Example of Modbus Communication Code Based on 51 Single Chip Microcomputer

The following is an example of Modbus communication code based on the 51 microcontroller, using Keil C compiler and STC89C52 microcontroller:

#include <reg52.h>

#define SlaveAddress 1      // 从机地址
#define FunctionCode 3      // 功能码
#define RegisterAddress 0   // 寄存器地址
#define RegisterLength 1    // 寄存器长度

sbit RS485_EN = P2^7;       // RS485芯片使能控制引脚

void delay(unsigned int i)  // 延时函数
{
    while(i--);
}

void UART_Init()            // 串口初始化函数
{
    SCON = 0x50;
    TMOD = 0x20;
    TH1 = 0xFD;
    TL1 = 0xFD;
    TR1 = 1;
}

void SendByte(unsigned char dat)  // 发送一个字节
{
    SBUF = dat;
    while(TI == 0);
    TI = 0;
}

void SendPacket(unsigned char address, unsigned char function, unsigned char* pData, unsigned char length) // 发送数据包
{
    unsigned char i, crcHi, crcLo;
    crcHi = 0xFF;
    crcLo = 0xFF;
    SendByte(0xFF);
    SendByte(0xFF);
    SendByte(address);
    SendByte(function);
    SendByte(RegisterAddress / 256);
    SendByte(RegisterAddress % 256);
    SendByte(RegisterLength / 256);
    SendByte(RegisterLength % 256);
    for(i = 0; i < length; i++)
    {
        SendByte(pData[i]);
        crcHi ^= pData[i];
        for(j = 0; j < 8; j++)
        {
            if(crcHi & 0x01)
            {
                crcHi >>= 1;
                crcHi ^= 0xA0;
            }
            else
            {
                crcHi >>= 1;
            }
        }
    }
    SendByte(crcHi);
    SendByte(crcLo);
}

void Modbus() // Modbus主函数
{
    unsigned char data[2] = {0, 0};
    SendPacket(SlaveAddress, FunctionCode, data, RegisterLength);
}

void main()
{
    UART_Init();
    while(1)
    {
        RS485_EN = 0;
        delay(1000);
        Modbus();
        delay(1000);
        RS485_EN = 1;
        delay(1000);
    }
}

Note: This code is only an example and for reference only. Specific implementation needs to be adjusted according to actual needs.