Cycle and reaction times
5.2 Cycle time
CPU 31xC and CPU 31x, Technical Data
5-4 Manual, 01/2006 Edition, A5E00105475-06
5.2.2 Calculating the cycle time
Introduction
The cycle time is derived from the sum of the following influencing factors.
Process image update
The table below shows the time a CPU requires to update the process image (process
image transfer time). The times specified might be prolonged as a result of interrupts or CPU
communication. The process image transfer time is calculated as follows:
Table 5-2 Formula for calculating the process image (PI) transfer time
The transfer time of the process image is calculated as follows:
Base load K + number of bytes in PI in module rack 0 x (A)
+ number of bytes in PO in module rack 1 to 3 x (B)
+ number of words in PO via DP x (D)
+ number of words in PO via PROFINET x (P)
= Transfer time for the process image
Table 5-3 CPU 31xC: Data for calculating the process image (PI) transfer time
Const. Components CPU 312C CPU 313C CPU 313C-2 DP CPU 313C-2 PtP CPU 314C-2 DP CPU 314C-2 PtP
C Base load 150 μs 100 μs 100 μs 100 μs
A Per byte in
the rack 0
37 μs 35 μs 37 μs 37 μs
B per byte in
module
racks 1 to 3 *
- 43 μs 47 μs 47 μs
D
(DP
only)
Per word in
the DP area
for the
integrated
DP interface
- - 1 μs - 1 μs -
* + 60 μs per rack
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