Allen-Bradley 1756 Series, User Manual, Page: 66 / 136

66 Rockwell Automation Publication 1756-UM013B-EN-P - October 2019
Chapter 4 1756-OBV8S Output Module Features
Field Power Loss Detection
The Field Power Loss Detection feature monitors for the loss of field power.
Keep in mind that all points on the module fault when a field power is lost.
Table 14 describes what happens when a field power loss
condition is detected
.
To correct the issue, you must reapply field power to the output module.
Table 15 describes what happens when a field power loss condition is resolved,
and the module is recovered.
Field Power Loss Detection has a corresponding tag that can be examined in the
user program if a fault occurs. For information on module tags, see Appendix B,
Module Tag Definitions on page 113 .
You can also monitor a point for the presence of a field power loss via the
diagnostics that are available in the Module Properties dialog box in Logix
Designer application.
IMPORTANT The 1756-OBV8S module supports Field Power Loss Detection.
The module receives field-side power via the DC power terminals on the
module. When field-side power is lost on the 1756-OBV8S, it is lost from the
DC power terminals.
See 1756-OBV8S Module Wiring Diagrams on page 123 .
Table 14 - Field Power Loss Detection
Output
Behavior Tag Value Diagnostic Value I/O Status Indicator State
• Faults
• Turns off
One of the following:
• I.Ptxx.FieldPowerOff tag = 1
• I.Ptxx.Fault tag = 1
FieldPowerOff diagnostic = 1 • Solid red
Table 15 - Field Power Loss Detection
Output
Behavior Tag Value Diagnostic Value I/O Status Indicator State
Restarts in its
commanded
state. One of the following:
• I.Ptxx.FieldPowerOff
tag = 0
• I.Ptxx.Fault tag = 0
FieldPowerOff diagnostic = 0 Off
IMPORTANT The module can require up to 2 seconds to complete the recovery.
IMPORTANT Keep in mind the following:
• For the 1756-OBV8S module, Field Power Loss detection is in regard to
loss of field power on the DC power terminals, or the presence of
Undervoltage or Overvoltage conditions on the DC Power bus.