How would I read the signal of something to emulate it?

I am trying to add a computer to various coin operated machines. Instead of inserting coins, I want the computer to decide whether or not credit should be added. I run into two situations, where one seems easier than the other. The easy one is the stand $0.25 arcade game coin mechs. How would I go about hooking up a coin mech to a computer so I could put a quarter in and see what signal is sent out to the JAMMA board to give credit? Once I figure out what the signal is, could I have a computer emulate that signal through the serial port?

The harder situation is a CoinCo 9370 coin mech with dollar bill acceptor. I figured the easier way is to plug the dollar bill acceptor into something to read the signal, but I am came to realize that the dollar bill acceptor probably sends the signal saying $1 has been inserted. Since the coin mech is set less than $1, it dispenses change. I don't want change given out every time the computer does something. The coin tubes has a ribbon cable going to the mech, but I believe that is for dispensing change. I am thinking I need to find out what the signal is coming from the 9-pin jones connector. But I need the signal when the exact change is entered. Some of the pins are probably to receive power to the coin mech. How would I go about figuring out what signals needed to be sent?

hi,
If you have an oscilloscope,[ a storage type would be best] you could examine the output waveforms of the coin machines.

This would give an indication of the voltage levels and the timings.

A logic analyser would be ideal.

When you have measured the waveforms, post them and we can suggest ways of connecting to a PC.