Re: [foxboro] Operator interface question

We use a similar technique here.  For our Allen-Bradley PLC's, we use the
Allen-Bradley PanelView local operator interface.  It has a built-in
capability to associate handshaking bits with each analog write.  We do a
similar thing with the Foxboro gateway, each analog output has a digital
output (handshaking bit) associated with it.  The PLC maintains three copies
of the setpoints, etc.  The one actually used by the PLC, the one from the
PanelView, and the one from the IA.  When either the IA or the PanelView
want to change a setpoint, etc., they write the analog value (actually the
values are always being written) and set the handshaking bit for short time.
There has to be logic in the PLC to scan the handshaking bits and move the
analog value into its final destination when the bit is set.  The downside
is that if a handshaking bit gets left on, the associated analog value
overrides the others, and it takes a little work on the IA side to make the
handshaking bit happen.  However, so far it has worked quite well, and I
might also add, that the IA is actually interfaced to a PLC over a leased
data line, which is relaying everything on to another PLC via radio.

Roger B. Smith
Process Control Engineering
Waste Water Services
City of Atlanta
404 350-4952

 -----Original Message-----
From:   foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
On Behalf Of Jones, Charles R. (Chuck)
Sent:   Monday, May 13, 2002 6:00 PM
To:     'foxboro@xxxxxxxxxxxxx'
Subject:        Re: [foxboro] Operator interface question


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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