Programmable Logic Controller (PLC)

Your Programmable Logic Controller (PLC) is the heart of the control of your process. The PLC basically starts by looking at the current state of all inputs and outputs. Then runs some code to determine what the next state of the outputs should be and then sets the outputs to that state. Then the cycle starts repeats. The time it takes to go through this cycle is called its scan rate.

Most PLC systems produced today have a scan rate of about 10ms to 100ms. The number of I/O points to be scanned and the length of the code directly affect the scan rate.

The code is stored and run in a special device called a Processor Module. Most code can be in sequential function chart or ladder diagram formats. Some systems will even allow you to use both formats in the same processor. Ladder diagram format is the older and most widely used format. It is easier to read and follow.

Click here to see an example of ladder diagram format for an Allen-Bradley Control Logix PLC.