Siemens SIMATIC S7-400 Reference Manual page 111

Calculation of the Actual Cycle Time
• Allowance of communication load:
10.23 ms * 100 / (100-40) = 17.05 ms.
• A time-of-day interrupt having a runtime of 0.5 ms is triggered every 100 ms.
The interrupt cannot be triggered more than once during a cycle:
0.5 ms + 0.35 ms (in Table 3-7) = 0.85 ms.
Allowance for communication load:
0.85 ms * 100 / (100-40) = 1.42 ms.
• 17.05 ms + 1.42 ms = 18.47 ms.
The actual cycle time is therefore 19 ms taking into account the time slices.
Calculation of the Longest Reaction Time
• Longest reaction time
19 ms * 2 = 38 ms.
• Delays in the inputs and outputs
– The digital input module SM 421; DI 32×DC 24 V has an input delay of not
more than 4,8 ms per channel
– The digital output module SM 422; DO 16×DC 24 V/2A has a negligible
output delay.
– The analog input module SM 431; AI 8×13 bit was assigned parameters for
50 Hz interference frequency suppression. This results in a conversion time
of 25 ms per channel. Since 8 channels are active, a cycle time of 200 ms
results for the analog input module.
– Analog output module SM 432; AO 8×13 bit was assigned parameters for
the measuring range from 0 to 10V. This results in a conversion time of 0.3
ms per channel. Since 8 channels are active, a cycle time of 2.4 ms results.
To this must be added the settling time for the resistive load, which is 0.1
ms. A response time of 2.5 ms therefore results for an analog output.
• All the components are plugged into the central rack; DP cycle times do not
therefore have to be taken into account.
• Case 1: When a digital signal is read in, an output channel of the digital output
module is set. This produces a reaction time of:
Reaction time= 38 ms + 4.8 ms = 42.8 ms.
• Case 2: An analog value is read in and an analog value output. This produces a
reaction time of:
Reaction time = 38 ms + 200 ms + 2.5 ms = 240.5 ms.
Automation System S7-400 CPU Specifications
A5E00165965-01
Cycle and Reaction Times of the S7-400
3-21