if you continue having issues you might consider using pytty. i helped write it and since then i've been using it for about 2 years in the HacDC occupancy sensor with no problems (due to pytty atleast ). it's much more straight forward to use and is pure python so it's a bit less hassle if you don't need to do more than read and write to the device.
Thanks! I'll be trying this as well, I do like pure python.