Allen-Bradley 1734-OB8S - Chapter 2 Safety Inputs, Safety Outputs, and Safety Data; Safe States

Rockwell Automation Publication 1734-UM013C-EN-P - August 2010 25
Safety Functions Chapter 2
Dual-channels, Equivalent
In Equivalent mode, both inputs of a pair should 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 equivalent
dual pair, the data bits for both channels will always be sent to the controller as
equivalent, both high or both low.
Figure 8 - Equivalent, Normal Operation and Fault Detection (not to scale)
ON
OFF
IN0
Safety Input 0
IN1
Fault Detected
Discrepancy Time
Safety
I/O
Network
Data Sent
to the
Controller
ON
OFF
ON
OFF
ON
OFF
ON
OFF
ON
OFF
ON
OFF
IN0
Safety Input
Status 0, 1
IN1
Fault Detection
ON
OFF
ON
OFF
ON
OFF
Discrepancy Time
Safety Input
Status 0, 1
Safety Input 1
Safety Input 1
Safety Input 0
Normal
Operation
Safety
I/O
Network
Data Sent
to the
Controller