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Subject: Re: CA problem from Vax
From: hill@luke.atdiv.lanl.gov (Jeff Hill)
Date: Mon, 20 Jun 94 10:19:07 MDT
Chris,

>   sorry, another problem for which we would like some advice. In running
> channel access from the Vax and setting a callback on a PV we have noticed
> that if the PV gets changed very quickly in the state code we get the wrong
> value back to the Vax (apparently at least). 

When a field in the database is modified and a CA client has registered
an event call back for that field then the value, time stamp, and alarm 
information are all pushed onto a queue. Next, the server is
notified by setting a semaphore. The, typically lower priority,
CA server runs when higher priority labor has completed. The
server sequentially converts each item in the queue and sends a
corresponding event message to the client. This procedure ensures
that the clients will see transient changes to a process variable
even if there are short bursts of CPU and network saturation.
It is important to offload the labor required to interface with external
CA clients from the high priority database scan tasks.

Is the native type of the field you are monitoring a string? This will
be the case when you are using the VAL field of a state record.
If so you should know that the CA server event system does not queue up 
changes to string fields. The event system also does not queue up changes 
to array fields. This is due to memory consumption constraints in the
IOCs. In this case the server will send the value present in the
database when the server dispatches the event message (not necessarily
the value present in the database when the event was triggered).


Regards


Jeff


______________________________________________________________________
Jeffrey O. Hill			Internet	johill@lanl.gov
LANL MS H820			Voice		505 665 1831
Los Alamos, NM USA 87545	FAX		505 665 5107


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