PureBasic Forums - English

Code: Select all

;-TOP

; Link: https://stackoverflow.com/questions/65734589/understand-modbus-rtu-crc
;
;  uint16_t calculate16CRC(uint16_t Length, uint8_t *Data)
; {
;     /* Reversed Modbus polynomial of 0x8005 -> CRC-16 POLYNOMIAL: P(x) = x16 + x15 + x2 + 1 */
;     uint16_t Poly = 0xa001;
; 
;     uint16_t CRCRegister = 0xFFFF; // Initialize CRC register with all 1's
; 
;     uint16_t b; // Carry Over Checker
;     uint16_t c; // Data Array Index
;     uint16_t i; // Bit Index
; 
;     for (c = 0; c < Length; c++) {
;         CRCRegister ^= Data[c]; // XOR CRC with Byte
;         for (i = 0; i < 8; i++) {
;             b = CRCRegister & 1;
;             CRCRegister >>= 1; // Shift to the right
;             if (b != 0){ // Carry over?
;                 CRCRegister ^= Poly; // Yes
;             }
;         }
;     }
; 
;     return CRCRegister;
; }

Structure ArrayOfByte
  StructureUnion
    a.a[0]
    b.b[0]
  EndStructureUnion
EndStructure

Structure ArrayOfAny
  StructureUnion
    a.a[0]
    b.b[0]
    u.u[0]
    w.w[0]
    l.l[0]
    q.q[0]
  EndStructureUnion
EndStructure

Procedure calculate16CRC(Length, *Data.ArrayOfByte)
  
  ;/* Reversed Modbus polynomial of 0x8005 -> CRC-16 POLYNOMIAL: P(x) = x16 + x15 + x2 + 1 */
  Protected Poly = $A001;
  
  Protected CRCRegister = $FFFF ; // Initialize CRC register with all 1's
  
  Protected b; // Carry Over Checker
  Protected c; // Data Array Index
  Protected i; // Bit Index
  
  For c = 0 To Length-1
    CRCRegister = (CRCRegister ! *Data\a[c]) & $FFFF; // XOR CRC with Byte
    For i = 0 To 7
      b = CRCRegister & 1;
      CRCRegister >> 1   ; // Shift to the right
      If (b <> 0)        ; // Carry over?
        CRCRegister = (CRCRegister ! Poly) & $FFFF; // Yes
      EndIf
    Next
  Next
  
  ProcedureReturn CRCRegister;
EndProcedure

; ----

Procedure bswap16(value.u)
  CompilerIf #PB_Compiler_Backend = #PB_Backend_C
    !return __builtin_bswap16(v_value);
  CompilerElse
    !xor eax,eax
    !mov ax, word [p.v_value]
    !rol ax, 8
    ProcedureReturn
  CompilerEndIf
EndProcedure

; ----

Procedure fc03request(Slave, Offset, Count, *SendData) ; Build Read Holding Registers
  Protected *request.ArrayOfAny = *SendData
  
  *request\a = Slave
  *request + 1
  *request\a = 3
  *request + 1
  *request\u = bswap16(Offset)
  *request + 2
  *request\u = bswap16(Count)
  *request + 2
  *request\u = calculate16CRC(6, *SendData)
  ProcedureReturn 8
EndProcedure

*fc04request = AllocateMemory(100)

len = fc03request(17, 107, 3, *fc04request)
Debug "Len to send = " + len

ShowMemoryViewer(*fc04request, 16)