Jitter Aspects

Performance in real-time applications is highly dependent on system-level behavior, and requires every element to perform reliably and predictably within a specific time window to avoid failure.

For applications concerned about “real-time” performance aspects, CODESYS provides a mechanism to view key timing parameters that are involved.

There are three metrics associated with the PLC’s timing performance (cycle time)- Jitter, Latency and Release Jitter.

A PLC Bus Cycle Process Loop is measured in cycle time of the particular task.

This loop starts from Reading the PLC Inputs, then Running the User Programs and finally Writing the Outputs. The time it takes to complete one(1) cycle is the PLC’s Task Cycle Time.

In many applications such as motion, the time variance from cycle-cycle must be kept as small as possible.

The deviation of the task cycle time from the desired set task cycle time (Tset) is referred to as Periodic Jitter (Jper).

Jper = Tper - Tset

The desired (ideal) cycle time Tset is specified in the Configuration of the Task as Interval.

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Latency (L) is the delay between the invocation of a task and the actual start of its release.

The Release Jitter (Jr) is the difference between the maximum and the minimum Latency that has ever occurred.

Jr = Lmax - Lmin

Note

If the sum of all negative Jper values and the sum of all positive Jper values does not equal zero(0), then a drift results.

The associated CODESYS Help section can be found here- Jitter

Display of Metrics

The current value, as well as the maximum and minimum values of the periodic jitter, can be viewed on the Monitor tab of the Task Configuration section for that specific task.

The Task Configuration include:

  • Task - name defined in the task configuration.

  • Status - current operation of the Task.

  • IEC-Cycle Count - (not supported)

  • Cycle Count - Number of executed cycles since logging in to the controller, even if the task is in STOP mode.

  • Configured Cycle Period - user defined value in configuration.

  • Last Cycle Time (us) - last measured cycle time.

  • Average Cycle Time (us) - average cycle time over all cycles.

  • Max. Cycle Time (us) -

  • Min. Cycle Time (us) -

  • Jitter (us) - current value of the Periodic Jitter.

  • Min. Jitter (us) - minimum measured periodic jitter.

  • Max. Jitter (us) - maximum measured periodic jitter.

Tip

Values can be reset by right clicking in the context area and selecting Reset.