Thursday, May 13, 2010

PLC Input Devices

A programmable logic controller consists of the following components:
• Central Processing Unit (CPU)
• Memory
• Input devices
• Output device
• Power supply

The components that will discuss in this article are PLC Input Device. A PLC is a control device. It takes information from inputs and makes decisions to energize or de-energized outputs. The decisions are made based on 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, etc. these input devices can be AC or DC. The input voltages can be high or low. The input signals can be analog or digital. Differing inputs require different input modules. An input module provides an interface between input devices and a PLC’s CPU, which is only use 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 24V AC, 48V AC, 120V AC, 220V AC, 24V DC, 48V DC, 120V DC, 220V 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.

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