A programmable logic contoller (PLC) is a solid-state device with digital processing capabilities designed for real-time control of residential applications. A PLC comprises of input and output units (I/O) and a central processing unit (CPU).The input and output components are built into the same physical box with the CPU. The package provides a small, light-weight, low-cost, and self-contained solution for a wide range of control applications.
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.
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.
