Honeywell Experion PKS Series A, Руководство пользователя, Страница: 49 / 560

3. Fieldbus Integration With Experion System
3.2. Control Integration
R400 Experion PKS Series A Fieldbus Interface Module User's Guide 49
July 2010 Honeywell
Builder automatically creates a CEE output agent to handle the analog output to the
fieldbus block. The block-like output agent maps the Cont OP with status parameter to
the fieldbus data structure (DS-65) of the CAS_IN parameter. It interprets the value
portion in Experion terms and converts it to fieldbus representation. The floating-point
representation is identical, in most cases, but the Ex/-infinity value must be
converted to a fieldbus representation. If the status of OP is "BAD", its value must be
converted from NaN to zero (0.0) for fieldbus representation or it may retain its previous
good value, as long as the fieldbus status byte indicates "BAD".
WARNING
The Fieldbus Foundation specifications do not clearly state the expectations
of fieldbus devices for handling Not-a-Number (NaN) values. This means that
some may behave as the user desires and others may not.
If you compute a data type float + status value that uses data structure (DS-
65), which includes all
F
OUNDATION
Fieldbus inputs, outputs and some
contained parameters, a resulting NaN value passed outbound through the
FIM sets the status element to BAD and changes the value element from
NaN to 0.0 (zero). If you compute a data type float value, which includes
many contained values such as alarm trip points, set points or output limits,
gains, and filter time values, a resulting NaN value that is to be passed
outbound through the FIM will not be written and usually results in retention of
the previous value. If the device would not behave in the application as you
desire, you must add appropriate function blocks or logic to test for and
replace the NaN with an acceptable value.
The CEE output agent also accepts a single BKCAL_OUT parameter with the fieldbus
data structure (DS-65) and maps it to the BACKCALIN parameter of the PID block in
Experion terms.
ATTENTION
The Experion Control Builder application automatically makes the appropriate
back calculation connections during configuration and the connections are
"hidden" in Control Chart views.
Like the FIM, the output agent supports both publish/subscribe and client/server
communication methods. The publish/subscribe method allows the FIM to appear as a
fieldbus device on the H1 link. The FIM publishes the output (OP) for subscribing
fieldbus device resident blocks such as Analog Output and Proportional, Integral,
Derivative (PID) through their CAS_IN parameter input connection. This connection is
generally used when the downstream control block is in the Cas (cascade) mode. This