Cpu Scan Cycle - Siemens CP 1243-1 PCC Operating Instructions Manual

5.6

CPU scan cycle

Structure of the CPU scan cycle
The cycle (including the pause) with which the CP scans the memory area of the CPU is
made up of the following phases:
● High-priority read jobs
For data points of the type "Input", which are configured with the "High priority" setting in
the data point configuration in "General > Priority in the scan cycle", the PLC tags are all
read in one scan cycle.
● Write jobs
In every cycle, the values of a certain number of unsolicited write jobs are written to the
CPU. The number of tags written per cycle is specified for the CP in the "Communication
with the CPU" parameter group with the "Max. number of write jobs" parameter. The tags
whose number exceeds this value are then written in the next or one of the following
cycles.
● Low-priority read jobs - proportion
For data points of the type "Input", which are configured with the "Low priority" setting in
the data point configuration in "General > Priority in the scan cycle", the values of a part
of the PLC tags are read in every scan cycle.
The number of tags read per cycle is specified for the CP in the "Communication with the
CPU" parameter group with the "Max. number of read jobs" parameter. The tags that
exceed this value and can therefore not be read in one cycle are then read in the next or
one of the following cycles.
● Cycle pause time
This is the waiting time between two scan cycles. It is used to reserve adequate time for
other processes that access the CPU via the backplane bus of the station.
Duration of the CPU scan cycle
Since no fixed time can be configured for the cycle and since the individual phases cannot
be assigned a fixed number of objects, the duration of the scan cycle is variable and can
change dynamically.
Siemens Automation
CP 1243-1 PCC
Operating Instructions, 02/2015, C79000-G8976-C384-01
Configuration and operation
5.6 CPU scan cycle
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