Re: Commands with synchronous answers from python

["Kasemir, Kay via Tech-talk" <tech-talk at aps.anl.gov>]

Hi: I may misunderstand, but when you say … I need a method to trigger the processing of a StreamDevice device in a python code, wait until the processing is done …, I see two parts to that. From the channel access client side, you can perform

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Hi:

 

I may misunderstand, but when you say … I need a method to trigger the processing of a StreamDevice device in a python code, wait until the processing is done …, I see two parts to that.

 

From the channel access client side, you can perform a “put callback” that writes to a record and waits until that record finishes processing.

From the command line, that would be something like

 

  caput -c -w 10 Variable 1

 

As Matt has shown, with pyepics it would be

 

    from epics import caput

    caput('Variable', 1, wait=True, timeout=10)

 

The thing is, however, that the server side needs to support that.

If you write to a motor record’s VAL field, the caput will indeed wait until the motor moved to the commanded position, maybe tried again if the error was too large etc., and completes once it did its best to reach the commanded position.

If you use stream device to simply write some setpoint to a device, the caput might wait until stream device sent that message out to the device, but not necessarily until the device did anything about that. If the device is a lakeshore temperature controller that’s asked to cool down from 300 to 30 Kelvin, that might take hours. You need database constructs which typically include the “busy” record to create logic that blocks until the device is indeed “done”. For an example see https://github.com/ornl-epics/lakeshore

That way you have a record that truly delays completion until the device reached the commanded state. You can then simply use the put-callback and you’ll know that the device is in the desired state once that put-callback completes.

 

-Kay