Re: [foxboro] Operator interface question
- From: "Jones, Charles R. (Chuck)" <CRJones@xxxxxxxx>
- To: "'foxboro@xxxxxxxxxxxxx'" <foxboro@xxxxxxxxxxxxx>
- Date: Mon, 13 May 2002 16:59:35 -0500
We do things like this in our plant. I have been involved in several
discussions on this topic, so I know that many others have done this in
several different ways. We use the older "gateway" method of data transfer.
There are better (and worse) ways to communicate between I/A and PLC.
If the I/A writes directly to the data table used by the PLC controller, the
PLC may overwrite the data coming in from the I/A on the next scan,
depending on where in the control loop you are entering the data. Or, the
I/A may overwrite input from a field station. To get around this problem,
we "buffer" the information from the I/A that we want to change within the
PLC. The information passed to the PLC goes into the buffer. Then we use a
Boolean signal to the PLC to know when to read the buffer. We also use
another method where the PLC scans for changes in this "input buffer".
Which means we create a second buffer to maintain the last known value of
the input buffer. Both of these methods have weaknesses that cannot be
ignored.
For field access, we use touch control panels that are cost effective and,
as you stated, connect directly to the PLC and not into I/A. We have had to
implement a set of Boolean switches into the displays in the field and in
the control room to let the PLC know which station has control.
It ain't pretty. There is more to know, but I'll stop until we know more of
what you have to work with. You may contact me directly, if you wish:
mailto:CRJones@xxxxxxxx
Chuck Jones
Refinery Automation Technologist
A.E. Staley Mfg. Co. -- Lafayette South Plant
765.477.5324 - Office | 877.536.9219 - Pager
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