51单片机modbus rtu从机源码

以下是一个简单的51单片机Modbus RTU从机的源代码示例：

#include <reg52.h>

#define BUF_SIZE 64

unsigned char const slave_address = 0x01;
unsigned char const function_code = 0x03;
unsigned char const start_address_hi = 0x00;
unsigned char const start_address_lo = 0x01;
unsigned char const num_of_points_hi = 0x00;
unsigned char const num_of_points_lo = 0x02;

unsigned char buf[BUF_SIZE];
unsigned char buf_len = 0;

void delay_ms(unsigned int count)
{
    unsigned int i;
    while (count--)
    {
        for (i = 0; i < 1000; i++)
            ;
    }
}

void init_serial()
{
    TMOD = 0x20; // Timer 1 in Mode 2
    TH1 = 0xFD;  // 9600 baud rate
    SCON = 0x50; // Serial port in Mode 1
    TR1 = 1;     // Start Timer 1
}

void send_byte(unsigned char byte)
{
    SBUF = byte;
    while (TI == 0)
        ;
    TI = 0;
}

void send_response()
{
    unsigned char i, crc_hi, crc_lo;
    unsigned short crc = 0xFFFF;

    send_byte(slave_address);
    send_byte(function_code);
    send_byte(num_of_points_lo * 2);

    for (i = 0; i < num_of_points_lo * 2; i += 2)
    {
        send_byte(buf[start_address_lo + i]);
        send_byte(buf[start_address_lo + i + 1]);

        crc ^= (unsigned short)(buf[start_address_lo + i + 1] << 8);
        for (j = 0; j < 8; j++)
        {
            if (crc & 0x8000)
            {
                crc = (crc << 1) ^ 0x8005;
            }
            else
            {
                crc <<= 1;
            }
        }
    }

    crc_hi = (unsigned char)(crc >> 8);
    crc_lo = (unsigned char)(crc & 0xFF);

    send_byte(crc_lo);
    send_byte(crc_hi);
}

void receive_request()
{
    unsigned char i, crc_hi, crc_lo;
    unsigned short crc = 0xFFFF;

    while (RI == 0)
        ;

    buf_len = 0;
    buf[buf_len++] = SBUF;

    while (RI && buf_len < BUF_SIZE)
    {
        buf[buf_len++] = SBUF;
    }

    // Check if the message is for us
    if (buf[0] != slave_address)
        return;

    // Check the CRC
    crc_hi = buf[buf_len - 2];
    crc_lo = buf[buf_len - 1];
    for (i = 0; i < buf_len - 2; i++)
    {
        crc ^= (unsigned short)buf[i];
        for (j = 0; j < 8; j++)
        {
            if (crc & 0x8000)
            {
                crc = (crc << 1) ^ 0x8005;
            }
            else
            {
                crc <<= 1;
            }
        }
    }
    if (crc_hi != (unsigned char)(crc >> 8) || crc_lo != (unsigned char)(crc & 0xFF))
        return;

    // Process the request
    if (buf[1] == function_code)
    {
        unsigned short start_address = ((unsigned short)buf[start_address_hi] << 8) | buf[start_address_lo];
        unsigned short num_of_points = ((unsigned short)buf[num_of_points_hi] << 8) | buf[num_of_points_lo];
        if (start_address + num_of_points > BUF_SIZE)
            return;
        for (i = 0; i < num_of_points * 2; i++)
        {
            buf[start_address + i] = 0x00; // Set the data to 0 for now
        }
        send_response();
    }
}

int main()
{
    init_serial();

    while (1)
    {
        receive_request();
        delay_ms(10); // Delay to avoid busy waiting
    }
}

这个源代码示例使用了一个简单的循环来等待和接收Modbus RTU请求，并根据请求的类型进行处理。在这个例子中，我们只实现了对读保持寄存器的响应，即读取从机上的一些数据。要实现其他功能码和更复杂的逻辑，需要进行更多的编程。