Chapter 9
197
Logic programming – Function blocks
9.9.8 Tolerant dual channel monitor
Function block diagram
General description
The Tolerant dual channel monitor function block can be used to evaluate
switches and sensors. It offers dual channel monitoring that is less restrictive than
the normal dual channel monitor on the input/output I/O modules, e.g. WS0-XTIO or
WS0-XTDI or the function blocks Switch evaluation, Emergency stop, Light grid
evaluation, Magnetic switch, Two-hand type IIIA and Two-hand type IIIC (see
section 9.9.2).
In the case of tolerant dual channel monitor the correct sequence of the two input
signals is evaluated. During this process it is expected, if one of the two signals has
caused the switch off, that the other signal will follow.
The tolerant dual channel monitor differs from the normal dual channel monitor in
the following points:
The switch off condition is allowed to be met on the two inputs with a delay. It is not
imperative for the switch off condition to be met simultaneously on both inputs at
least at one point in time.
Optionally an AND mode can be activated to make the evaluation even more
tolerant in certain circumstances. In this case even switching off only one input is
accepted as a correct sequence without the need for the other input to follow. This
situation may be acceptable if the hazardous machine parts (actuator) are safety
shut down at this point in time. For this purpose the optional Actuator released
input is connected to the signal in the logic that controls the output for the safety-
related actuator release. If necessary the duration of the AND mode can be limited.
Optionally, a brief switch off on one or both inputs can be ignored with the aid of the
off-delay. If necessary the off-delay can be enabled using the Off-delay timer input.
The discrepancy time monitoring can be activated separately for switching on and
for switching off.
Figure 185:
Logic connections for the
tolerant dual channel
monitor function block
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