Crash in heartbeat protocol due to toggle bit

[eyalcohen]

Hi, great library, thanks for the effort.

When reading NMT messages under the heartbeat protocol, occasionally the highest bit is set for state. I believe this is resulting in a crash because bit 8 is set for the state (PRE-OPERATIONAL state is 0xFF instead of 0x7F for example):

  File "/home/user/miniconda2/envs/abc/lib/python2.7/site-packages/canopen/nmt.py", line 103, in state
    self.send_command(code)
  File "/home/user/miniconda2/envs/abc/lib/python2.7/site-packages/canopen/nmt.py", line 138, in send_command
    super(NmtMaster, self).send_command(code)
  File "/home/user/miniconda2/envs/abc/lib/python2.7/site-packages/canopen/nmt.py", line 72, in send_command
    NMT_STATES[self._state], NMT_STATES[new_state])
KeyError: 255

nmt.py defines the codes for states as below 128

NMT_STATES = {
    0: 'INITIALISING',
    4: 'STOPPED',
    5: 'OPERATIONAL',
    80: 'SLEEP',
    96: 'STANDBY',
    127: 'PRE-OPERATIONAL'
}

The CANopen docs in section 7.2.8.3.2.2 Protocol heartbeat say that bit 8 of slave state is a toggle bit which is toggled with two consecutive NMT messages, so seeing a slave send 0xFF for PRE-OPERATIONAL state could be expected.

If I understand the issue correctly, can we have the above error mask out the toggle bit?

[eyalcohen]

Thanks!