I been looking at this for a little while. It looks like the coin box is using 12 volts and is pulling low.
So it looks like the board that sends out the coin counts and volume is maybe limiting the inputs to a safe level to a micro.
Why i think that is because it tells you not to use a coin box that pulls high so the coin output could basically be floating at the uC Vdd and then be pulled low.
We get a coin it sends a F1, reject F12, vol+ F9, vol- F8, sevice Tab and Credit F2.
Now for the hard part I need to know what happens when a coin is dropped for sure I need the voltage at no coin and the voltage when the OP drops a coin.
The chip there using for USB to serial is a 18F14k50 and from them images I've found it's inputs have just a resistor on them to let the chips input more then likely use the clamping diodes float at Vdd and be pulled low. 10K the pin would see 1.25 mA