A PLC is a control device. It takes information from inputs and makes decisions to energize or de-energize outputs.
The decisions are made based on the statuses of inputs and outputs and the ladder logic program that is being executed. The input devices used with a PLC include pushbuttons, limit switches, relay contacts, photo sensors, proximity switches, temperature sensors, and the like.
These input devices can be AC (alternating current) or DC (direct current). The input voltages can be high or low. The input signals can be digital or analog. Differing inputs require different input modules.
An input module provides an interface between input devices and a PLC’s CPU, which uses only a low DC voltage.
The input module’s function is to convert the input signals to DC voltages that are acceptable to the CPU.
Standard discrete input modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and transistor-transistor logic (TTL) level.
The devices controlled by a PLC include relays, alarms, solenoids, fans, lights, and motor starters.
These devices may require different levels of AC or DC voltages.
Since the signals processed in a PLC are low DC voltages, it is the function of the output module to convert PLC control signals to the voltages required by the controlled circuits or devices.
Standard discrete output modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and TTL level.