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So you've picked up a sweet Socket 7 motherboard to get yourself rolling on the retro computing bandwagon, but it didn't come with a bracket for connecting a PS/2 mouse. You could just use a serial mouse, but you won't be able to enjoy the benefits of high precision mouse tracking, and it doesn't help that few serial mice have scroll wheels handy. But where do you get the right PS/2 mouse connector for what your motherboard connector expects? Turns out you don't need the exact one, you can just pick up a generic 2x5 connector online and rework it accordingly!
There's a few things you'll need upfront to get started:
Which Pins Go Where?
PS/2 mouse header pinouts have never been consistent across motherboards, even those of the same manufacturer. However, a lot of them should be good enough to provide a diagram in a manual indicating exactly which pin is to be connected where. Given PS/2 mouse ports themselves always use the same pinout, you can use the information in the manual to determine the function of each pin on the header by using the continutity test function of a multimeter.
A PS/2 mouse port uses four of six pins for VCC/5V power, clock, ground, and data. Review the diagram pictured above and check the pin descriptions in the manual; you should be able to see where certain pins are supposed to go, so keep the manual or a printout handy when you go to rearrange the pins.
Of course, wire colors on PS/2 mouse headers are not standardized, so that's where the multimeter comes into play...
Insert a metal needle into one of the pins used by the PS/2 mouse port. Since most multimeter leads are too large to go into any pin, the needle helps stick out the conductivity of a pin for a lead to be able to reach it.
Take one lead and place it on the needle inserted in the pin, then take the other lead and keep touching different pins on the header until you find continuity. A light may come on, you may hear a tone, and/or the information on the LCD may change. Keep holding continuity at the pin and take note of the wire's color. Record your finding on some paper, taking note of the function of the pin you tested. Move the needle to another pin, then test the other pins until continuity is found again. Repeat this for all pins but the last, as the last pin's function will be obvious at that point.
Now should be a good time to deatch the wires from the header for rearrangement. You could have done this already, but I prefer to do it after the continuity tests so no pins are shorting each other. Insert the needle underneath the latch holding a wire in place, and gently pry it upward. Pull the wire out, then repeat for all other wires.
Using the motherboard documentation and the notes you've taken, insert each colored wire into the correct position in the header. The wires should audibly lock into place when inserted; if they don't, gently tug then to ensure nothing's coming loose, and push down on the latches to tighten their grip if necessary.
That's it! If all went well, your PS/2 mouse should be detected. You can test the mouse in the MS-DOS text editor (assuming a mouse driver is loaded), any GUI operating system, or even AMI WinBIOS if available. If the mouse is not working, you'll have to go back and rearrange some pins as needed. Windows 9x users will see an error dialog if that is the case. Most of the time, getting the pins wrong is not of risk compared to working with USB headers, but I would double-check your pin arrangement before even testing for mouse functionality.
On some motherboards, you may need to set a jumper to enable the PS/2 mouse port, as is the case with plenty of 486 motherboards.
Side note: Some motherboards such as the Shuttle HOT-433 may not actually have PS/2 mouse support despite having a header due to not having an AMIKEY chip or something like that. Check your motherboard's manual and search the internet for details on your particular board.