Rockwell Automation 1794-OB16D FLEX I/O Diagnostic Modules User Manual - Manual (Page 32)

Publication 1794-UM061A-EN-P - July 2006
30 About the FLEX I/O Diagnostic Output Module
The output open-wire current monitor detects a fault condition if the output
OFF-state current drops below 0.1 mA. Output channel shorts are protected
by a current limit and over-temperature thermal sensor built into the output
device. For overcurrents, the output device’s internal current limit is tripped
and the output voltage begins to collapse.
For a sustained overcurrent or direct output short the output device’s thermal
limit is tripped and the output is automatically turned off. This shorted
condition is monitored by the on-state output channel short monitor. It
detects a short if the output voltage drops below 2V. If the on-state output
channel short is removed, the output will automatically recover and voltage
will appear at the output, thus driving the attached load on.
Optocouplers isolate FLEX I/O system-side logic voltages from the user
power supply and output channels. This provides protection against field-side
voltages and transients.
Module Protection
Functions
The module protection functions consist of output device short protection,
external power supply reverse voltage protection, and power supply loss
detection
Output Device Short Protection
Diagnostic digital outputs have internal electronics to prevent too much
current from flowing through the module. This feature protects the module,
attached wiring, and load from damage. Current limit and over-temperature
thermal sensor built into the output device protect against output shorts. For
an overcurrent, the output device’s internal current limit trips and output
voltage collapses. The output device’s thermal limit is tripped and the output is
automatically turned off if an output short circuit is detected. This shorted
condition is monitored by the on-state output channel short monitor. It will
detect a short if the output voltage drops below 2V. Under shorted conditions,
the shorted channels indicator turns red, the module fault indicator turns red
and the short fault bit and module error bit are set. If the on-state output
channel short is removed, the output automatically recovers and voltage
appears at the output, thus driving the attached load on.
ATTENTION
A shorted output channel turns off and deenergizes a connected
load. Following removal of the short the output channel
becomes active and will reenergize a connected load. Be
careful for unexpected machine operation following removal of
an active output channel short.