Reading Serial data

[bowlofred]

perhaps the pi leaves the serial line down for short periods of time?.

I might suggest just wrapping the block in try/except and retrying. If that works okay, probably best you can do.

try:
    if (ser.inWaiting()>0): 
        data_str = ser.read(ser.inWaiting())
        print(data_str)
except OSError:
    # ignore or log...  Let the loop retry.
    pass