Siemens SIMATIC S7-400H - Minimum Signal Duration of Input Signals During the Update

Linking and Synchronizing
6-6
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Minimum signal duration of input signals during the update
During the update, program scanning is stopped for a certain time (we will discuss
this subject in greater detail later). So that the change of an input signal can be
reliably detected by the CPU even during the update, the following condition must
be satisfied:
Min. signal duration > 2 ×× time required for I/O update (for DP only)
+ call interval of the priority class
+ processing time for the program of the priority class
+ time for the update
+ processing time for programs of high-priority
priority classes
Example:
Minimum signal duration of an input signal that is evaluated in a priority class > 15
(for example, OB 40).
Minimum signal duration
For DP only:
Time for updating the
I/O (worst case 2x)
Call interval
of the priority class
– for example, OB
40
Processing time for the pro-
gram of the priority class – for
example, run time OB 40
Time for updating
(75 ms + 0.7 ms per
KB for modified data
blocks)
Processing time
of high-priority
priority classes
Figure 6-3 Example of minimum signal duration of an input signal during updating