Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 107)

Cycle and reaction times
5.6 Sample calculations
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
5-25
Calculating the longest response time
Longest response time:
6.8 ms x 2 = 13.6 ms.
•
I/O delay can be neglected.
• Neither PROFIBUS DP, nor PROFINET IO are being used, so you do not have to make
allowances for any DP cycle times on PROFIBUS DP or for PROFINET IO update times.
• Interrupts are not processed.
5.6.2 Sample of response time calculation
Installation
You have configured an S7­300 and equipped it with the following modules in two racks:
•
a CPU 314C-2
Configuring the cycle load as a result of communication: 40 %
• 4 digital input modules SM 321; DI 32 x 24 VDC (4 bytes each in the PI)
• 3 digital output modules SM 322; DO 16 x 24 VDC/0.5 A (2 bytes each in the PI)
• 2 analog input modules SM 331; AI 8 x 12-bit (not in the PI)
• 2 analog output modules SM 332; AO 4 x 12 bit (not in the PI)
User program
According to the instruction list, the user program runtime is 10.0 ms.
Calculating the cycle time
In this example, the cycle time is equivalent to the sum of the following times:
•
User program execution time:
approx. 10 ms x CPU-specific factor 1.10 = approx. 11 ms
• Process image transfer time
Process image of inputs: 100 μs + 16 bytes x 37 μs = approx. 0.7 ms
Process image of outputs: 100 μs + 6 bytes x 37 μs = approx. 0.3 ms
• Operating system runtime at the scan cycle checkpoint:
approx. 0.5 ms
The sum of the listed times is equivalent to the cycle time:
Cycle time = 11.0 ms + 0.7 ms + 0.3 ms + 0.5 ms = 12.5 ms.