I need to be able to calculate a CRC in order to communicate with a machine. I'm pretty new to calculating CRC's so I'm sorry if this exact question has already been answered as I'm not sure what I am looking for. Anyway, I need to compute a CCIT 16-bit CRC in Python. My message is a binary stream that looks like:
b'G18M1314D4417,2511165'
The message also begins with a STX and ends with a ETX character. I tried using this Python module:
import crc16
print(crc16.crc16xmodem(b'G18M1314D4417,2511165'))
Which returned:
61154
When the correct CRC was:
A855
What am I missing?
Edit: I'm using Python 3.3. I got the 'correct' CRC from a message sent from the machine. I'm not sure whether STX/ETX characters should be included as I have never worked with CRC's before.
I believe that your machine is using a parametrised CRC algorithm named CRC-16/MCRF4XX.
You can use the crcmod module which can be installed with pip. The CRC can be calculated using the predefined algorithm crc-16-mcrf4xx. STX and ETX should be included in the CRC calculation.
import crcmod
import crcmod.predfined
STX = b'\x02'
ETX = b'\x03'
data = b'G18M1314D4417,2511165'
message = STX + data + ETX
crc16 = crcmod.predefined.Crc('crc-16-mcrf4xx')
crc16.update(message)
crc = crc16.hexdigest()
>>> crc
'A855'