CODEWITHSUNDEEP
pythoncrc
Raeven
Raeven

Reputation: 623

Calculating a CRC

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.

Upvotes: 0

Views: 5248

Answers (1)

mhawke
mhawke

Reputation: 87074

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'

Upvotes: 2

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