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2-7GFK-0787B Chapter 2 Input Subsystem
Line Monitoring for Discrete Inputs
Normally-closed inputs on GMR-configured blocks can be monitored for short circuit
faults. Normally-open inputs on blocks which are not configured in GMR mode can be
monitored for open circuit faults.
Normally-closed Inputs
For applications such as Emergency Shutdown (ESD), normally-closed inputs are generally
monitored for short circuits across the lines, since that represents a fail to danger condition
(that is: trip is not detected). In general, these inputs are powered from +24V, and a field
short to ground is interpreted as a trip condition.
Source
Genius
Block
+24V
Typical Normally-closed Input
Normally-open Inputs
For applications such as Fire and Gas Detection, normally-open inputs are generally
monitored for open circuits on the lines, since that represents a fail to danger condition
(that is: trip is not detected). In general, these inputs are powered from +24V, and a field
short to +24V is interpreted as a trip condition.
Source
Genius
Block
Typical Normally-open Input
+24V
When a block is configured (with a Hand-held Monitor) as a GMR block, its input thresholds
change to those listed below.
Input Voltage Input Status Input State
Source Blocks
tristate inputs
<30% V
dc
off 0
Source Blocks
tristate inputs
>50% V
dc
< V
dc
–7V
on 1
< V
dc
–4V
short circuit fault 1
bi-state inputs
<30 V
dc
off 0
bi-state inputs
>50% V
dc
on 1
Sink Blocks
tristate inputs
<4V short circuit fault 1
Sink Blocks
tristate inputs
>7V
<50% V
dc
on 1
>70% V
dc
off 0
bi-state inputs
<50% V
dc
on 1
bi-state inputs
>70% V
dc
off 0
Input Filter Time
For any circuit configured as a tristate input, the Input Filter Time configured for the block
(using a Hand-held Monitor) must be at least 30mS.