Chapter 9
188
Logic programming – Function blocks
9.9.2 Dual-channel evaluation (1 pair) and discrepancy time
This section relates to the Safety gate monitoring, Emergency stop, Light curtain
monitoring, Magnetic switch, Two hand control type IIIA and Two hand control type
IIIC function blocks.
It does not relate to the Tolerant dual channel monitor function block.
Note that the safety I/O modules, e.g. WS0-XTIO or WS0-XTDI, can carry out a dual-
channel evaluation when predefined input elements from the Elements window (e.g.
RE27, C4000, …) are connected to them. If such an input element is selected, you do
not need a separate function block for dual-channel evaluation (e.g. light curtain
monitoring, safety gate monitoring or magnetic switch). For detailed information on
discrepancy monitoring on the safety I/O modules see Section 10.1.
Alternatively, you can connect non pre-evaluated input signals to both input channels
of a function block with a dual-channel input configuration. In this case the dual-
channel evaluation takes place in the function block.
The disadvantage of this alternative is that it requires one function block more in the
logic which can lead to a higher logic execution time. The advantage is that a
discrepancy time error is made available via the output of the function block and can
be evaluated in the logic.
The following function blocks generate the same output value for a dual-channel input
signal that was pre-evaluated by the I/O device.
The dual-channel evaluation evaluates the correct sequence of the two input signals.
It is expected that if one of the two signals has caused a switching off, the other signal
will follow accordingly. Which values the two signals must have depends on the type
of the dual-channel evaluation. There are two possibilities:
equivalent evaluation
complementary evaluation
An optional discrepancy time can be configured. The discrepancy time defines for how
long the two inputs may have discrepant values after one of the both input signals has
changed without this being considered as an error.
Note
Figure 175:
Dual-channel evaluation
with safety I/O module or
with function block
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