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

Subject: RE: Fetching return value from external program call in StreamDevice
From: "Mooney, Tim M." <[email protected]>
To: Christian Pauly <[email protected]>, "[email protected]" <[email protected]>
Date: Thu, 5 Feb 2015 17:56:59 +0000
I just wrote some python code (caputRecorder) that accidentally does the kinds things you would have to do to implement a python daemon that does the following:

1) monitor an EPICS PV, which I'll call READ
2) read the sensor whenever READ==1
3) write the sensor value to an EPICS PV, which I'll call VALUE
4) write 0 to READ.

I would implement the READ PV with a "busy" record, so that whomever writes 1 to it can find out when the read operation is done by waiting for a completion callback.

I would use PyEpics' camonitor() function to monitor READ, specifying a callback function I'll call monRead()
I would have monRead() put the READ value on a message quere if it's 1
I would have a separate thread read from that message queue, do the actual sensor read, write to VALUE, and  write 0 to READ

The database might look like this:

record(busy, "READ") {
    field(ZNAM, "Done")
    field(ONAM, "Read")
}
record(ao, "VALUE") {
}

The python code might look like this:

import threading
import epics
import Queue

msgQueue = Queue.Queue(maxsize=10)

def reader():
    value = msgQueue.get()
    currValue = yourFunction()
    epics.caput("VALUE", currValue)
    epics.caput("READ", 0)

readThread = threading.Thread(target=reader)
readThread.daemon = True
readThread.start()

def monRead(pvname, value, char_value, **kwd):
    if (value):
        msgQueue.put(value)

epics.camonitor("READ",callback=monRead)

Tim Mooney ([email protected]) (630)252-5417
Software Services Group (www.aps.anl.gov)
Advanced Photon Source, Argonne National Lab


________________________________________
From: [email protected] [[email protected]] on behalf of Christian Pauly [[email protected]]
Sent: Thursday, February 05, 2015 10:58 AM
To: [email protected]
Subject: Fetching return value from external program call in StreamDevice

Hi

I want to read out a little pressure sensor, for which i have already a
small python script which reads the sensor (via i2c bus) and prints the
sensor value to the terminal. Alternatively, i could also give the
sensor value as return value upon program exit.


Now i want to read this sensor inside an EPICS IOC.
Is it possible, to use StreamDevice for this, by calling the external
script / program and evaluating the return value ?
Maybe using the "exec" statement in the protocol file ?

Reading the sensor takes some time (500ms), so it would be good to do it
asynchronously, without blocking the IOC.

Or is there any other easy way, without reimplementing all the i2c
sensor readout eg inside an ASYN driver in C++ code ?


Thanks for any hints or help

Best regards,
Christian



--
Dr. Christian Pauly
Bergische Universität Wuppertal
Fachbereich C - Physik
Gaußstraße 20, D-42119 Wuppertal
Tel: +49 (0)202 439-2632,
Fax: +49 (0)202 439-2662




Replies:
Re: Fetching return value from external program call in StreamDevice Kasemir, Kay
References:
Fetching return value from external program call in StreamDevice Christian Pauly

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