Open wire detection, Clamping and limiting, Clamp and limit alarms – Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules User Manual
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Rockwell Automation Publication 1756-UM533C-EN-P - February 2011
Chapter 6
1756-OF8H HART Analog Output Module
Open Wire Detection
This feature detects when current flow is not present at any channel. The
1756-OF8H module must be configured for 0…20 mA or
4…20 mA operation to use this feature. At least 0.1 mA of current must be
flowing from the output for detection to occur.
When an open wire condition occurs at any channel, a status bit named
ChxOpenWire is set for that channel.
Clamping and Limiting
Clamping limits the output from the analog module to remain within a range
configured by the controller, even when the controller commands an output
outside that range. This safety feature sets a high clamp and a low clamp.
Once clamps are set for a channel, any data received from the controller that
exceeds those clamps sets a limit alarm and transitions the output to that limit but
not beyond the configured clamp value. For example, an application may set the
high clamp on a module for 8V and the low clamp for -8V. If a controller sends a
value corresponding to 9V to the module, the module will only apply 8V to its
screw terminals. The signal value actually applied is reflected in the Input Tag
ChxData field.
Clamping limits are entered in engineering units.
Clamp and Limit Alarms
This function works directly with clamping. When a module receives a data value
from the controller that exceeds clamping limits, it applies the clamping limit to
the signal value and sends a status bit to the controller, notifying it that the
commanded output data value exceeds the clamping limits.
Using the example, if a channel has clamping limits of 8V and -8V but then
receives data to apply 9V, only 8V is applied to the screw terminals and the
module sends a status bit back to the controller informing it that the 9V value
exceeds the channel’s clamping limits.
Clamping alarms can be disabled or latched on a per channel basis. Clamping
limits are entered in engineering units.