Experimental Physics and
Industrial Control System

Mark Rivers via Tech-talk <tech-talk at aps.anl.gov> · Thu, 16 Jul 2020 12:27:26 +0000

Hi Christian,

> So: What is the general concept how to control such a device ? (This  must be a "standard case").
> Use SCAN for each status variable ?

I don't think there is a standard case.  But here are a couple of popular options.

1) Have a status record #1 that is periodically scanned, e.g. SCAN="1 second".  That record has its FLNK field set to status record #2, which in turn has a FLNK to status record #3, etc.  You can then expose the SCAN field of status record #1 in the OPI so the use can control the readback rate of all of the status records.

2) For each output record have its FLNK field point to the readback record for that parameter.  Then every time it writes a new value it reads it back into a different PV.  There are no periodically scanned records.

Mark

________________________________
From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Christian Pauly via Tech-talk <tech-talk at aps.anl.gov>
Sent: Thursday, July 16, 2020 3:02 AM
To: tech-talk at aps.anl.gov
Subject: Stream device - general concept how to use

Dear experts

I am struggling right now with programming a control interface for a
device using the streamdevice module.
The device has several status variables (like eg "temperature"), and
several control variables (like eg "pump speed").

My approach is, to have for each control variable two records:

record getPumpSpeed  (type analog input, ai)
record setPumpSpeed  (type analog output, ao)

These are connected to two different protocol functions to set - or to
read back the value.

"caput setPumpSpeed xxx" works fine : The protocol gets executed, the
pump speed is set.

but "caget getPumpSpeed" does not execute the protocol, instead it just
returns the record value.
I could do a "caput getPumpSpeed xxx" first (with some arbitrary number
xxx, to execute the record), then the protocol is executed, and a
subsequent "caget getPumpSpeed" gives me the latest value... But that is
not really what i want. (Naively) i am looking for a "realtime" readback
everytime i do a caget.

But from the documentation i see, that this is the intended behaviour.

I could set the SCAN field of the caget record to have regular polls of
the pump speed.
But if i now imagine to have hundred such variables, i would get quite
some traffic, and the return values are still not really "real time".

So: What is the general concept how to control such a device ? (This
must be a "standard case").
Use SCAN for each status variable ?
Or combine the set- and get-record to a single record "PumpSpeed" ?
And just assume, that the latest set value (which is then returned upon
caget) is still the actual real value ?

Or am i lacking some understanding of streamdevice ?

Any help and advice greatly appreciated !

Thanks,
Christian

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System