[foxboro] Operator interface question

We are in the process of migrating one process units worth of controls
operator interface from a "application enabler" to a Foxboro system.

The existing controls are AB PLC5 based, and will stay that way for this
migration. There are some fairly simple PID loops in this unit which
are controlled by PID instructions in the PLC's.

The existing operator interface for this has an "input field", and a 
"read back field" for each of the analog values for these loops that can 
be set by the operator (SP and CV). In addition it has some 
"increment/decrement" buttons, but they are implemented by sending a bit 
down to the PLC's and doing the actual change of value there. This part of 
the existing interface design does not work particularly well, and we plan 
on changing it to use the more tradition Foxboro "look and feel". 
That is having the buttons increment and decrement a variable in the
Foxboro itself.

In addition we plan on implementing a field located CRT operator station, 
which will communicate directly with he PLC's. This station should also be 
able to modify the analog values, and of course there is always the possibility 
of changing them from the PLC programming system.

The problem we are having with this design is that we see problems with timing 
races here. The Foxboro needs to track, say the CV, as it is changed 
within the PLC, but yet the Foxboro needs to be able to read the current 
value, increment, or decrement it, and write it back to the PLC.  As you 
can see this is fraught with timing issues.

I thought I would seek the collective wisdom of this group, and see if 
anyone else had implemented something similar to this? If so how did you 
do it, and how well has it worked out?  Any thoughts on this would be 
greatly appreciated.


-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                                                -- Benjamin Franklin
 
 
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