What is SCAN in PLC? What is Scan Time?

What is SCAN in PLC?

The PLC scan, commonly referred to as the PLC program scan, is the procedure whereby the PLC controller continually performs the following steps:

Step-1: Reads input device status & updates the input table.

Step-2: Executes the user’s program logic.

Step-3: Updates the output devices dependent on program execution.

The PLC scan is a cyclical process that involves reading inputs, executing the program, & updating outputs.

What is Scan Time?

The scan time is how long it takes for the PLC to perform one full scan cycle. Scan times may differ depending on the following factors:

  • The PLC program is complicated.
  • Number of inputs & outputs.
  • The processing capability of the PLC
  • PLC scan periods can vary from a few milliseconds to 10s of milliseconds, based on the model and application.

A slow scan time might cause problems because it prevents the PLC from reacting adequately to rapidly changing input conditions. This can result in issues such as missed input changes and delayed output updates.

The PLC scan is the primary cyclical process via which the PLC reads inputs, performs program logic, & updates outputs.

The scan time is the time required to complete 1 full scan cycle and is an important performance indicator for PLC systems.

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