I've got a small program that I'm using to try and connect up to a Rigol Spectrum Analyzer.

import usbtmc
listOfDevices = usbtmc.list_devices()
for device in listOfDevices:
print hex(device.idVendor), hex(device.idProduct)

instrument = usbtmc.Instrument(0x0400,0x09c4)
print instrument
print instrument.ask("*IDN?")

When running it I get this output. Even after chmodding 777 /dev/usbtmc2 I still need to sudo which is interesting.

sandford@MikeLinux ~/projects $ sudo python usbtmc_test.py
0x400 0x9c4
<usbtmc.usbtmc.Instrument object at 0x7f3a78e1d750>
Traceback (most recent call last):
File "usbtmc_test.py", line 11, in
print instrument.ask("*IDN?")
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 346, in ask
return self.read(num, encoding)
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 341, in read
return self.read_raw(num).decode(encoding).rstrip('\r\n')
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 309, in read_raw
msgid, btag, btaginverse, transfer_size, transfer_attributes, data = self.unpack_dev_dep_resp_header(bytearray(resp))
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 259, in unpack_dev_dep_resp_header
msgid, btag, btaginverse = self.unpack_bulk_in_header(data[0:4])
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 255, in unpack_bulk_in_header
msgid, btag, btaginverse = struct.unpack('BBBx', data[0:4])
struct.error: unpack requires a string argument of length 4
sandford@MikeLinux ~/projects $

So then I modified the source to print out the data going in to unpack_bulk_in_header, did "sudo python setup.py install" and re-ran and saw this:

sandford@MikeLinux ~/projects $ sudo python usbtmc_test.py
0x400 0x9c4
<usbtmc.usbtmc.Instrument object at 0x7fd0cc66c750>
[bytearray(b'\x02\x02\xfd\x00')]
Traceback (most recent call last):
File "usbtmc_test.py", line 11, in
print instrument.ask("*IDN?")
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 347, in ask
return self.read(num, encoding)
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 342, in read
return self.read_raw(num).decode(encoding).rstrip('\r\n')
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 310, in read_raw
msgid, btag, btaginverse, transfer_size, transfer_attributes, data = self.unpack_dev_dep_resp_header(bytearray(resp))
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 260, in unpack_dev_dep_resp_header
msgid, btag, btaginverse = self.unpack_bulk_in_header(data[0:4])
File "/usr/local/lib/python2.7/dist-packages/python_usbtmc-0.1-py2.7.egg/usbtmc/usbtmc.py", line 256, in unpack_bulk_in_header
msgid, btag, btaginverse = struct.unpack('BBBx', data[0:4])
struct.error: unpack requires a string argument of length 4
sandford@MikeLinux ~/projects $

There are four bytes in that array but the last one is a null. I'm not terribly familiar with the code or usbtmc as a standard so I'm afraid I don't really know what to do here. Any help is greatly appreciated.