Cpu Scan Cycle - Siemens SIMATIC NET S7-1200 Operating Instructions Manual

Configuration and operation

5.6 CPU scan cycle

Format (memory Data point type
requirements)
Floating-point Measured value, short floating point
number (32 bits) number
Measured value, short floating point
number with time tag
Set point command, short floating
point number
Block of data Bitstring of 32 bits
(1...32 Bit)
1)
Bitstring of 32 bits with time tag
Bitstring of 32 bits
For information on the format of the time stamp, refer to the next section.
1)
With these data point types, contiguous memory areas up to a size of 32 bits can be transferred. All S7 data types with a
2)
size between 1 and 32 bits are compatible. If the array is modified later, the data point must be recreated.
Time stamp with the IEC CP
IEC CP time stamps are transferred according to the IEC specification in the "CP56Time2a"
format. Note that in the frames only the first 3 bytes for milliseconds and minutes are
transferred.
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.
38
IEC type
<13>
<36>
1)
<50>
<7>
2)
<33>
1) 2)
<51>
2)
Direction S7 data types
in REAL
in REAL
out REAL
in
1)
in
1)
out
1)
CP 1243-1 DNP3, CP 1243-1 IEC
Operating Instructions, 02/2014, C79000-G8976-C312-02
Address area
Q, M, DB
Q, M, DB
Q, M, DB
I, Q, M, DB
I, Q, M, DB
I, Q, M, DB