Allen-Bradley 1756-L6 Series, Reference Manual, Page: 13 / 134

Rockwell Automation Publication 1756-RM001I-EN-P - May 2012 13
SIL Policy Chapter 1
• If the application cannot tolerate an output that fails open (de-energized),
then an external means such as a manual override or output must be wired
in parallel. (Refer to
Wiring ControlLogix Digital Output Modules on
page 44 for more information). The user must supply the alternative means
and develop the application program to initiate the alternate means of
removing or continuing to supply power in the event the main output fails.
• This manual override circuit is shown in Figure 1 . It is composed of a
hard-wired set of contacts from a selector switch or push-button. One
normally-open contact provides for the bypass of power from the
controller output directly to the actuator. The other is a normally-closed
contact to remove or isolate the controller output.
• An application program needs to be generated to monitor the diagnostic
output modules for dangerous failures such as shorted or open-output
driver channels. Diagnostic output modules must be configured to hold
last state in the event of a fault.
• A diagnostic alarm must be generated to inform the operator that manual
control is required.
• The faulted module must be replaced within a reasonable time frame.
• Any time a fault is detected, the system must annunciate the fault to an
operator by some means (for example, an alarm light).
Figure 1 - Manual Override Circuit
L1
L2 or Ground
Actuator
Manual Override
43379
Alarm to Operator
Fault
Allen-Bradley Motors