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What is the Difference between Continuous Scan and Periodic Scan in a ControlLogix PLC?

Last Update: 11/7/2018

The scan modes refer to the way that the ControlLogix PLC scans the ladder and processes communications.

In Continuous Scan Mode, the PLC processes on a 10 millisecond cycle. A percentage of this cycle is dedicated to processing the ladder program and another percentage is dedicated to communications. This percentage is set using the System Overhead Time Slice (SOTS) setting in the PLC. In typical applications, this setting is configured between 20 to 50% for communications.

In Periodic Scan Mode, cycle time is set. Any time that remains after the ladder program is processed will be dedicated to communications. Users must ensure that the amount of time left over does not exceed 50% of the total cycle time; otherwise, they run the risk of overloading the processor on the PLC network card. For example, if it takes 14 milliseconds to process the ladder, the Periodic Scan Rate should not be greater than 28 milliseconds. Users should note that communications are blocked while the ladder is being scanned. If the ladder takes 100 milliseconds to process, requests from the server may be rejected during that time.

Note: It is possible to have mixed scan modes. For more information, refer to Allen-Bradley's documentation on scan modes in the PLC.