Re: [foxboro] Operator interface question

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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