Pyserial not getting response every time, input/output errors

[Rainy]

Hello!

I'm having some trouble with pyserial package, I'm sending commands
and reading responses from a custom pcb, and sometimes I get a proper
response, at other times I get nothing, and sometimes I get about half
of the response string with beginning cut off. About half the time an
empty string is returned, the other ~half time good response, and more
rarely I get partial response.

When I try to use the same Serial instance to send/receive a few
times, I end up getting an input/output error.

Here are some examples:

>>> ser = serial.Serial('/dev/ttyAM1', 115200, timeout=0.1)
>>> ser.write(chr(2) + chr(1) + chr(6) + chr(12) + chr(34) + chr(0) + chr(0))
>>> ser.read(60)
''
>>> ser.close()
>>> ser = serial.Serial('/dev/ttyAM1', 115200, timeout=0.1)
>>> ser.write(chr(2) + chr(1) + chr(6) + chr(12) + chr(34) + chr(0) + chr(0))
>>> ser.read(60)
'\x02\x00\'\x0c"\x00My Thermostat R1.0B'

>>> ser.write(chr(2) + chr(1) + chr(6) + chr(12) + chr(34) + chr(0) + chr(0))
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
 File "/usr/lib/python2.3/site-packages/serial/serialposix.py", line
372, in write
   n = os.write(self.fd, d)
OSError: [Errno 5] Input/output error


I tried timeout values from 0.1 to 3sec. Doesn't make a difference.

Aside from the possibility of hw issue (I'm investigating this now),
what else could it be? We tested this from a different computer from a
Ferret terminal program using a different cable, and it worked without
problem. I tried many different things, using miniterm.py provided
with pyserial, getting newest version of pyserial, sending one char at
a time, etc etc. I get same intermittent problems..

If anyone have an idea on what else I can try, please help! thanks!