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<== Date ==> <== Thread ==>

Subject: Re: streamDevice: how to read a value as fast as possible?
From: Andrew Johnson <[email protected]>
To: [email protected]
Date: Fri, 21 Dec 2012 15:49:55 -0600
Hi Martin,

On 2012-12-21 Martin Konrad wrote:
> 2. A free-running mode (device automatically starts the next acquisition
> as soon as the first one is finished). In this mode the interface is
> never blocked and data can always be read out with the "FETCH?" command
> which immediately returns the last reading. Using this command with
> SCAN=".1 second" leads to a lot of unneeded record processing (as well
> as CA traffic to clients that monitor this PV and also archiving load).
> Is it possible to avoid processing of my ai record if the VAL field did
> not change? It seems to me that this is a limitation caused by the
> asynchronous behavior of Asyn/StreamDevice...
> 
> Any ideas?

You can avoid passing the changing data up through CA by setting the MDEL 
field of your ai record, but you can't stop the record processing.  The 
default value of MDEL=0 makes it only send values that are not bit-for-bit 
identical, but if you have random noise on your input you may need to increase 
it to whatever value is sufficient to suppress that, as long as that doesn't 
affect the signal you're reading out.

- Andrew
-- 
Computer science is as much about computers as astronomy is about
telescopes. -- Edsger Dijkstra

References:
streamDevice: how to read a value as fast as possible? Martin Konrad

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