problem with usbtmc-communication

[Jean Dubois]

On 10 dec, 16:34, w... at mac.com wrote:
> On Dec 10, 2012, at 8:31 AM, Jean Dubois <jeandubois... at gmail.com> wrote:
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> > As you can see this approach suffers from the same "buffer problem" as
> > the approach with readline did. One now good argue as a workaround:
> > get rid of the first data pair and add an extra measure command for
> > the missing data pair, however this still does not explain why this
> > problem is there in Python and not in Octave and I also fear I'll get
> > more trouble when sending combined commands e.g. such as that to
> > create a staircase current
> > So my question is, how to modify the Python-code such that the first
> > data pair is indeed the first data pair
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> > thanks,
> > jean
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> > Here follows the new code:
> > #!/usr/bin/python
> > import time
> > import os
> > import sys
> > measurementcurr=''
> > measurementvolt=''
> > timesleepdefault=5
> > print "Enter a numofchar (11 =<numchar =<4095):",
> > numofchar = int(raw_input())
> > filename ='mydata.txt'
> > usbkeith = open('/dev/usbtmc1','r+')
> > usbkeith.flush()
> > usbkeith.write("*IDN?\n")
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> It seems like a real leap of faith to be opening /dev/usbtmc1 as though it were a file-oriented device.  I've never heard of ANY instrument interface implemented this way.
> Where did you see example code that did that.
I found examples in the usbtmc kernel driver documentation (the
examples there are given in C):
http://www.home.agilent.com/upload/cmc_upload/All/usbtmc.htm?&cc=BE&lc=dut


>  Have you tried to access /dev/usbtmc1 as though it were a serial device?
Yes, I did, as I used to do when communicating with rs232 devices. I
first tried to communicate to with the Keithley using cutecom but I
soon discovered you can't work that way because as soon as you open
the device it closes immediately thereafter. You really have to use
usbtmc (unfortunately) I'm missing the correct "flushing commands" to
do it correctly in Python...Maybe I should try to call the octave code
from within Python?


thanks
jean
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> > #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.read(numofchar)
> >   print 'Measured current %d: ' % number, measurementcurr
> >   usbkeith.write(":MEAS:VOLT?\n")
> >   measurementvolt=usbkeith.read(numofchar)
> >   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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