problem with usbtmc-communication

[Jean Dubois]

On 11 dec, 15:34, w... at mac.com wrote:
> On Dec 11, 2012, at 1:58 AM, Jean Dubois <jeandubois... at gmail.com> wrote:
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> > 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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> >> [byte]
> >>> 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 theusbtmckernel driver documentation (the
> > examples there are given in C):
> >http://www.home.agilent.com/upload/cmc_upload/All/usbtmc.htm?&cc=BE&l...
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> OK - I see where the examples came from, and I notice -
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>         int my_inst;
>         my_inst=open(“/dev/usbtmc1”,O_RDWR);
>         write(my_inst,”*RST\n”,5);
>         close(my_inst);
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> and similarly in another place -
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>         retval=write(myfile,"*IDN?\n",6);
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> Note that both write commands contain a byte count of the number of characters to be written (\n counts as one character).
> Again, the read commands contain byte counts.  I'm very suspicious that a write command with no byte count writes nothing, but does move a buffer pointer.
>
> -Bill

Does Python support/implement simular commands? Can I use
usbkeith.write("*IDN?\n",6) and  something simular for the reading
commands?

thanks,
jean