Rockwell Automation T7485 ICS Regent+Plus Isolated Guarded Output Modules - Manual (Page 3)

Isolated Guarded Digital Output Module (T7485)
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field programmable gate arrays (FPGA). Each FPGA
independently operates one of the two output control switches.
The two output switches are connected in series with the load.
When both output switches are on, current will flow through
the output and energize a field load. If either switch is off,
current will not flow through the output and the load will be
de-energized. This combination of series output switches and
independent drive signals produces fail-safe activation of the
load. Single failures can only affect one of the output drive
signals or switches. A single failure will result in either
continued correct control or a fail-safe output as shown in
Table 1.
Table 1. Output States After Switch Failure.
Case
Commanded
Output State
Switch
Failed
State
Actual
Output
to Load Remarks
1 On On On Continued correct control.
Automatic testing detects
stuck-on switch. If output is
subsequently commanded
off, output will turn off.
2 On Off Off Fail-safe output. Automatic
testing detects stuck-off
switch.
3 Off On Off Continued correct control.
Automatic testing detects
stuck-on switch. If output is
subsequently commanded
on, output will turn on.
4 Off Off Off Fail-safe output. Automatic
testing detects stuck-off
switch. If output is subse-
quently commanded on,
output will remain off.
To achieve fault tolerance, two isolated Guarded output
modules are used with their outputs connected in parallel.
This configuration provides for continued correct control even
when one output switch fails off (cases two and four in
Table 1). The module failure is automatically detected and
the module can be removed and replaced without interrupting
output control.