Siemens SIMATIC PCS 7 - Page 372

Supplementary information
18.16 CPU 410 cycle and reaction times
CPU 410 Process Automation/CPU 410 SMART
372 System Manual, 05/2017, A5E31622160-AC
Calculating the cycle time
The theoretical cycle time for the example is derived from the following times:
● As the CPU-specific factor is 1.2, the user program execution time is:
approx. 12.0 ms
● Process image transfer time (4 x double-word access and 3 x word access)
Process image: 9 µs + 7 ×25 µs = approx. 0.184 ms
● Operating system runtime at scan cycle checkpoint:
approx. 0.31 ms
The total of the listed times is equivalent to the cycle time:
Cycle time
= 12.0 ms + 0.184 ms + 0.31 ms
= 12.494 ms
.
Calculation of the actual cycle time
● Allowance for communication load:
12.494 ms * 100 / (100–40)
= 20.823 ms
.
● A time-of-day interrupt with a runtime of 0.5 ms is triggered every 100 ms.
The interrupt can be triggered a maximum of one time during a cycle:
0.5 ms + 0.490 ms (from table 16-9)
= 0.99 ms.
Allowing for communication load:
0.99 ms * 100 / (100–40)
= 1.65 ms
.
● 20.823 ms + 1.65 ms
= 22.473 ms
.
Taking into account the time slices, the actual rounded up cycle time is
22.5 ms
.
Calculating the longest response time
● Longest response time
22.5 ms * 2
= 45 ms
.
● Delay of inputs and outputs
– The maximum input delay of the digital input module SM 421; DI 32×DC 24 V is 4.8
ms per channel
– The output delay of the digital output module SM 422; DO 16×DC 24 V/2A is
negligible.
– An interference frequency suppression of 50 Hz was assigned for the analog input
module SM 431; AI 8×13Bit. The result is a conversion time of 25 ms per channel. As
8 channels are active, a cycle time of the analog input module of 200 ms results.
– Analog output module SM 432; AO 8×13Bit was assigned for measuring range 0 ... 10
V. This results in a conversion time of 0.3 ms per channel. Since 8 channels are
active, the result is a cycle time of 2.4 ms. The transient time for a resistive load of 0.1
ms must be added to this. The result is an analog output response time of 2.5 ms.
● All components are installed in the central controller, so DP cycle times can be ignored.