Siemens SIMATIC S7-1200 - 5.1.4 Monitoring and configuring the cycle time

PLC concepts
5.1 Execution of the user program
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
109
5.1.4
Monitoring and configuring the cycle time
The cycle time is the time that the CPU operating system requires to execute the cyclic
phase of the RUN mode. The CPU provides two methods of monitoring the cycle time:
● Maximum scan cycle time
● Minimum scan cycle time
Scan cycle monitoring begins after the startup event is complete. Configuration for this
feature appears under the "Device Configuration" for the CPU under "Cycle time".
The CPU always monitors the scan cycle and reacts if the maximum scan cycle time is
exceeded. If the configured maximum scan cycle time is exceeded, an error is generated
and is handled one of two ways:
● If the user program does not include a time error interrupt OB, then the timer error event
generates a diagnostic buffer entry, but the CPU remains in RUN mode. (You can change
the configuration of the CPU to go to STOP mode when it detects a time error, but the
default configuration is to remain in RUN mode.)
● If the user program includes a time error interrupt OB, then the CPU executes it.
The RE_TRIGR instruction (Page 309) (re-trigger cycle time monitoring) allows you to reset
the timer that measures the cycle time. If the elapsed time for the current program cycle
execution is less than ten times the configured maximum scan cycle time, the RE_TRIGR
instruction retriggers the cycle time monitoring and returns with ENO = TRUE. If not, the
RE_TRIGR instruction does not retrigger the cycle time monitoring. It returns ENO = FALSE.
Typically, the scan cycle executes as fast as it can be executed and the next scan cycle
begins as soon as the current one completes. Depending upon the user program and
communication tasks, the time period for a scan cycle can vary from scan to scan. To
eliminate this variation, the CPU supports an optional minimum scan cycle time. If you
enable this optional feature and provide a minimum scan cycle time in ms, then the CPU
delays after the execution of the program cycle OBs until the minimum scan cycle time
elapses before repeating the program cycle.
In the event that the CPU completes the normal scan cycle in less time than the specified
minimum cycle time, the CPU spends the additional time of the scan cycle performing
runtime diagnostics and/or processing communication requests.
In the event that the CPU does not complete the scan cycle in the specified minimum cycle
time, the CPU completes the scan normally (including communication processing) and does
not create any system reaction as a result of exceeding the minimum scan time. The
following table defines the ranges and defaults for the cycle time monitoring functions:
Table 5- 18 Range for the cycle time
Cycle time
Range (ms)
Default
Maximum scan cycle time
1
1 to 6000
150 ms
Minimum scan cycle time
2
1 to maximum scan cycle time Disabled
1
The maximum scan cycle time is always enabled. Configure a cycle time between 1 ms to 6000
ms. The default is 150 ms.
2
The minimum scan cycle time is optional, and is disabled by default. If required, configure a cycle
time between 1 ms and the maximum scan cycle time.