Re: [foxboro] Forcing an ABSCAN to write data

On Mon, Dec 09, 2002 at 02:19:57PM -0600, David Johnson wrote:
> 
> Hi list,
> 
> Does anyone know of a way to force a write when using the ABSCAN block in 
> the "event driven" mode?  I have an Integrator 30 set up with an ABSCAN 
> block in a compound that has the SCANTYP = 1 (event driven mode)  I want 
> the data to be written whenever there is a change.  This works fine.  I 
> also want to periodically write all the data, even if there wasn't a 
> change.  This way I can reset values if the PLC writes over them, or there 
> is a communications fault, or whatever.  Turning the compound off and back 
> on DOESN'T work.  Apparently, the ABSCAN block first reads, then writes the 
> data. The documentation tends to support the fact that this attempted cheat 
> won't work.
> 

This has lead to an interesting discussion here. I missed originally that
the ABSCAN block was in event driven mode. I guess I never thought of using
that for data written from the DCS to the PLC (as opposed to the other
direction, where it made sense to me). So, is the advantage of doing this
on data written from the DCS to the PLC, to eliminate the periodic read
traffic to determine the actual PLC values?

What I'm wondering is if there is a big advantage in doing this, as opposed
to being in pooled mode, and just letting the reads occur, since write will
only occur when the data read back from the PLC does not agree with the
current value of the tag in the DCS anyway. 

One of my co-workers also suggested an application level workaround for
this. The assumption here is that if anything is written everything is
written with a given ABSCAN block. If this is true, then you could using a
bit that the PLC never referenced in the ABSCAN block, and periodically toggle
that bit in the DCS.

Can anyone confirm whether the whole ABSCAN block is written for any change,
or not?

-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                                                -- Benjamin Franklin
 
 
_______________________________________________________________________
This mailing list is neither sponsored nor endorsed by Invensys Process
Systems (formerly The Foxboro Company). Use the info you obtain here at
your own risks. Read http://www.thecassandraproject.org/disclaimer.html
 
foxboro mailing list:             http://www.freelists.org/list/foxboro
to subscribe:         mailto:foxboro-request@xxxxxxxxxxxxx?subject=join
to unsubscribe:      mailto:foxboro-request@xxxxxxxxxxxxx?subject=leave

Other related posts: