Re: [foxboro] Operator interface question
- From: Alan J Schaff <schaffa@xxxxxxxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Tue, 14 May 2002 12:00:57 -0400
We have had the same types of problems before we decided to move all the
control to the I/A and use a PID block for the actual process control. The
Gateway reads the process value into an AIN block that is connected to a
PID block. The output of the PID block goes to the AOUT block in the
Gateway (kind of like a "normal" PID loop). The PLC puts the process data
into the Word to be read by the DCS and monitors the word that is updated
by the DCS. The PLC just processes the input and output information to the
field devices, it does no control. Granted you can only do this on a
noncritical loop that is not hurt by the time delay, but you don't have to
worry about who has control of the data. Also, the operator interface can
be done the same way as all your other PID loops, including the
increment/decrement option.
Alan Schaff
BASF Corp.
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| Subject: Re: [foxboro] Operator
interface question |
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On Tue, May 14, 2002 at 08:43:44AM -0500, Corey R Clingo wrote:
>
>
> One characteristic of most DCS PLC gateways is that the output values are
> silently read back behind the scenes so the operator is able to see
exactly
> what is in that PLC. This causes confusion sometimes (i.e., when
operator
> actions have no effect because the PLC logic obliterated their change),
but
> is a reflection of the real world, and helps in situations like this. I
> believe the Integrator 30 at least behaves in this manner.
>
I think I'm begning to gat a picture of how this should work.
However I still have a rather basic question. How can I read a value from
the PLC, and increment/decrement this value to write it back? If the AIN in
the gateway is in auto, I can't change the value in the DCS, right? If
instead, i make a copy, modify it, and write that back to the PLC, which
then detects the new data, and sends it back to the AIN, that I am
modifying, don't I have a timing race?
--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin
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