Why Do 'Cycle Time May Increase (0601:000044)' or 'Possible Cycle Time Violation (0601:000042)' Messages Appear in TIA Portal STEP 7?

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When monitoring the status of a block in TIA Portal STEP 7, you may encounter the messages "Cycle time may increase (0601:000044)" or "Possible cycle time violation (0601:000042)". These messages are generated by the CPU during block status monitoring to indicate potential impacts on the program cycle time.

Why does the message "Cycle time may increase (0601:000044)" appear?


The CPU evaluates the cycle time load caused by enabling block status monitoring. If the expected cycle time falls within the critical range (as depicted in the shaded area of Figure 2), the message "Cycle time may increase (0601:000044)" is displayed. This warning signals a potential increase in cycle time due to the additional load from monitoring.


Why does the message "Possible cycle time violation (0601:000042)" appear?


This message is triggered when the calculated program cycle time exceeds the configured maximum cycle time minus a 5 ms buffer. It serves as a warning of a potential cycle time violation. To mitigate this risk, you can:
  • Configure OB 80 to prevent the CPU from stopping due to a cycle time violation.
  • Adjust the maximum cycle time in the hardware configuration.
The decision criteria for this message are illustrated in Figure 4. If the expected value lies within the shaded area (Figure 3), the message is generated.


Scenarios Leading to These Messages
The messages in Figures 1 and 3 typically occur when the monitored block is called frequently within a program cycle, such as:
  • Recursive calls to the block within a loop.
  • Frequent use of shared blocks.
Conditional monitoring does not mitigate the issue and may introduce additional load, further impacting cycle time.
Example Calculation
Monitoring a block once adds approximately 5 µs to the cycle time. While a single instance has negligible impact, repeated monitoring can accumulate significant load. For example:
  • If a block is monitored 1,000 times in a single cycle, the total additional load is 5 ms.
  • If the original cycle time was ≤ 10 ms, this increase triggers the "Cycle time may increase" message.
Recommended Solutions
To address these messages, consider the following adjustments to the user program:
  • Limit the monitored path to a single execution.
  • Display relevant values in the variable table.
  • Inspect block parameters at the call point.
Firmware and Software Improvements
The mechanism for calculating when these messages are displayed has been optimized starting with CPU firmware version V2.9.2. The following enhancements have been implemented in TIA Portal V16 Update 6, V17 Update 4, and V18:
  • When monitoring is enabled, a dialog box appears, warning of potential cycle time increases.
  • Users can choose to proceed or cancel monitoring via this dialog.
  • To suppress the dialog for future project sessions, select the "Do not show again" checkbox and confirm with "Yes".