Rockwell Automation Publication 1794-UM061B-EN-P - March 2020
31
About the FLEX I/O Diagnostic Output Module
Chapter
3
You must connect dummy resistors to unused sensor power ports to mask the
diagnostic function. If these external resistors are not used, the module’s sensing
circuitry will not detect the intended voltage or current and signal a module fault.
The channel fault indicator and the module’s fault indicator will light, and the
module’s open and error bits are set, thus rendering fault detection of the
remaining channels useless. The recommended value of this dummy resistor is
10 k
(+10%), 1/8 W (or larger). The resistor is wired between unused output
channels and common. The figure depicts wiring of a dummy resistor.
Diagnostic Fault Detection
The module monitors current and voltage at each output channel terminal, and
monitors the user supply for reverse supply voltage. The figure below shows the
location of these fault monitors within the 1794-OB16D diagnostic output
module.
User Power
Open Wire
Detect
Current
Shorted
Output
Monitor
Fault Indicator
-
Reverse
Short
Open
Output
V+
Open Wire
Monitor
Reverse
Voltage
Monitor
Output Indicator
Output
Fl
ex
bu
s In
te
rf
ac
e
+
40117C
Current Limiting
Thermal Prot.
Output FET