Code:
'**********************************************************************************************************************
' I needed these CRC-16 routines to calculate the CRC for each byte of data in a string, accumulate the value in
' CrcReg??, and then use the final value of CrcReg?? to calculte a final Block Check Character (two; one byte each).
' These may be useful to someone. Should be easy to modify. Kev Peel and Tom Hanlin (and possibly others) may
' remember helping me in translating some of these from C. Thanks again. Feel free to use/modify this code.
'
' - CRC-16 Routines
' - Create a unique CRC-16 lookup table by changing the values of c%(0) through c%(7)
' - Calculate CRC value by passing data one byte at a time to CalculateCRC16 as databyte? until all bytes are passed
' Can be used to calculate CRC16 for your transmit string, or to calculate the CRC16 of a received string
' Last two bytes of received string (for my purposes) are the Most Significant Byte and then Least Significant
' Byte of of the final Block Check Character - can be omitted if you don't need them.
' - Final BCC (Block Check Character) composed of two one byte pieces of data, BCC1?, and BCC2?
' Convert these to byte sized bit patterns with MKBYT$ before appending to end of string
'
'
'
'
'**********************************************************************************************************************
'create a 256 element look up table based on a unique mask/rule set
FUNCTION MakeLookUpTable () AS LONG
GLOBAL CrcReg??
GLOBAL table?? ()
LOCAL c% ()
LOCAL d$
LOCAL i&, j&, mask&
REDIM table??(0 TO 255)
REDIM c%(0 TO 7) 'these hex values create a unique look up table, change values to change the table
c%(0) = &h1001
c%(1) = &h2002
c%(2) = &h4004
c%(3) = &h8118
c%(4) = &h1221
c%(5) = &h2482
c%(6) = &h48A4
c%(7) = &h3AB8
FOR i& = 0 TO 255
table??(i&) = 0
mask& = &h0001
FOR j& = 0 TO 7
IF(i& AND mask&) THEN table??(i&) = table??(i&) XOR c%(j&)
SHIFT LEFT mask&, 1
NEXT j&
d$ = HEX$(table??(i&))
NEXT i&
FUNCTION = 0
END FUNCTION
'*************************************************************************************************************
FUNCTION ResetCRC () AS LONG
CrcReg?? = 0 'reset CrcReg?? before calculating new string
END FUNCTION
'*************************************************************************************************************
' calculate the CRC-16; call this routine until all bytes of data have been passed to it, then call GetBCC1BisyncByte
FUNCTION CalculateCrc16 (databyte?) AS LONG
LOCAL s??, CrcCopy??
CrcCopy?? = CrcReg?? 'make a copy because SHIFT changes the value of CrcReg??
s?? = 0
SHIFT RIGHT CrcCopy??, 8
s?? = (databyte? AND &h00ff) XOR (CrcCopy?? AND &h00ff) 'databye? is one byte of data extracted from data string
SHIFT LEFT CrcReg??, 8
CrcReg?? = (CrcReg?? XOR table??(s??))
END FUNCTION
'********************************************************************************************************************
'get first block check character (MSB(yte))
FUNCTION GetBCC1BisyncByte () AS LONG
STATIC BCC1?
LOCAL a$, CRC??
CRC?? = CrcReg?? 'make a copy of the value of CrcReg??
SHIFT RIGHT CRC??, 8
BCC1? = NOT (CRC?? AND 255)
CALL GetBCC2BisyncByte (BCC1?)
END FUNCTION
'******************************************************************************************************************************
'get second block check character (LSB(yte))
FUNCTION GetBCC2BisyncByte (BCC1?) AS LONG
STATIC BCC2?
BCC2? = NOT(CrcReg?? AND 255)
blockcheck$ = MKBYT$(BCC1?) + MKBYT$(BCC2?) 'blockcheck$ is a global variable declared elsewhere
END FUNCTION
' ********************************************************************************************************************
Mark Pruitt
markspruitt@yahoo.com
[This message has been edited by mark pruitt (edited July 30, 2003).]