Re: [foxboro] Sequence code (MON, DEP, IND) opinions

  • From: <tom.vandewater@xxxxxxxxxxxxxx>
  • To: <foxboro@xxxxxxxxxxxxx>
  • Date: Tue, 15 May 2007 16:48:02 -0400

Chuck Jones said:
" When I was first learning this job, I went to an area Users' Group
meeting at the plant where Duc and Tom work.  (At the time, I didn't
know they were world famous...)  What they were doing with sequence
blocks 10 years ago still humbles me when I look at my notes from that
meeting.  (I know, 'cause I just dug them out.)"

Wow Chuck!  Duc and I got a good ribbing from the rest of the guys here
at work because they didn't know that we were "WORLD FAMOUS" either.
I'm impressed that you could still lay hands on that propaganda we put
out at the users group meeting 10 years ago.

We do use a lot of sequence blocks and have developed a modular sequence
strategy that loosely adheres to the S88 standards of UNITS, EQUIPMENT,
OPERATIONS, and PHASES.  It provides a standard HMI interface to allow
operators to transition equipment from one operation to another by
selecting operational targets via the DM based HMI.  We run a
combination of continuous and batch in our processes and I agree that
Foxboro IA provided a lot of ways to accomplish our control goals. =20
        Early on we used a lot of ladder logic for discrete device
manipulations and sequence blocks to coordinate the analog and discrete
devices to run our batch and continuous processes.  We have been
replacing a lot of the discrete valve and motor control that used to be
done in ladder logic with CIN/GDEV/COUT blocks that are more easily
documentable/traceable.  We only seldom need the millisecond speed that
the ladder logic provides as Gabriel O'Dwyer mentioned.  All of the
options have value and can be employed if needed.  The sequence blocks
are the only block that allows "connectionless" reads and writes.  That
ability has served us well but not without considerable documentation
pain.  When getting or setting a value from a sequence block, Foxboro
has provided no way to document or search for them. =20
        Today, thanks to Duc and Mike Stewart, we have a browser based
queriable database that not only shows the CMPD:BLK.PARAM references
made via direct connections between blocks but also all of the SOFT
"connectionless" gets and sets we make in our sequence code. We always
hoped that FOXCAE, IACC, or IEE would provide that capability because it
is essential for us to know where every control reference originates.
If I want to delete a block from our system I first query the database
to see ALL of the references to it.  If I deleted the block and a
sequence somewhere on our system was trying to write one of its
parameters the sequence could fail/hang-up/or delay based on how much
smarts I included in the code.  I know I will get ripped by many CALC
block / PLB block aficionados for saying it, but I believe the sequence
blocks are the most powerful, understandable, documentable blocks that
Foxboro offers.  There is, however, a lot to know about them.  They
really needed to be optimized on CP-10's and 30's to avoid over running
the CP's.  The optimization on CP40's, 60's, and 270's is still a good
practice but not as critical to the CP if you have two or three poorly
optimized ones.  In situations where high speed batch control is not
required I have used sequence blocks to control an entire process, even
in a CP-10!

Cheers,
Tom VandeWater
Control Systems Developer/Analyst
Dow Corning Corporation
Carrollton, KY   USA



 
 
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