Most of this has probably been said already, but here goes anyway.
RS-232 (Standard serial, what the COM port on your computer uses) is full duplex with an assumed frequency (though it varies between devices). there are 3 pins needed--Tx, Rx, Gnd. (the other 6 are status indicators). TTL is very similar, just uses 0v and 5v instead of -3v to -25v and 3v to 25v of RS-232. (
Beyond Logic has some great
RS-232 resources)
The PS/2 format is also serial, but is half-duplex and requires a clock. While it
can connect to a standard serial setup (either TTL or RS-232), if the clock speed varies a little, you're screwed. A better solution may be to take a cheap PIC/Stamp/etc. and wire the clock to an interupt and the data to a digital IO, and have it convert to/from TTL/RS-232 and PS/2.
The actual data format of PS/2 devices has long been standardized (at least for basics). Beyond Logic has an article on the
AT Keyboard. Here's a list of Mouse format articles I found via
Google:
There is, of course, plenty more to find online.
