Siemens Simatic S7-1500 Function Manual page 24

Cycle control point
When the cycle control point is reached, the CPU has completed the cycle program and is
no longer executing OBs. All user data are consistent at this time. Requirement is that no
communication that modifies user data (such as HMI communication or PUT/GET
communication) is active.
The cycle control point marks:
● The end of a cycle and its cycle time statistics
● The start of the next cycle and its cycle time statistics
● The restart of the monitoring of the configured maximum cycle time
(time-out counter is reset)
The cycle control point is reached depending on which of the following events occurred last:
● End of the last cyclic OB
● Expiry of the minimum cycle time (if configured)
After the cycle control point has been reached, the CPU executes the following steps:
1. Writes the process image outputs to the output modules
2. Reads the state of the inputs into the input modules
3. Executes the first cyclic OB
Cycle and response times
Function Manual, 10/2018, A5E03461504-AD
Cyclic program execution
3.1 Cycle
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