Experimental Physics and
Industrial Control System

Christian Pauly via Tech-talk <tech-talk at aps.anl.gov> · Thu, 16 Jul 2020 10:02:35 +0200

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