XT motherboard? How about vice versa, where an AT motherboard would presumably be connecting the "key" pin on the PSU connector to +5V (assuming that the AT motherboard is not otherwise drawing more current than the PC or XT PSU can supply)?

2 Answers 2


In PC and XT manuals, the "KEY" pin is mentioned elsewhere, it is also called "N.C." in some diagrams, e.g. in page 19 of the PDF you linked, "System Unit 1-7". It can mean "Not Connected", "No Contact", "No Connection" or other similar things, but basically, it is unused and has no connection on PC and XT. On the motherboard, the power connectors are P1 and P2, but on the supply, they are P9 and P8.

It is kind of impossible to guess now, but maybe the intention was to use the pin 2 as you describe, as a keying to prevent connectors from being plugged incorrectly. Maybe some other connector was originally considered, with possibility for no pin on motherboard and plugged hole on the power supply.

But since these connectors already were keyed in their plastic mold, it was not necessary to use a contact position as the key.

The connectors are keyed differently so it is impossible to swap them. P8 has a tab in the middle while P9 has a tab near the edge. The motherboard connectors have slots that match, making it impossible to plug connectors in incorrectly, at least if you check if they were fully inserted.

Perhaps different connectors were first thought that were without plastic keying but then these connectors were selected and they happened to allow keying in the plastic shroud. Maybe they were not even available from manufacturer with missing pins and plastic pins to plug the unused hole, because the shrouds are already keyed.

When AT appeared, the unused KEY or N.C. pin of the PC and XT was used as extra +5V pin. As all +5V pins are connected together on the AT motherboard, it simply allows for more current, or to be more exact, more current with same voltage drop, or same current with less voltage drop, because you have more paralleled conductors and more pins with some max rated current and contact resistance.

Technically an AT supply could power an XT motherboard just fine, and XT supply could power an AT motherboard, assuming motherboards do not exceed ratings of the supplies.

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    IIRC, it was to stop you accidentally connecting P9 to where P8 should go, and vice-versa. If it wasn't prevented, you could fry the motherboard
    – CSM
    Commented Feb 6 at 18:50
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    @CSM It was maybe supposed to; but as said, the unused pin was never removed, and the unused hole was never plugged, because, the connectors already had keyed plastic shroud which made the removal of unused pin and plugging the unused hole redundant.
    – Justme
    Commented Feb 6 at 19:05
  • Except plugging the unused hole isn't redundant because if you accidentally reverse the connectors you connect the PSU 12V lines to the motherboard 5V bus.
    – cjs
    Commented Feb 6 at 21:07
  • @cjs But I explained it is redundant. The connectors cannot be reversed because they are already keyed. You can't swap them. So since you can't reverse them anyway, why plug the unused hole, and if you do plug the hole, then you need to remove the pin from the motherboard side connector too.
    – Justme
    Commented Feb 6 at 21:15
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    @cjs The images in the question have arrows pointing to the plastic keys at different location on P8 and P9. The motherboard connectors have matching slots, one approximately in the middle and the other near edge. They are not interchangeable.
    – Justme
    Commented Feb 6 at 21:22

The original connector of the IBM PC, in the prototype pictures (as seen here), was the same as the System/23 Datamaster. It was a 12-pin MOLEX structured in three columns of four pins. It contained every single signal the production connectors eventually got and had a single N.C. pin (pin 10).

System/23 PSU

I can only speculate that the original one was phased out due to the friction when plugging/unplugging it made it not practical for the end user and also was too big, occupying too much space in the board.

System/23 simplified board layout

The case is, the IBM PC started as a modified System/23 and for this reason it still has quirks that can only be explained by studying its predecessor. This is just one of them.

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