Not the best subject line - unless you are thinking of using something from a Sony PlayStation
I base the following on the assumption that you're using a PC keyboard with PS/2 protocol/connections.
Is there any way to interface an old PS/2 Keyboard? There is like 100 juicy inputs available from each key. I need some way to simulate a keypress for different sensors.
Just need a way to trigger a keypress with TTL levels.
PC keyboards will be slower than a parallel port, especially if you run a multitasking OS, but if you can live with that, it's quite easy to use them.
The keys are scanned in a matrix, so you need to poke around in the circuit and try cridging a row and a column with a signal diode (1N4148, 1N914 etc.) to find which lin is the most positive.
When you have established the direction the diode needs to be in, each node can be replaced with an NPN transistor with the emitter where the cathode of the diode was (when it gave an input).
In case neither of the lines is at ground potential when scanned, you'll need to use an optocoupler to avoid shorting the keyboard processor, but it's not very likely to happen.
To "press" a key, just put the logic level (not TTL level) via a base resistor to the transistor.
Even though you have the keyboards, it might not be the best solution, but hey, you decide