so: you 're inputing quantity of coins in computer , then what?
I don't get , what is the purpuse of counting. Suggestion you have REAL coins 10, 20 for example
for 10 is 0v, for 20 is +5v , you put coins in the hopper , hopper gives signas 0v, +5v to computer, and computer counts how many signals there are , so you're having resut how many 10 coins and how many 20 coins

if so then what you need to output from PC? indeed you need to read signals from hopper

in the case you want to create signals from PC to hopper (i don't understand why), then i guess you need to know other language than C , cuz C is "high-objective" language , and you need "low-objective" language. ( Assembler for example. or Pascal i don't know for sure)
maybe in C there is such possibility, try to find this command in MSDN