Communication - Siemens Simatic S7-300 Technical Data Manual

Cycle and reaction times
5.6 Sample calculations
Calculating the physical cycle time
Under consideration of communication load:
12.5 ms * 100 / (100-40) = 20.8 ms.
Thus, under consideration of time-sharing factors, the actual cycle time is 21 ms.
Calculation of the longest response time
• Longest response time = 21 ms * 2 = 42 ms.
• I/O delay
• Neither PROFIBUS DP, nor PROFINET IO are being used, so you do not have to make
• Response times plus I/O delay:
5-26
– The maximum delay of the input digital module SM 321; DI 32 x 24 VDC is 4.8 ms per
channel.
– The output delay of the digital output module SM 322; DO 16 x 24 VDC/0.5 A can be
neglected.
– The analog input module SM 331; AI 8 x 12 bit was configured for an interference
suppression at 50 Hz. The result is a conversion time of 22 ms per channel. With the
eight active channels, the result is a cycle time of 176 ms for the analog input module.
– The analog output module SM 332; AO 4 x 12-bit was programmed for the measuring
range of 0 ...10 Hz. This gives a conversion time of 0.8 ms per channel. Since
4 channels are active, the result is a cycle time of 3.2 ms. A settling time of 0.1 ms for
a resistive load must be added to this value. The result is a response time of 3.3 ms
for an analog output.
allowances for any DP cycle times on PROFIBUS DP or for PROFINET IO update times.
– Case 1: An output channel of the digital output module is set when a signal is received
at the digital input. The result is a response time of:
Response time = 42 ms + 4.8 ms = 46.8 ms.
– Case 2: An analog value is fetched, and an analog value is output. The result is a
response time of:
Longest response time = 42 ms + 176 ms + 3.3 ms = 221.3 ms.
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05