Chapter 9
189
Logic programming – Function blocks
The following truth table describes the discrepancy conditions for the dual-channel
equivalent and the dual-channel complementary input evaluation:
Evaluation
type
Input
1A
Input
1B
Discrepancy
timer
*1
Evaluation
status
Enable output
Discrepancy
error output
Equivalent 0 0 0 Inactive 0 Unchanged
*2
0 1
< Discrepancy
time
Discrepant 0 Unchanged
*2
1 0
< Discrepancy
time
Discrepant 0 Unchanged
*2
1 1 0 Active
*3
1 0
x x
≥ Discrepancy
time (timeout)
Error 0 1
Complementary 0 1 0 Inactive 0 Unchanged
*2
0 0
< Discrepancy
time
Discrepant 0 Unchanged
*2
1 1
< Discrepancy
time
Discrepant 0 Unchanged
*2
1 0 0 Active
*3
1 0
x x
≥ Discrepancy
time (timeout)
Error 0 1
*1 If the discrepancy time is active (> 0), the discrepancy timer is restarted on the first signal change
resulting in a discrepant status. If the discrepancy time is inactive (= 0), the discrepancy timer does
not start, i.e. a timeout will never occur.
*2 Unchanged = Last status is preserved.
*3 If the correct sequence has been observed.
For the change between the different states of the dual-channel evaluation the
following rules apply:
A dual-channel evaluation can only change to Active (Enable output changes from
Low to High), if …
since the last Active status at least once the status was Inactive, i.e. it is not
possible to switch from Active to Discrepant and back to Active, and
the discrepancy time has not elapsed or the discrepancy time is deactivated, and
A discrepancy error (timeout) is reset, if the Active status has been reached, i.e. the
Enable output changes to High.
When defining values for the discrepancy time the following must be observed: The
discrepancy time …
must be greater than the logic execution time,
has a tolerance of +/- 10 ms in addition to the logic execution time. The logic
execution time depends on the number and type of function blocks used and is
shown in the Setting and Monitoring Tool in the logic editor on the FB info tab and
also in the report.
Table 81:
Dual-channel evaluation
Note
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