Sample Of Response Time Calculation - Siemens CPU 31 Series Technical Data Manual

Calculating the cycle time
The cycle time for the example results from the following times:
• User program execution time:
• Process image transfer time
• Operating system runtime at scan cycle checkpoint:
Cycle time = 5.5 ms + 0.4 ms + 0.4 ms + 0.5 ms = 6.8 ms.
Calculation of the actual cycle time
• There is no active communication.
• There is no interrupt handling.
Hence, the physical cycle time is 6 ms.
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
• There is no interrupt handling.
5.6.2

Sample of response time calculation

Design
You have configured an S7300 and equipped it with the following modules in two racks:
• a CPU 314C-2
• 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)
CPU 31xC and CPU 31x, Technical Data
Manual, 01/2006 Edition, A5E00105475-06
approx. 5 ms x CPU-specific factor 1.10 = approx. 5.5 ms
Process image of inputs: 100 μs + 8 Byte x 37 μs = approx. 0.4 ms
Process image of outputs: 100 μs + 8 Byte x 37 μs = approx. 0.4 ms
Approx. 0.5 ms
allowances for any DP cycle times on PROFIBUS DP or for PROFINET IO update times.
Configuring the cycle load as a result of communication: 40 %
Cycle and reaction times
5.6 Sample calculations
5-23