I made a program that dispenses coins in 345 lines. It tells you if the coin tray is in place, and if it didn't dispense enough coins it will tell you how many successfully dispensed. It also has ACK\NAK communication. It's written in C for a specialized microcontroller.

I have written code that talks to 4 devices, but this is the only one I have written the code for. In 2 years it has never had a bug, and always responds quickly and correctly via RS 232.