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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: problem with usbtmc-communication
Date: Thu, 06 Dec 2012 15:28:20 -0500
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On 12/6/2012 10:44 AM, Jean Dubois wrote:

> I followed your suggestion an now the code looks like this:
> #!/usr/bin/python
> import time
> import os
> import sys
> measurementcurr=''
> measurementvolt=''
> timesleepdefault=2
> filename ='mydata.txt'
> usbkeith = open('/dev/usbtmc1','r+')
> usbkeith.flush()
> usbkeith.write("*IDN?\n")
> #strip blank line:
> identification=usbkeith.readline().strip()
> print 'Found device: ',identification
> usbkeith.write("SYST:REM" + "\n")
> usbkeith.write(":SENS:VOLT:PROT 1.5\n")
> keithdata = open(filename,'w')
> usbkeith.write(":OUTP:STAT ON\n")
> for number, current_in in enumerate(('0.025', '0.050', '0.075',
> '0.100'), 1):
>     usbkeith.write(":SOUR:CURR %s\n" % current_in)
>     time.sleep(timesleepdefault)
>     usbkeith.write(":MEAS:CURR?\n")
>     measurementcurr=usbkeith.readline()
>     print 'Measured current %d: ' % number, measurementcurr
>     usbkeith.write(":MEAS:VOLT?\n")
>     measurementvolt=usbkeith.readline()
>     print 'Measured voltage %d: ' % number, measurementvolt
>     keithdata.write(measurementcurr.strip()+' '+measurementvolt)
> usbkeith.write(":OUTP:STAT OFF\n")
> print "Goodbye, data logged in file:"
> print filename
> usbkeith.close()
> keithdata.close()
>
> Still there is a "buffer-problem" as you can see in the output below:
> 0.00639725 0.0104065; these values are completely wrong
> 0.0248976 0.262959; these should have been be the first values
> 0.0500431 0.516602: these should been the second values
> 0.0749168 0.772616; these are the 3rd values
>                       4th values are missing
>
> any idea why this does what it does in Python?

I am not familiar with the protocol at all, but my guess (without 
looking at the octave code) is that two of these three commands

 > usbkeith.write("SYST:REM" + "\n")
 > usbkeith.write(":SENS:VOLT:PROT 1.5\n")
 > usbkeith.write(":OUTP:STAT ON\n")

before the loop have responses that you need to read (and toss?)

usbkeith.readline(); usbkeith.readline()

so that the first values you read in the loop are the one that should be 
first. In other words, make sure the read buffer is clear before the loop.

-- 
Terry Jan Reedy