Rockwell Automation T3468A ICS Regent DC Guarded Digital Output Modules - Manual (Page 3)

(T3461A, 62A, 68A) DC Guarded Digital Output Modules
P D - 6 0 1 9 M a r - 0 6
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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-off 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 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.
Testing and Diagnostics
The voter and I/O bus interface logic of the Guarded output
modules is automatically tested by the processor modules.
Discrepant data are sent through one of three legs of the I/O
Safetybus to determine whether the module’s voters are able
to outvote the incorrect data. A failure to return the correct