MVI56-DEM
♦
ControlLogix Platform
Reference
Honeywell DE Communication Module
Page 50 of 80
ProSoft Technology, Inc.
March 6, 2008
5.2.5 PV Value Integrity
A great deal of effort is put into the task of assuring the integrity of the PV value
presented to the PLC. Two forms of the PV and SV variables are made available
to the PLC:
PV: Last Good Value
This value is the PV value unconditioned by the Data Base Mismatch. Care
should be exercised when using this PV value as the scaling, which is based on
the URV/URL/LRV parameters, may not be accurate.
During the normal operation, the DEM module performs integrity checks of the
health of the PV value. Should a condition be detected which could affect the
integrity, the Bad PV Flag is set (Device Status Word 1/13). The four conditions
which will cause the Bad PV Flag to be set are:
PV Update Timeout
If the PV value has not been updated within 6 seconds (24 PV update cycles),
the flag will be set
Data Base Mismatch
If any data base mismatch condition is detected, the flag will be set
SFC Write Detected
If an SFC write to the instrument data base is detected, the flag will be set. Note
that in a redundant application, the SFC Write Detected condition will be detected
when a Download command is executed from the other module or from the SFC
unit.
FTA Not Present
If the FTA connector or the 24 VDC power supply is disconnected, the flag will be
set.
In order to clear the Bad PV Flag, the offending condition must be cleared, and
under most circumstances will have to wait until a new data base has been
received from the instrument.
5.2.6 Data
Base
Mismatch
Data base mismatch testing is performed by the DEM module on the write
parameters received from the PLC. The mismatch status is returned to the PLC
for use by the ladder logic or for status available to the PLC in via the Data
Mismatch Active Flag in Device Status Word #1.
Bit Description
0
Mismatch - LRV
1
Mismatch - URV
2
Mismatch - URL
3
Mismatch - Damping
