problem with usbtmc-communication

Jean Dubois jeandubois314 at gmail.com
Thu Dec 6 15:05:09 CET 2012


On 4 dec, 20:55, Terry Reedy <tjre... at udel.edu> wrote:
> On 12/4/2012 7:14 AM, Jean Dubois wrote:
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> > The following test program which tries to communicate with a Keithley
> > 2200 programmable power supply usingusbtmcin Python does not work as
> > expected. I have connected a 10 ohm resistor to its terminals and I
> > apply 0.025A, 0.050A, 0.075A en 0.1A,
> > I then measure the current and the voltage en write them in a file
> > De data produced looks like this:
> > 0.00544643 0.254061; first current value is wrong, voltage value is
> > correct
> > 0.0250807 0.509289; second current value is wrong, but it corresponds
> > to the first, second voltage is correct
> > 0.0501099 0.763945; 3rd current value is wrong, but it corresponds to
> > the second, 3rd voltage is right
> > 0.075099 1.01792; 4th current value is wrong,  it corresponds to the
> > 3rd, 4th voltage is right
> >                              4th correct current value is missing
>
> > But is should be (numerical inaccuracy taking into account)  (these data
> > were produced by a similar octave-program):
> > 0.0248947 0.254047
> > 0.0499105 0.509258
> > 0.0749044 0.764001
> > 0.0998926 1.01828
>
> > Here is the python-program:
> > #!/usr/bin/python
> > import time
> > import os
> > import sys
> > measurementcurr=''
> > measurementvolt=''
> > timesleepdefault=1
> > 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')
> > #start first measurement
> > usbkeith.write(":SOUR:CURR 0.025\n")
> > usbkeith.write(":OUTP:STAT ON\n")
> > time.sleep(timesleepdefault)
> > usbkeith.write(":MEAS:CURR?\n")
> > time.sleep(timesleepdefault)
> > measurementcurr=usbkeith.readline()
> > print 'Measured current 1: ',measurementcurr
> > usbkeith.write("MEAS:VOLT?\n")
> > time.sleep(timesleepdefault)
> > measurementvolt=usbkeith.readline()
> > print 'Measured voltage 1: ',measurementvolt
> > keithdata.write(measurementcurr.strip()+' '+measurementvolt)
>
> [3 near repetitions snipped]
>
> This sort of repetitious code without even line breaks is painful for me
> to read. Python has looping statements for a reason. Replace all four
> nearly identical blocks with the following. (If you are not familiar
> with built-in enumerate, you should be. Read its entry in the library
> manual.)
>
> for number, current_in in enumerate(
>      ('0.025', '0.050'. '0.075', '0.100'), 1)
>    usbkeith.write(":SOUR:CURR %s\n" % current_in)
>    ...
>    print 'Measured current %d: ' % number, measurementcurr
>    ...
>    print 'Measured voltage %d: ' % number, measurementvolt
>
> Now you can make changes in only one place and easily add more test values.
>
> > print "Goodbye, data logged in file:"
> > print filename
> > usbkeith.close()
> > keithdata.close()
>
> > can anyone here what is going wrong and how to get it right?
>
> No, but if both the python and octave programs used loops, it would be
> easier to see if both are doing the same things before, within, and
> after the loop.
>
> --
> Terry Jan Reedy

Thank you for you reply. Of course you are right this kind of code is
not the normal way to program but as I mentioned in my initial post
this is merely a test program, I did avoid using a loop on purpose
because timing can be critical in this kind of application and I
didn't want to add unnecessary uncertainty in the timing which could
be caused by using a loop.
The final version will be more like you suggested here above.
As Bill stated in another e-mail the real trouble here is some kind of
"buffer-problem",  according to him it could be caused by the use of
the readline()-statements. So if anyone could tell me how to
substitute those by some Python commands which read characters instead
does the Octave-code I'd appreciate that very much.

regards,
Jean



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