Rockwell Automation Publication 1734-UM013N-EN-P - September 2017 35
Safety Inputs, Safety Outputs and Safety Data Chapter 2
When a channel is configured for module sensor power, a sensor power
diagnostic is executed on that channel at powerup. The diagnostic is used to
make sure that the sensors are not drawing over- or under-current and that
channel-to-channel shorts are not present.
Channel Offset
You can configure an offset when differences in the sensors nominal input signals
would otherwise exceed the desired discrepancy deadband. Use the Channel
Offset if you are using two sensors of different types to measure the same variable.
Sensors from two different vendors potentially give slightly differing data values
for a given temperature or pressure. Use the Channel Offset to bring the data
values back together. You can also use the Channel Offset with two identical
sensors that are physically offset from each other.
The channel offset is applied before the channel discrepancy is evaluated.
Process Alarms
Process alarms alert you when an analog input value has exceeded the configured
high or low limits for each channel. Process alarms are set at four configurable
trigger points.
• High High alarm
• High alarm
• Low alarm
• Low Low alarm
When a sensor power over-current condition occurs, it can take as much as 15
seconds longer than the configured latch time for channel status to recover
after the over-current condition is cleared.
If you use an external power supply, you must monitor the system for the
following:
• The supply voltage must be within the operating range of the sensor.
• The current draw of the sensors must not be over- or under-current, which
could indicate a problem with the components of the sensor.
• Channel-to-channel shorts must be detected, if they occur.
The Channel Offset is applied only during the evaluation of discrepancy between
two channels that are configured for Dual Channel and is not applied to any of the
Process Alarms. Therefore, if you are using two sensors to measure the same process
variable, and these sensors read different values, you potentially need to set the
Process Alarms to different values based on the sensor readings.
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