From your discription , I am presuming that you have 12 terminals all sending data back to the main PC

I personally would use RS485 as the transmission layer , with a CAN based protocol.

You could not use RS232 , as even with CAT5 you would exceed the 2500pf cable limit..
Radio could be used , as could carrier pidgeons , or radio controlled rats carrying data packet's..This is how the internet works.