Just one question, the cable you are using, is it 9 pin on both ends? If one end is a USB plug I cannot help. I never could get a USB rs232 cable to work with machines or anything. I have an old pc with rs232 plugs due to this. I am told it can be done but I left the industry before finding out?

Coin plug missing issue shouldn't be a problem as you just connect your plug straight into the back of the rfm pc. If you are using two bought cables, one to the rfm pc and attached to the coin door and then attaching another to the coin door and to your pc, this can be a problem. Your send/receive wires may not be correct by doing this (not crossed). The rfm send wire goes to your pcs receive pin etc. So if both cables are identical then you are double crossing making the receive go to the others devices receive and so on. Its a bit like two negatives make a positive or think of it as both devices talking and on one listening. Hope this may trigger some ideas.

If you have done this try using only the one cable and see what happens. My guess the original rfm cable to its pc had wires that were not crossed.