Input and Output Units (I/O)

Inputs are wired to sensing devices such as button keypads, selector switches, photocells, motion detectors, etc. If an input senses that a sensor is closed, the input converts the voltage to a logic 1 signal understood by the CPU to be ON. A logic 1 signal indicates an On or CLOSED state, and a logic 0 signal indicates an OFF or OPEN state.

Ouputs are wired to switching devices such as lights, garage doors, heating systems, and other household appliances. Outputs switch the supplied control voltage that energizes or de-energizes the switching devices. If an output is turned ON by the CPU, the control voltage is switched to activate the switching device.

Central Processing Unit (CPU)

Within the CPU are the digital processor, memory, and power supply. These components interact to solve application logic and pass control signals to the outputs. The CPU reads the converted input signals, executes the user logic program stored in its memory, then writes the appropriate output signals to the switching devices. The application program is written in a language called ladder logic.

PLC Block Diagram

System Block Diagram

The block diagram shows the major components of a PLC. The PLC monitors the state of the sensing devices by receiving signals from its inputs, solves a user logic program stored in its CPU, and then directs switching device activity by sending control signals to its outputs.

This article is a quick overview of a Programmable Logic Controller.For more information on this type of home automation controller, check out the following sites;

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http://www.ab.com/ – Rockwell
http://www.modicon.com – Schneider
http://www.industry.net/c/mn/08vqbap2 – GE Fanuc