Do you have a multimeter?

PS/2 uses 4 pins actively: DATA, CLK, Vcc and GND.

Vcc is easy to find: that's the one that outputs +5V relative to ground (anywhere on the board/system). GND is the one with continuity (very low resistance) to ground elsewhere.

With an oscilloscope you can find CLK, but once you know Vcc and GND you can just brute-force the rest.

dionb wrote on 2025-12-27, 02:40:

Do you have a multimeter? […]
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Do you have a multimeter?

PS/2 uses 4 pins actively: DATA, CLK, Vcc and GND.

Vcc is easy to find: that's the one that outputs +5V relative to ground (anywhere on the board/system). GND is the one with continuity (very low resistance) to ground elsewhere.

With an oscilloscope you can find CLK, but once you know Vcc and GND you can just brute-force the rest.

I do have one! Thanks for that info. I might give that a shot tomorrow.

While I've had some experience in small electrical repairs, it was all based on information I already had and knowing what everything is ahead of time. I have never probed to find certain points myself before. When searching Vcc, I just probe a known ground point anywhere else on the board while it's powered while going along each pin on the PS/2 header until I find +5V? Same with finding GND, but with the meter in continuity mode?

Provided I find Vcc and GND, that leaves me with four more pins to test for the other two (one of the rows on the board header only has two pins). I don't have an oscilloscope. While maybe not being able to tell which is DATA and which is CLK specifically, will I be able to find that a pin is at least *one* of these with the multimeter so I can narrow it down to two pins? Will it hurt anything if these two are swapped while I'm figuring it out? Neither of them would be incorrectly swapped to Vcc or GND as I will already know which ones those are that is at this point.

Phastor wrote on 2025-12-27, 04:11:

Will it hurt anything if these two are swapped while I'm figuring it out? Neither of them would be incorrectly swapped to Vcc or GND as I will already know which ones those are that is at this point.

No and that's the point, identifying the dangerous pins and then just trying the remaining combinations, dionb described the process nicely.

That said, the manual of 5200 may not contain the pinout but the manual of 5250 does, and it's also a 4x2 header with the second row having only 2 pins. So if you measure and find out that your ground and vcc pins match the 5250, I'd say try the other two like this with a good chance to hit them on the first try and save some time.

Who the hell chose that numbering scheme? It's worse than the seats in a Romanian train compartment...

konc wrote on 2025-12-27, 08:58:

No and that's the point, identifying the dangerous pins and then just trying the remaining combinations, dionb described the pro […]
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Phastor wrote on 2025-12-27, 04:11:

Will it hurt anything if these two are swapped while I'm figuring it out? Neither of them would be incorrectly swapped to Vcc or GND as I will already know which ones those are that is at this point.

No and that's the point, identifying the dangerous pins and then just trying the remaining combinations, dionb described the process nicely.

That said, the manual of 5200 may not contain the pinout but the manual of 5250 does, and it's also a 4x2 header with the second row having only 2 pins. So if you measure and find out that your ground and vcc pins match the 5250, I'd say try the other two like this with a good chance to hit them on the first try and save some time.

The attachment ATC 5250 PS2.png is no longer available

This is the standard pinout for 4x2. The reason it is standard is the same reason that the quoted image has seemingly bizarre pin numbering - it matches the footprint of the through hole pins for a female mini-din 6 connector. Those are the pin numbers of the actual mini-din 6 connector.

The idea was you could put this footprint in the board edge and either solder a connector or pin header in the spot.

BTW as for finding Vcc (+5V) with a multimeter - the only reliable way to do this is to check for continuity to the +5V power rail of the motherboard with the system off. You can check from an ISA slot or from the power supply input connectors.

If you try to just look for 5V with respect to ground with the system powered on you will see it everywhere. Anything at a logic high state will read around 5V with respect to ground (including the floating mouse clock and data lines)

maxtherabbit wrote on 2025-12-27, 14:30:

BTW as for finding Vcc (+5V) with a multimeter - the only reliable way to do this is to check for continuity to the +5V power rail of the motherboard with the system off.

Any sane designer would have put a polyfuse on the ps/2 VCC rail. Sometimes those fuses dropped enough to not trigger a multimeter's continuity beeper, but they were at least easy to spot.

maxtherabbit wrote on 2025-12-27, 14:30:

This is the standard pinout for 4x2. The reason it is standard is the same reason that the quoted image has seemingly bizarre pin numbering - it matches the footprint of the through hole pins for a female mini-din 6 connector. Those are the pin numbers of the actual mini-din 6 connector.

The idea was you could put this footprint in the board edge and either solder a connector or pin header in the spot.

Oh interesting info! So while there was not any official standard, many companies used one in practice. Seeing this odd numbering I always thought that companies just did whatever with the header, but apparently the oddness is... consistent.

This was the right pinout. Thank you for that.

konc wrote on 2025-12-27, 08:58:

No and that's the point, identifying the dangerous pins and then just trying the remaining combinations, dionb described the pro […]
Show full quote

Phastor wrote on 2025-12-27, 04:11:

Will it hurt anything if these two are swapped while I'm figuring it out? Neither of them would be incorrectly swapped to Vcc or GND as I will already know which ones those are that is at this point.

No and that's the point, identifying the dangerous pins and then just trying the remaining combinations, dionb described the process nicely.

That said, the manual of 5200 may not contain the pinout but the manual of 5250 does, and it's also a 4x2 header with the second row having only 2 pins. So if you measure and find out that your ground and vcc pins match the 5250, I'd say try the other two like this with a good chance to hit them on the first try and save some time.

The attachment ATC 5250 PS2.png is no longer available