24 Rockwell Automation Publication 1794-UM061B-EN-P - March 2020
Chapter 2 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 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.
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