On 12/4/2012 7:14 AM, Jean Dubois wrote:
The following test program which tries to communicate with a Keithley
2200 programmable power supply using usbtmc in 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
--
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