Re: [foxboro] Determining Forces in PLB + the perils of ladder logic

Wow...it is amazing how some of our decisions regarding I/A have turned 
out to be so prescient.


We never used PLB much here (none in my plants) for various reasons, one 
of which was because it always seemed to be a "grafted-on" solution.  For 
high-speed stuff, lots of sequencing, skid units, etc. we used A-B PLCs 
and tied them in with Integrator 30s (unfortunately, the Integrators have 
had problems themselves, and seem to be another Foxboro orphan, especially 
now that they have FDSIs).


We also never used HLBL much, primarily because of all the IND block 
horror stories (I did try a bit of it once, and it will bury a CP if you 
leave things at their defaults).  This was somewhat of a change for me, as 
I was quite comfortable with the HLBL equivalent on the Honeywell systems 
I used to work on.  Thanks goodness I/A has CALC blocks :)


Corey Clingo
BASF Corporation






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Re: [foxboro] Determining Forces in PLB + the perils of ladder logic






Thanks Kevin,
Yes we have installed that, and many other QF's and have now
minimized, but not eliminated that problem.  Foxboro has been
continually tweaking the iom82 file that is the EEPROM image for the
FCM10E, DCM10E, FBI10E, and WFCM10E.  We have changed the iom82 from
1.02, 1.03, 1.04..... all the way to the current 1.16.  Sometimes we
were told to go backward, (from 1.13 to 1.11).  We have tried it all as
Foxboro continues their "Search for the Holy Grail.
At this point we are moving everything to the MESH using 200
Series I/O with very minimal use of PLB Ladder Logic.  When we started
configuring them in 1988 my Modicon PLC background made ladder logic
understandable and very useable to me.  In addition, the extended
discrete FBM's were cheaper to buy.  I didn't realize we would
continually be plagued by Foxboro's inability to support/manage the
software issues associated with it.  During that time, we have twice
received new software that failed to correctly map the CIN/CO's within
PLB's running in discrete FBM's.  This really screwed us up one time as
we were unable to restart our process after a software upgrade.  The
latest incident that I recall was when we received a new FBM242 that was
EEPromed at 1.05 instead of 1.04 like the one we were replacing and the
I/O mapping to CIN/CO's was messed up in the ladder.  Luckily we were
able to burn it back to 1.04 and all was well.
The PLB/Ladder Logic still provides a solution for applications
that require single digit ms control response but all associated I/O
must be able to fit in a single FBM in order to accomplish the task.
For ms control of equipment requiring larger discrete I/O counts David
Johnson's suggestion of using PLC's and interfacing them to Fox IA with
FDSI's running a controllogix interface is a possible solution.
In the end, it has been our experience that PLB ladder logic is
treated like an orphan-step-child from within the Foxboro organization.
Rather than fitting it with a new set of clothes they keep deciding to
put additional patches over the holes.  The phrase "You can't fight City
Hall" comes to mind here and so we are attempting to eliminate the use
of PLB/Ladder Logic.  My feeling is that if Foxboro could wave a magic
wand and make all PLB/Ladder Logic disappear from all of their installed
systems they would do it in a heart beat.  That would mean they wouldn't
even have to put a patch on their orphan-step-child's clothes any more!
Unfortunately, I don't think they have a reasonable and cost effective
way to replace the functionality the PLB Block provides.
Just one mans opinion, and you know what they say about opinions...

Cheers,
Tom VandeWater

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