Dual-channel mode and discrepancy time, Dual-channels, equivalent – Rockwell Automation 1791DS-IBxxxx Guard I/O DeviceNet Safety Modules User Manual
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Rockwell Automation Publication 1791DS-UM001J-EN-P - May 2013
Chapter 2
Understand the Operation of Safety Functions
Dual-channel Mode and Discrepancy Time
To support redundant-channel safety devices, the consistency between signals on
two channels can be evaluated. Either equivalent or complementary can be
selected. This function monitors the time during which there is a discrepancy
between the two channels.
If the length of the discrepancy exceeds the configured discrepancy time
(0…65,530 ms in increments of 10 ms), the safety input data and the individual-
safety input status turns off for both channels.
This table shows the relation between input terminal states and controller input
data and status.
Dual-channels, Equivalent
In Equivalent mode, both inputs of a pair should typically be in the same
(equivalent) state. When a transition occurs in one channel of the pair prior to
the transition of the second channel of the pair, a discrepancy occurs. If the
second channel transitions to the appropriate state prior to the discrepancy time
elapsing, the inputs are considered equivalent. If the second transition does not
occur before the discrepancy time elapses, the channels will fault. In the fault state
the input and status for both channels are set low (off ). When configured as an
IMPORTANT
The dual-channel function is used with two consecutive inputs that are
paired together, starting at an even input number, such as inputs 0 and 1, 2
and 3, and so on.
IMPORTANT
Do not set the discrepancy time longer than necessary. The purpose of the
discrepancy time is to allow for normal differences between contact
switching when demands are placed on safety inputs. For this testing to
operate correctly, only a single demand on the safety input is expected
during the discrepancy time. If the discrepancy time is set too high, and
multiple demands occur during this time, then both safety input channels
will fault.
Table 1 - Terminal Input Status and Controller I/O Data
Dual-channel Mode
Input Terminal
Controller Input Data and Status
Dual- channel
Resultant
Data
Dual-
channel
Resultant
Status
IN0
IN1
Safety
Input 0 Data
Safety
Input 1 Data
Safety
Input 0 Status
Safety
Input 1 Status
Dual-channels, Equivalent
OFF
OFF
OFF
OFF
ON
ON
OFF
Normal
OFF
ON
OFF
OFF
OFF
OFF
OFF
Fault
ON
OFF
OFF
OFF
OFF
OFF
OFF
Fault
ON
ON
ON
ON
ON
ON
ON
Normal
Dual-channels,
Complementary
OFF
OFF
OFF
ON
OFF
OFF
OFF
Fault
OFF
ON
OFF
ON
ON
ON
OFF Normal
ON
OFF
ON
OFF
ON
ON
ON Normal
ON
ON
OFF
ON
OFF
OFF
OFF
Fault