problem with usbtmc-communication

[Jean Dubois]

On 6 dec, 21:28, Terry Reedy <tjre... at udel.edu> wrote:
> On 12/6/2012 10:44 AM, Jean Dubois wrote:
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> > 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()
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> > 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
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> > any idea why this does what it does in Python?
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> 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
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>  > usbkeith.write("SYST:REM" + "\n")
>  > usbkeith.write(":SENS:VOLT:PROT 1.5\n")
>  > usbkeith.write(":OUTP:STAT ON\n")
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> before the loop have responses that you need to read (and toss?)
No they don't need to have there responses be red,
first command sets remote mode
second command sets limit on voltage across output terminals
3rd command connects terminals with DUT


> usbkeith.readline(); usbkeith.readline()
This doesn't work because nothing is sent back
> 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

I'll look at it further monday

regards,
jean