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Friday, January 8, 2010

How The PLC Work?

The PLC is a highly functional that can control external devices (indicator lamp, solenoid, motor, and others) according to the signals received from external input devices (pushbutton switches, limit switches, sensors, and others). Also, desired program can be written into the internal memory of the PLC using the handheld programmer or PC software program. Below is the illustration of the PLC construction.


PLC comprises a microcomputer (CPU : Central Processing Unit) and a memory (storage element).

Firstly the PLC will read the status ON/OFF of all input terminals before the program is executed. Then the PLC CPU will do various processing and calculations that performed according to the details written to the program. Lastly the results of program execution are output from the output terminals.
The flow of operations from input processing through to output processing is called a “scan”. This scan is repeatedly executed on ladder programs.
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