I’m dealing with a device that sends a string of data followed by a Checksum Byte. Can I use the CLACom CRC routines to compute this Checksum?

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CRC’s are normally 2 or 4 bytes, whereas a Checksum is a single byte that is computed by adding together the value of each character in a buffer. The high 8 bits are thrown away leaving an 8 byte checksum.

Instead of simply adding the values together, some implementations Exclusive OR the values, which offers slightly better error correction.

The following function can be used to compute a Checksum and with a slight modification, an XOR Checksum. This example uses a CSTRING as the source of data since it is assumed you will be working with data received from the Serial Port.

Prototype:

GetChecksum(*CSTRING),BYTE

Procedure:

    GetChecksum FUNCTION(RStr)

    Cnt         SHORT
    StrLen      SHORT
    Tally       SHORT
    ChkSum      BYTE

       CODE

       Tally  = 0

       StrLen = Len(Clip(RStr))

       ! Tally up the Checksum
       ! Comment Out or Remove the Line inside
       ! the Loop that you do not need.

       Loop Cnt = 1 to StrLen
          ! Compute a Regular Checksum

          Tally += Val(RStr[Cnt])

          ! Compute an Exclusive OR Checksum

          Tally = BXOR(Val(RStr[Cnt]),Tally)
       End

       ChkSum = Tally    ! Checksum = lower 8 bits

       Return(ChkSum)