CRC

A cyclic redundancy check (CRC) is an error-detecting code.

HDLC like framing

The http://www.ietf.org/rfc/rfc1662.txt has two examples of CRC calculations.

Polynom CRC-16: 0x8408 ( CRC-16-CCITT Reversed )

Polynom: x⁰ + x⁵ + x¹² + x¹⁶

CRC-16-IBM

Bisync, Modbus, USB, ANSI X3.28, SIA DC-07, many others; also known as CRC-16 and CRC-16-ANSI

Polynom: x¹⁶ + x¹⁵ + x² + 1

Normal	Reversed	Reversed reciprocal
0x8005	0xA001	0xC002

   1     def crc16ANSI(self,message):
   2         crc = 0x0000
   3         bitOne=0x0001
   4         crcpoly=0xA001
   5         idx = 0
   6         while idx < len(message):
   7             origByte = ord(message[idx])
   8             bitCounter = 0
   9             
  10             while bitCounter < 8:
  11                 crcFirstBit=(crc & bitOne)
  12                 origByteFirstBit=(origByte & bitOne)                
  13                 crcShiftRight=crc >> 1                
  14                 #check if first bit of crc is different than the first of the original byte 
  15                 if(crcFirstBit ^  origByteFirstBit== 1):
  16                     crc = crcShiftRight ^ crcpoly
  17                 else:
  18                     crc = crcShiftRight                    
  19     
  20                 bitCounter = bitCounter + 1
  21                 origByte = origByte >> 1
  22                 
  23             idx = idx + 1
  24                     
  25         res=crc & (0xFFFF)        
  26         return res
  27 '''
  28 'A' 0x30C0
  29 'ABC'  0x4521       
  30 'THE,QUICK,BROWN,FOX,0123456789' 0xB96E 
  31 '''