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Continuous Scan versus Periodic Scan in ControlLogix PLCs

What is the difference between continuous scan and periodic scan in a ControlLogix PLC and how will it affect my communications?

This refers to the way the PLC scans the ladder and processes communications.

In Continuous Scan mode, the PLC processes tasks on a 10 MS cycle. A percentage of this cycle is dedicated to processing the ladder program and a percentage is dedicated to communications. This percentage is set using the SOTS (System Overhead Time Slice) setting in the PLC. Typical applications see this being set between 20-50% for communications.  The default setting is 10%, which means you will want to increase this to at least 20% if you are going to be using Continuous Scan mode for your PLC processes.

In Periodic Scan mode, cycle time is set. Any time left over after the ladder program is processed is dedicated to communications.

Do not be fooled, though.  You still need to make sure that the amount of time left over after the ladder is scanned does not exceed 50% of the total cycle time or you run the risk of overloading the processor on the PLC Network card. So if it takes 14msec to process the ladder you do not want the Periodic Scan rate to be greater than 28 MSEC. Additionally, when the ladder is being scanned communications is blocked so if the ladder takes 100 ms to process that will be time when requests from the server can be rejected.

It is also possible to have mixed scan modes.

If Periodic Scan Mode tasks are configured to have a short trigger period (periodic scan rate) and have a long scan time, there will be little time available for overhead, i.e. communications from applications such as TOP Server.  This is especially dangerous for ControlLogix projects with large numbers of configured tasks/programs.

Recommendations:

  1. Eliminate any unnecessary tasks by consolidating your PLC code into the fewest number of tasks/programs possible.
  2. Increase the scan rate(s) of the periodic task(s) as much as possible so the task(s) only run as often as necessary.
  3. Increase the System Overhead Time Slice (for ControlLogix project using continuous scan tasks).

For more details see the Allen-Bradley documentation regarding scan modes in the PLC.