With a multimeter you can use its continuity setting to find the ground pin, by putting one lead to the metal chassis and running the other one through all the pins until you find the one that beeps (with the computer off). Knowing which one is the ground pin should lead you to the correct pinout.

Just as an academic exercise, TX could be found using a bi-color LED in series with a ~2k7 resistor as a probe: one side of the contraption to ground and the other to the pin under test. Using a terminal program set to a very low baud rate (say, 75 baud) by typing some characters in the terminal window you should see the LED change color when you probe the TX pin. Following that, RX could be found by bridging TX through a resistor to the rest of the pins until you get character loopback in the terminal window. Flow control pins could be identified using the same 'LED probe' and software capable of letting you toggle the lines individually.

Using realterm you could nicely toggle certain pins on or off.

133MHz wrote:

With a multimeter you can use its continuity setting to find the ground pin, by putting one lead to the metal chassis and running the other one through all the pins until you find the one that beeps (with the computer off). Knowing which one is the ground pin should lead you to the correct pinout.

Thanks I'll start with this and see how I go!

Harekiet wrote:

Using realterm you could nicely toggle certain pins on or off.

Nice idea but this is a 486, so it's running Windows 3.1 😀