On 5 dec, 16:26, w...@mac.com wrote:
> On Dec 4, 2012, at 11:12 AM, Jean Dubois <jeandubois...@gmail.com> wrote:
>
> > On 4 dec, 15:33, w...@mac.com wrote:
> >> On Dec 4, 2012, at 7:14 AM, Jean Dubois <jeandubois...@gmail.com> wrote:
>
> >>> The following test program which tries to communicate with a Keithley
> >>> 2200 programmable power supply using usbtmc in Python does not work as
>
> Is usbtmc a software layer (library of some sort) or some sort of hardware
> adapter?
This is the information concerning usbtmc from the National
Instruments site:
USBTMC stands for USB Test & Measurement Class. USBTMC is a protocol
built on top of USB that allows GPIB-like communication with USB
devices. From the user's point of view, the USB device behaves just
like a GPIB device. For example, you can use VISA Write to send the
*IDN? query and use VISA Read to get the response. The USBTMC protocol
supports service request, triggers and other GPIB specific operations.
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> >>> 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()
>
> Without knowing anything about the usbtmc hardware/software it is hard to
> make real recommendations, but it seems pretty clear that you are being
> stepped on by a buffer problem of some sort. I'm VERY suspicious of using of
> readline() as a way of getting the data out of the usbtmc thingy. That makes
> python treat the Keithley as a file-like object and there are way too many
> ways the file pointer may not be where you think it is.
>
> I note that in your Octave example you are reading characters rather than
> lines. It seems to me that you have two choices here, either do the
> equivalent in python or dig through the Keithley documentation to find the
> hex codes that usbtmc is presumably translating your commands into. If you
> can find those, and if you are interested, I can send you off-line the
> handler I wrote a couple of years ago that used a dictionary to translate
> English commands into hex, then assembled those with a checksum and sent the
> string out to a Keyspan usb to serial converter.
If you could show me how to "do the equivalent in Python" I'd
appreciate that very much
best regards,
jean
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