[newbie] problem with usbtmc-communication

[Jean Dubois]

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)
#start second measurement
usbkeith.write("SOUR:CURR 0.050\n")
time.sleep(timesleepdefault)
usbkeith.write("MEAS:CURR?\n")
time.sleep(timesleepdefault)
measurementcurr=usbkeith.readline()
print 'Measured current 2: ',measurementcurr
usbkeith.write("MEAS:VOLT?\n")
time.sleep(timesleepdefault)
measurementvolt=usbkeith.readline()
print 'Measured voltage 2: ',measurementvolt
keithdata.write(measurementcurr.strip()+' '+measurementvolt)
#start 3rd measurement
time.sleep(timesleepdefault)
usbkeith.write("SOUR:CURR 0.075\n")
time.sleep(timesleepdefault)
usbkeith.write("MEAS:CURR?\n")
time.sleep(timesleepdefault)
measurementcurr=usbkeith.readline()
print 'Measured current 3: ',measurementcurr
usbkeith.write("MEAS:VOLT?\n")
time.sleep(timesleepdefault)
measurementvolt=usbkeith.readline()
print 'Measured voltage 3: ',measurementvolt
keithdata.write(measurementcurr.strip()+' '+measurementvolt)
#start 4th measurement
time.sleep(timesleepdefault)
usbkeith.write("SOUR:CURR 0.1\n")
time.sleep(timesleepdefault)
usbkeith.write("MEAS:CURR?\n")
time.sleep(timesleepdefault)
measurementcurr=usbkeith.readline()
print 'Measured current 4: ',measurementcurr
usbkeith.write("MEAS:VOLT?\n")
time.sleep(timesleepdefault)
measurementvolt=usbkeith.readline()
print 'Measured voltage 4: ',measurementvolt
keithdata.write(measurementcurr.strip()+' '+measurementvolt)
usbkeith.write(":OUTP:STAT OFF\n")
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?

thanks
jean