Siemens S7-1200 - CPU Scan Cycle

Configuration and operation
4.7 CPU scan cycle
CP 1243-7 LTE
50 Operating Instructions, 01/2015, C79000-G8976-C381-01
4.7
CPU scan cycle
Priority in the scan cycle
The cyclic updating of the values of input data points of the CP by reading the current values
of the assigned PLC tags on the CPU can be prioritized.
Less important input data points do not need to be read in every CPU scan cycle. Important
input data points, on the other hand, can be prioritized for updating in every CPU scan cycle.
You can prioritize the data points in STEP 7 in the data point configuration in the "General"
tab with the "Priority in the scan cycle" parameter. There you will find the two following
options for input data points:
● High priority
● Low priority
The data points are read according to the method described below.
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
The values of input data points with the scan priority "High-priority" are read in every scan
cycle.
●
Low priority read jobs
Some of the values of input data points with the scan priority "Low-priority" are read in
every scan cycle.
The number of values 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 values
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.
●
Write jobs
In every cycle, the values of a certain number of unsolicited write jobs are written to the
CPU. The number of values 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 values whose number exceeds this value are then written 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.