Rockwell Automation 1794-OB16D FLEX I/O Diagnostic Modules User Manual - Manual (Page 24)

Publication 1794-UM061A-EN-P - July 2006
22 About the FLEX I/O Diagnostic Input Module
Sensor Power Open and Short Circuit Detection
The sensor-power open-wire current monitor detects a fault condition if the
sensor-power current drops below 50
μ A. Sensor-power shorts are protected
by the positive temperature coefficient (PTC) resistor in series with each
sensor power output (16 total). For overcurrents or shorts, the PTC resistor
heats up, the resistance increases and the sensor power output opens, similar to
a fuse opening. This shorted condition is monitored by the sensor power
voltage monitor. It detects a short if the sensor power voltage disappears.
When the short is removed, the PTC resistor cools down and the previously
shorted sensor-power port, sensor power is automatically restored and normal
operation continues.
The module’s embedded monitors detect open and short conditions.
When a fault is detected:
• the corresponding channel’s red indicator lights.
• the module’s red fault indicator lights.
• the module’s open, short or reverse error bit is set.
• the module error bit is set.
The input circuit is a conventional sinking style with an input voltage dropping
resistor. The yellow input indicator is in series with the input signal (field-side
indication). FLEX I/O system-side logic voltages are isolated from the user
power supply and input channels by optocouplers. This provides protection
against field side voltages and transients.