Re: [foxboro] Operator interface question
- From: stan <stanb@xxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Tue, 14 May 2002 08:18:08 -0400
Thanks for both of your suggestions.
Roger I like your approach, _but_ I don't think it will solve the problem
of letting us use the increment/decrement buttons on the Foxboro. Or if it
will I'm missing something in how to implement it.
To increment/decrement we need to start from a known value. In this case it
should be the actual working value for the PLC (not the last Foxboro written
value). I can't quite get a handle on how to read this value, increment it,
write it back, and set the "accept new Foxboro value" digital bit.
This is probably because I'm just coming to grips with configuring the
Foxboro ate all. I'm comfortable with how to program the PLC side of this,
but I don't see how to configure the Foxboro system to do this
Can anyone provide insight as to how to do this?
On Tue, May 14, 2002 at 07:17:59AM -0400, Roger Smith wrote:
>
> 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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