Allen-Bradley 1756-L6 Series, Reference Manual, Page: 55 / 134

Rockwell Automation Publication 1756-RM001I-EN-P - May 2012 55
ControlLogix I/O Modules Chapter 5
Use the Floating Point Data Format
ControlLogix analog output modules perform on-board alarm processing to
validate that the input signal is within the proper range. These features are only
available in Floating Point mode. To use the Floating Point Data format, select
the Floating Point Data format in the Module Properties dialog box.
Program to Respond to Faults Appropriately
When programming the SIL 2 system, verify that your program examines the
appropriate module fault, channel fault, and channel status bits and responds by
initiating the appropriate fault routine.
Each module communicates the operating status of each channel to the controller
during normal operation. Application logic must examine the appropriate bits to
initiate a fault routine for a given application. For more information on faults, see
Chapter 8 , Faults in the ControlLogix System on page 87 .
Configure Outputs to De-energize in ESD Applications
For typical emergency shutdown (ESD) applications, outputs must be configured
to de-energize. When configuring any ControlLogix output module, each output
must be configured to de-energize in the event of a fault and in the event of the
controller going into Program mode. For exceptions to the typical ESD
applications, see
Chapter 1 , SIL Policy on page 11 .
Monitor Channel Status
You must wire each analog output to an actuator and then back to an analog
input to monitor the output’s performance, as shown in
Figure 28 . The
application logic must examine the analog input (feedback value) associated with
each analog output to make sure that the output from the controller was received
correctly at the actuator. The analog output value must be compared to the
analog input that is monitoring the output to make sure the value is within an
acceptable range for the application.
In the ladder diagram in Figure 27 , a user-defined percentage of acceptable
deviation (that is, tolerance) is applied to the configured range of the analog
input and output and the result is stored (that is, delta). This delta value is then
added to and subtracted from the monitoring analog input channel; the results
define an acceptable high and low limit of deviation. The analog Output Echo is
then compared to these limits to determine if the output is working properly.
The output’s OK bit preconditions a Timer run that is preset to accommodate an
acceptable fault response time and any communication filtering, or output, lags
in the system. If the monitoring input value and the Output Echo miscompare
for longer than the preset value, a fault is registered with a corresponding alarm.
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