Rockwell Automation T3480 ICS Regent Guarded Analog Output Modules - Manual (Page 11)

Guarded Analog Output Modules (T3480)
P D - 6 0 2 6 M a r - 0 6
11
Effects of Open Circuit Field Wiring
When a Guarded analog output module is controlling the
outputs (as opposed to the standby mode) an open circuit in
the field wiring connected to a load device will be detected as a
failure. This causes the controlling module to switch off,
annunciate a fault and enter the standby mode. Once in the
standby mode, all of its outputs are shunted to AGND internal
to the module, thus eliminating the open circuit and the
module’s comparators become healthy. This will have one of
two effects based upon single or dual module configuration:
Single Guarded Module Configuration
In a single guarded output configuration the module will
toggle back and forth between the controlling and standby
modes about every one or two seconds. The ON status LED on
the face of the module will turn on and off repeatedly until the
open circuit field wiring is corrected. During this time the
module FAULT status LED will also be on and the system will
report a fault for this module.
Each time the single module enters the standby mode all six
of the output channels are taken off line and the field signals
become 0 mA. When the module switches back to the control
mode the field signals return to their controlled values. This
causes the output loads that have healthy wiring connections
to experience an output signal that toggles between 0 mA and
their controlled values which often produces undesirable
effects. To avoid this, you should monitor the fault status
system control relay for the output module in your application
program and use it to set all of the analog output channels to
zero. Turning all of the outputs off will often be safer than
letting them toggle between 0 mA and a controlled value.
Dual Guarded Module Configuration
In a dual module configuration the two modules will switch
back and forth between control and standby modes. The ON
status LED will appear to “ping-pong” back and forth between
the two modules about every one or two seconds. This will
continue until the open circuit field wiring is corrected.
During this time the module FAULT status LED will also be
on for each of the modules and the system will report a fault
for both modules.
Important!