CRC-16 Routines

[mark pruitt]

    '**********************************************************************************************************************
    '   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).]