The working of a PLC can be easily understood as a cyclic scanning method known as scan cycle.

A PLC Scan Process includes the following steps

• The operating system starts cycling and monitoring of time.
• The CPU starts reading the data from the input module and checks the status of all the inputs.
• The CPU starts executing the user or application program written in relay-ladder logic or any other PLC-programming language.
• Next, the CPU performs all the internal diagnosis and communication tasks.
• According to the program results, it writes the data into the output module so that all outputs are updated.
• This process continues as long as the PLC is in run mode.