Rockwell Automation Publication 1734-UM013J-EN-P - July 2014 35
Safety Inputs, Safety Outputs, and Safety Data Chapter 2
Dual-channel Equivalent Mode
The 1734-IE4S module supports Dual-channel Equivalent mode. In
Dual-channel Equivalent mode, the values of both inputs of a pair must be within
a configured tolerance range (discrepancy deadband). If the difference between
the channel values exceeds the deadband for longer than the configured
discrepancy time, a discrepancy fault is declared. When a dual-channel
discrepancy fault occurs, the input status values for both channels are set low
(off ) and the actual input values are reported. The fault is cleared when the
difference between the channel’s values falls back within the discrepancy
deadband tolerance range for the discrepancy time.
Figure 14
illustrates module operation in dual-channel equivalent mode. At A,
the difference between the channel values exceeds the discrepancy deadband
tolerance range and the discrepancy timer starts. When the timer expires at B, a
dual-channel discrepancy fault occurs and the inputs status bits are set low. At C,
the values fall back within the discrepancy deadband and the discrepancy timer
starts again. When the timer expires at D, and the values are still within the
discrepancy deadband, the fault is cleared. At E, the difference between the
channels exceeds the discrepancy deadband and the discrepancy timer starts. A
discrepancy fault occurs again at F, when the timer expires and the difference
between the channel values remains greater than the discrepancy deadband.
Figure 14 - Timing Diagram
If you are using the module with a GuardLogix controller, set the module’s
inputs to Single (default). Do not use the modules’ dual-channel mode as this
functionality is provided by the GuardLogix safety application instructions.
1
0
A B C
0
1
D
250 ms
250 ms
E F
250 ms
High High Alarm
High Alarm
Low Low Alarm
Low Alarm
Channel A
Channel B
Discrepancy Time = 250 ms
Fault Present
Input Status
Deadband
Difference between Channel A and Channel B
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