Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 96)

96 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 5 1756-OF8I Isolated Analog Output Module
Clamping/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 determined for a module and enabled, any data received from
the controller that exceeds those clamps sets an appropriate limit alarm and
transitions the output to that limit but not beyond the requested value.
For example, an application can 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 applies only 8V to its screw terminals.
You can disable or latch clamping alarms on a per channel basis. The alarms are
disabled by default.
IMPORTANT Clamp values are in engineering units and are not automatically
updated when the scaling high and low engineering units are changed.
Failure to update the clamp values can generate a very small output signal
that could be misinterpreted as a hardware problem.
For example, a 1756-OF8I module channel that uses a Current (mA) output
type with Clamping enabled has the following configuration parameters:
• Scaling values:
– High Engineering = 100.0000%
– Low Engineering = 0.0000%
• Clamp Limits:
– High Clamp = 100.0000%
– Low Clamp = 0.0000%
If you change the Scaling High Engineering value to 90.0000%, the High
Clamp value remains at 100.0000.
You must change the High Clamp value to 90.0000 to make sure the
application continues to operate as expected.