From: "Lars Truijens" <ebc@wxs.nl>
Subject: Re: XModem CRC confusion
Date: 30 Jun 1999 00:00:00 GMT
Message-ID: <7ld321$rmt4@forums.borland.com>
References: <7lcnka$r5v16@forums.borland.com>
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Newsgroups: borland.public.delphi.objectpascal

Hi,

After searching the depts of internet, much testing and a huge amount of
different sources in even more languages
I think I have the answer to my own question:

The polynome is X^16 + X^12 + X^5 + 1 according to
http://www.ora.com/reference/dictionary/terms/C/Cyclic_Redundancy_Code.htm

And here is a working source for calculating XModem CRC's with the 'fast'
implementation:

unit XModemCRC16;

interface

const
  // X^16 + X^12 + X^5 + 1
 XModem CRC16Table: array[0..255] of Word = (
    $0000,$1021,$2042,$3063,$4084,$50A5,$60C6,$70E7,
    $8108,$9129,$A14A,$B16B,$C18C,$D1AD,$E1CE,$F1EF,
    $1231,$0210,$3273,$2252,$52B5,$4294,$72F7,$62D6,
    $9339,$8318,$B37B,$A35A,$D3BD,$C39C,$F3FF,$E3DE,
    $2462,$3443,$0420,$1401,$64E6,$74C7,$44A4,$5485,
    $A56A,$B54B,$8528,$9509,$E5EE,$F5CF,$C5AC,$D58D,
    $3653,$2672,$1611,$0630,$76D7,$66F6,$5695,$46B4,
    $B75B,$A77A,$9719,$8738,$F7DF,$E7FE,$D79D,$C7BC,
    $48C4,$58E5,$6886,$78A7,$0840,$1861,$2802,$3823,
    $C9CC,$D9ED,$E98E,$F9AF,$8948,$9969,$A90A,$B92B,
    $5AF5,$4AD4,$7AB7,$6A96,$1A71,$0A50,$3A33,$2A12,
    $DBFD,$CBDC,$FBBF,$EB9E,$9B79,$8B58,$BB3B,$AB1A,
    $6CA6,$7C87,$4CE4,$5CC5,$2C22,$3C03,$0C60,$1C41,
    $EDAE,$FD8F,$CDEC,$DDCD,$AD2A,$BD0B,$8D68,$9D49,
    $7E97,$6EB6,$5ED5,$4EF4,$3E13,$2E32,$1E51,$0E70,
    $FF9F,$EFBE,$DFDD,$CFFC,$BF1B,$AF3A,$9F59,$8F78,
    $9188,$81A9,$B1CA,$A1EB,$D10C,$C12D,$F14E,$E16F,
    $1080,$00A1,$30C2,$20E3,$5004,$4025,$7046,$6067,
    $83B9,$9398,$A3FB,$B3DA,$C33D,$D31C,$E37F,$F35E,
    $02B1,$1290,$22F3,$32D2,$4235,$5214,$6277,$7256,
    $B5EA,$A5CB,$95A8,$8589,$F56E,$E54F,$D52C,$C50D,
    $34E2,$24C3,$14A0,$0481,$7466,$6447,$5424,$4405,
    $A7DB,$B7FA,$8799,$97B8,$E75F,$F77E,$C71D,$D73C,
    $26D3,$36F2,$0691,$16B0,$6657,$7676,$4615,$5634,
    $D94C,$C96D,$F90E,$E92F,$99C8,$89E9,$B98A,$A9AB,
    $5844,$4865,$7806,$6827,$18C0,$08E1,$3882,$28A3,
    $CB7D,$DB5C,$EB3F,$FB1E,$8BF9,$9BD8,$ABBB,$BB9A,
    $4A75,$5A54,$6A37,$7A16,$0AF1,$1AD0,$2AB3,$3A92,
    $FD2E,$ED0F,$DD6C,$CD4D,$BDAA,$AD8B,$9DE8,$8DC9,
    $7C26,$6C07,$5C64,$4C45,$3CA2,$2C83,$1CE0,$0CC1,
    $EF1F,$FF3E,$CF5D,$DF7C,$AF9B,$BFBA,$8FD9,$9FF8,
    $6E17,$7E36,$4E55,$5E74,$2E93,$3EB2,$0ED1,$1EF0);

function CalcXModemCRC16(P: PChar; Length: Word): Word; overload;
function CalcXModemCRC16(Str: String): Word; overload;

implementation

function CalcXModemCRC16(P: PChar; Length: Word): Word;
var I: Word;
begin
  Result := 0;
  for I := 1 to Length do begin
    Result := (Byte(Result) shl 8) xor XModemCRC16Table[Hi(Result)] xor
Byte(P^);
    Inc(P);
  end;
end;

function CalcXModemCRC16(Str: String): Word;
begin
  Result := CalcXModemCRC16(PChar(Str), Length(Str));
end;

end.