For Siemens S7-300/400 series PLCs, the cyclic scan time of the CPU is not related to the instruction, but for the S7-1500 series PLC, the program that uses the extended instruction program to transfer data may result in an increase in cycle time.
What is the cycle time?- four parts:
1 main loop program complete processing time
2 interrupt the main loop program, execute the processing time of the higher priority program (current cycle)
3 time required to update the process image area
4 operating system communication process time required
Asynchronous instructions can create a communication load, which can cause fluctuations in the cycle period. All asynchronous commands that access the memory card (SIM Memory Card, SMC) can cause fluctuations in the cycle time. This fluctuation depends on:
1 version or type of memory card
2 data capacity
3 number and size of files
4 number of files in each directory
For example, the asynchronous instructions "READ_DBL" and "WRIT_DBL" in the following figure access the load store (SMC). The memory card SMC has reached its own storage limit, which increases the management load on data consistency protection during power failure, resulting in loss of read and write performance of the memory card, and then causes an increase in the cycle period.
1 use a large capacity memory card
2 If using a cyclic interrupt OB, increase its clock period if necessary (eg from 5 ms to 30 ms).
3 If the program includes many asynchronous instructions (READ_DBL), do not configure the STEP 7 (TIA Portal) program loop monitoring time limit.
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