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configured, set by an operator, higher level device, or calculated. For example, the
GRANT_DENY parameter can be used to check access rights, the MODE_BLK
parameter can be used to request mode changes, and the RCAS_IN and RCAS_OUT
parameters can be used to control set points in the Remote Cascade mode.
If the contained parameter has a DS-65 or DS-66 data structure, you must use multiple
SCM Step outputs to write to the different fields in the parameter's data structure. For
example, if you use a Step output to write to the SP parameter of a fieldbus PID block,
when it is in Auto mode, you will need Step outputs for the related parameter attributes
of value, status, substatus and limits, as defined by its structure. A typical SCM write SP
scenario would include the following actions.
Transition checks if Target MODE of the fieldbus block is AUTO.
Step output sets Target MODE to AUTO, if required.
Step output sets SP value and GOOD (Non Cascade) status.
Optional Step output to set SP Limits.
Optional Transition to verify that the value got stored in the fieldbus device.
The SCM will issue a fail alarm, if the store fails for any reason.
ATTENTION
Avoid SCM configurations that would push a NaN (Not a Number) value as
an output to a fieldbus device. If this is not possible, be sure to test the field
device to observe how it reacts to the possible NaN value.
Fieldbus status data details
According to Fieldbus Foundation specifications, every fieldbus function block input and
output connection must support a status byte that provides the following status
indications.
Data Quality (usability)
Bad Data Cause
Degraded Data Cause
Limit Conditions
Cascade Control Initialization, Rejection
R400
Experion PKS Series A Fieldbus Interface Module User's Guide
July 2010
3. Fieldbus Integration With Experion System
Honeywell
3.2. Control Integration
55
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