Đúng là với Modbus RTU thì phần tính CRC là có vẻ phức tạp hơn cả. Còn lại, nếu cứ theo Modbus chuẩn thì tài liệu về nó rất nhiều do Modbus khad đơn giản và lại là chuẩn mở. Giờ các thiết bị hỗ trợ Modbus TCP còn thực hiện dễ hơn nữa.
Để gửi 1 command tới 1 Slave, bắt buộc phải tính CRC, nếu không Slave sẽ ko trả lại Respond.
Có thể tính CRC rất đơn giản như sau (mình tham khảo được)
//-----------------------------------------------------------------------
HiTable: array[0..255] of Byte = (
$00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0, $80,
$41, $00, $C1, $81, $40, $01, $C0, $80, $41, $00, $C1,
$81, $40, $00, $C1, $81, $40, $01, $C0, $80, $41, $01,
$C0, $80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40,
$01, $C0, $80, $41, $00, $C1, $81, $40, $01, $C0, $80,
$41, $01, $C0, $80, $41, $00, $C1, $81, $40, $01, $C0,
$80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40, $01,
$C0, $80, $41, $00, $C1, $81, $40, $01, $C0, $80, $41,
$01, $C0, $80, $41, $00, $C1, $81, $40, $00, $C1, $81,
$40, $01, $C0, $80, $41, $01, $C0, $80, $41, $00, $C1,
$81, $40, $01, $C0, $80, $41, $00, $C1, $81, $40, $00,
$C1, $81, $40, $01, $C0, $80, $41, $01, $C0, $80, $41,
$00, $C1, $81, $40, $00, $C1, $81, $40, $01, $C0, $80,
$41, $00, $C1, $81, $40, $01, $C0, $80, $41, $01, $C0,
$80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40, $01,
$C0, $80, $41, $01, $C0, $80, $41, $00, $C1, $81, $40,
$01, $C0, $80, $41, $00, $C1, $81, $40, $00, $C1, $81,
$40, $01, $C0, $80, $41, $00, $C1, $81, $40, $01, $C0,
$80, $41, $01, $C0, $80, $41, $00, $C1, $81, $40, $01,
$C0, $80, $41, $00, $C1, $81, $40, $00, $C1, $81, $40,
$01, $C0, $80, $41, $01, $C0, $80, $41, $00, $C1, $81,
$40, $00, $C1, $81, $40, $01, $C0, $80, $41, $00, $C1,
$81, $40, $01, $C0, $80, $41, $01, $C0, $80, $41, $00,
$C1, $81, $40);
LoTable: array[0..255] of Byte = (
$00, $C0, $C1, $01, $C3, $03, $02, $C2, $C6, $06, $07,
$C7, $05, $C5, $C4, $04, $CC, $0C, $0D, $CD, $0F, $CF,
$CE, $0E, $0A, $CA, $CB, $0B, $C9, $09, $08, $C8, $D8,
$18, $19, $D9, $1B, $DB, $DA, $1A, $1E, $DE, $DF, $1F,
$DD, $1D, $1C, $DC, $14, $D4, $D5, $15, $D7, $17, $16,
$D6, $D2, $12, $13, $D3, $11, $D1, $D0, $10, $F0, $30,
$31, $F1, $33, $F3, $F2, $32, $36, $F6, $F7, $37, $F5,
$35, $34, $F4, $3C, $FC, $FD, $3D, $FF, $3F, $3E, $FE,
$FA, $3A, $3B, $FB, $39, $F9, $F8, $38, $28, $E8, $E9,
$29, $EB, $2B, $2A, $EA, $EE, $2E, $2F, $EF, $2D, $ED,
$EC, $2C, $E4, $24, $25, $E5, $27, $E7, $E6, $26, $22,
$E2, $E3, $23, $E1, $21, $20, $E0, $A0, $60, $61, $A1,
$63, $A3, $A2, $62, $66, $A6, $A7, $67, $A5, $65, $64,
$A4, $6C, $AC, $AD, $6D, $AF, $6F, $6E, $AE, $AA, $6A,
$6B, $AB, $69, $A9, $A8, $68, $78, $B8, $B9, $79, $BB,
$7B, $7A, $BA, $BE, $7E, $7F, $BF, $7D, $BD, $BC, $7C,
$B4, $74, $75, $B5, $77, $B7, $B6, $76, $72, $B2, $B3,
$73, $B1, $71, $70, $B0, $50, $90, $91, $51, $93, $53,
$52, $92, $96, $56, $57, $97, $55, $95, $94, $54, $9C,
$5C, $5D, $9D, $5F, $9F, $9E, $5E, $5A, $9A, $9B, $5B,
$99, $59, $58, $98, $88, $48, $49, $89, $4B, $8B, $8A,
$4A, $4E, $8E, $8F, $4F, $8D, $4D, $4C, $8C, $44, $84,
$85, $45, $87, $47, $46, $86, $82, $42, $43, $83, $41,
$81, $80, $40);
function AnsiDataCRC16(sData: AnsiString; iLength: integer): Word;
var
HiCRC: byte;
LoCRC: byte;
iCount: integer;
iIndex: Word;
begin
HiCRC := $FF;
LoCRC := $FF;
for iCount := 1 to iLength do
begin
iIndex := HiCRC xor byte(sData[iCount]);
HiCRC := LoCRC xor HiTable[iIndex];
LoCRC := LoTable[iIndex];
end;
Result := (HiCRC shl 8 or LoCRC);
end;
//-----------------------------------------------------------------------
PS. Sorry, mình code trên Delphi nên hơi khác tí xíu, nhưng chắc ai cũng có thể chuyển sang C dc nhỉ
