Siemens SIMATIC S7-400 Reference Manual page 110

Cycle and Reaction Times of the S7-400
Calculation of the Longest Reaction Time
• Longest reaction time
19.03 ms * 2 = 38.06 ms.
• The delay in the inputs and outputs is negligible.
• All the components are plugged into the central rack; DP cycle times do not
therefore have to be taken into account.
• There is no interrupt handling.
Rounded off, the longest reaction time is thus = 38 ms.
Example II
You have installed an S7-400 with the following modules:
• One CPU 414-2
• Four digital input modules SM 421; DI 32×DC 24 V (4 byte each in PA)
• Three digital output modules SM 422; DO 16×DC 24 V/2A (2 byte each in PA)
• Two analog input modules SM 431; AI 8×13 bit (not in PA)
• Two analog output modules SM 432; AO 8×13 bit (not in PA)
CPU Parameters
The CPU has been assigned parameters as follows:
• Cycle load due to communications: 40%
User Program
According to the Instruction List, the user program has a runtime of 10.0 ms.
Cycle Time Calculation
The theoretical cycle time for the example results from the following times:
• Since the CPU-specific factor is 1.0, the user program processing time remains:
approx. 10.0 ms
• Process image transfer time
Process image: 20 ms + 22 byte×1.9 ms = approx. 0.06 ms
• Operating system runtime at scan cycle checkpoint:
approx. 0.17 ms
The cycle time for the example results from the sum of the times listed:
Cycle time = 10.0 ms + 0.06 ms + 0.17 ms = 10.23 ms.
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Automation System S7-400 CPU Specifications
A5E00165965-01