Understanding the PLC scan cycle
The PLC scan cycle is a good way to explain how the PLC works. Let’s have a look at it to have a better understanding of the operation of a PLC.
The PLC scan cycle is the cycle in which the PLC reads the inputs, runs the PLC programs, performs diagnostic and communication tasks, and updates the output. It’s a repetitive process. The time taken by the PLC to complete one scan cycle is referred to as the scan time and is measured in milliseconds.
The following is a PLC scan cycle:
Figure 7.9 – PLC scan cycle
PLC scan cycle may vary slightly depending on the PLC in use. Basically, the PLC starts by reading the inputs, that is, checking the state of the connected switches and sensors to know which one is ON or OFF or the current value if it’s analog. It then executes the user program based on the state of the inputs. Next, the PLC performs communication tasks, which include communicating necessary...