Siemens S7-1200 System Manual page 98

PLC concepts
4.1 Execution of the user program
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 4- 15 Range for the cycle time
Cycle time
Maximum scan cycle time
Minimum scan cycle time
The maximum scan cycle time is always enabled. Configure a cycle time between 1 ms to 6000 ms. The default is 150
1
ms.
The minimum scan cycle time is optional, and is disabled by default. If required, configure a cycle time between 1 ms
2
and the maximum scan cycle time.
Configuring the cycle time and communication load
You use the CPU properties in the Device configuration to configure the following
parameters:
● Cycle: You can enter a maximum scan cycle monitoring time. You can also enable and
enter a minimum scan cycle time.
● Communication load: You can configure a percentage of the time to be dedicated for
communication tasks.
For more information about the scan cycle, see "Monitoring the cycle time". (Page 97)
98
Range (ms)
1 to 6000
1
1 to maximum scan cycle time
2
Default
150 ms
Disabled
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG