I/O Modules
I/O modules, or input/output modules, are often physically and electronically connected to a PLC system's CPU via a backplane, but can sometimes be placed in an entirely different area than the CPU if supported by the PLC's hardware. I/O modules serve as the necessary relays between the PLC's CPU and the industrial equipment the PLC controls. I/O modules are sometimes also referred to as blocks. While most racks and backplanes are available with a preset number of slots for the number of I/O modules necessary for a given application, additional I/O modules can be attached to a PLC via expansion modules.