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  • Interfacing Modbus RTU with PIC

    shahrul

    Member

    Updated: Oct 21, 2024
    Views: 1.4K
    Anybody know the modbus protocol? It can connect to PIC by RS232 connection and PIC can do UART coding. Just want to know how the modbus RTU operation? I keep exploring this and will update the progress.
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  • shahrul

    Member • Feb 19, 2010

    Now, I see the pattern.

    Example read holding register.
    Request protocol:
    Slave ID: 1 byte
Function code: 1 byte
Starting Address: 2 bytes
Quantity of address: 2 bytes
CRC-16: 2 bytes
    Response protocol:
    Slave ID: 1 byte
Function code: 1 byte
Byte count: 1 byte
Register value: (2*Quantity of address) bytes
CRC-16: 2 bytes
    This is the example Tx and Rx data
    Tx: 01 03 00 00 00 03 05 CB
Rx: 01 03 06 00 01 00 01 00 04 4C B6
    Last 2 bytes for Tx and Rx is CRC-16 checking. My further question is, how to calculate CRC-16?
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  • shahrul

    Member • Feb 23, 2010

    Now, I get the CRC-16 function.
    Example
    unsigned int data[]={0x01,0x03,0x00,0x00,0x00,0x03};

int CRC16(int DataLength){
unsigned int i,j,CheckSum;
CheckSum=0xFFFF;
for (j=0;j<DataLength;j++){
    CheckSum=CheckSum^response[j];
    for(i=8;i>0;i--){
        if((CheckSum)&0x0001==1) CheckSum=(CheckSum>>1)^0xA001;
        else CheckSum=CheckSum>>1;}}
CRC_high=(CheckSum>>8)&0x00FF;    
CRC_low=CheckSum&0x00FF;
CheckSum=((CheckSum>>8)&0x00FF)|((CheckSum<<8)&0xFF00);
return CheckSum;
}
    will return value CRC=0x05CB
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  • shahrul

    Member • Mar 4, 2010

    Now, I have problem on parity bit. I have done 8bit UART, that is NONE PARITY.
    The parity bit is setup with 9th bit. I try send this, but it fail at certain data.
    response[0]=0x01;
    response[1]=0x03;
    response[2]=0x06;
    response[3]=0x00;
    response[4]=0x00;
    response[5]=0x00;
    response[6]=0x01;
    response[7]=0x00;
    response[8]=0x02;
    CRC16(9);
    response[9]=CRC_low;
    response[10]=CRC_high;}

    for(i=0;i<=10;i++){
    TX9D=even_parity(response);
    uart_transmit(response);}
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  • Ashraf HZ

    Member • Mar 4, 2010

    This is quite informative stuff. Keep us updated on your progress 😀
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