c语言编写modbus rtu master程序示例_图文讲解Modbus通讯协议，内容有点多，但是讲得很详细...

很高兴回答你的问题。在这里，我将提供一个简单的C语言程序示例，用于实现Modbus RTU主站的通信。

首先，让我们来了解一下Modbus RTU通信协议。Modbus是一种通信协议，用于在工业自动化系统中进行通信。Modbus RTU是Modbus协议的一种变体，它使用二进制编码，并通过串行通信进行传输。在Modbus RTU通信中，主站发送请求，从站响应请求并提供数据。请求和响应之间的通信使用CRC校验和进行校验。

接下来，让我们看一下C语言程序示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>

#define SLAVE_ADDRESS 0x01
#define FUNCTION_READ_COILS 0x01
#define FUNCTION_READ_HOLDING_REGISTERS 0x03
#define FUNCTION_WRITE_SINGLE_COIL 0x05
#define FUNCTION_WRITE_SINGLE_REGISTER 0x06

int open_port(void)
{
    int fd; 
    fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
    if (fd == -1)
    {
        perror("open_port: Unable to open /dev/ttyUSB0 - ");
    }
    else
    {
        fcntl(fd, F_SETFL, 0);
    }
    return (fd);
}

void set_port(int fd)
{
    struct termios options;
    tcgetattr(fd, &options);
    cfsetispeed(&options, B9600);
    cfsetospeed(&options, B9600);
    options.c_cflag |= (CLOCAL | CREAD);
    options.c_cflag &= ~PARENB;
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    options.c_iflag &= ~(IXON | IXOFF | IXANY);
    options.c_oflag &= ~OPOST;
    options.c_cc[VMIN] = 0;
    options.c_cc[VTIME] = 10;
    tcsetattr(fd, TCSANOW, &options);
}

unsigned short calculate_crc(unsigned char *buffer, int length)
{
    unsigned short crc = 0xFFFF;
    int i, j;

    for (i = 0; i < length; i++)
    {
        crc ^= buffer[i];
        for (j = 0; j < 8; j++)
        {
            if (crc & 0x0001)
            {
                crc >>= 1;
                crc ^= 0xA001;
            }
            else
            {
                crc >>= 1;
            }
        }
    }
    return crc;
}

int read_coils(int fd, unsigned char address, unsigned short start_address, unsigned short quantity, unsigned char *response)
{
    unsigned char request[8];
    unsigned short crc;
    int i, n;

    request[0] = address;
    request[1] = FUNCTION_READ_COILS;
    request[2] = start_address >> 8;
    request[3] = start_address & 0xFF;
    request[4] = quantity >> 8;
    request[5] = quantity & 0xFF;
    crc = calculate_crc(request, 6);
    request[6] = crc & 0xFF;
    request[7] = crc >> 8;

    n = write(fd, request, 8);
    if (n < 0)
    {
        perror("write");
        return -1;
    }

    usleep(50000);

    n = read(fd, response, 5 + quantity / 8 + 1);
    if (n < 0)
    {
        perror("read");
        return -1;
    }

    crc = calculate_crc(response, n - 2);
    if (crc != ((response[n - 2] << 8) | response[n - 1]))
    {
        return -1;
    }

    return n;
}

int read_holding_registers(int fd, unsigned char address, unsigned short start_address, unsigned short quantity, unsigned char *response)
{
    unsigned char request[8];
    unsigned short crc;
    int i, n;

    request[0] = address;
    request[1] = FUNCTION_READ_HOLDING_REGISTERS;
    request[2] = start_address >> 8;
    request[3] = start_address & 0xFF;
    request[4] = quantity >> 8;
    request[5] = quantity & 0xFF;
    crc = calculate_crc(request, 6);
    request[6] = crc & 0xFF;
    request[7] = crc >> 8;

    n = write(fd, request, 8);
    if (n < 0)
    {
        perror("write");
        return -1;
    }

    usleep(50000);

    n = read(fd, response, 5 + quantity * 2 + 1);
    if (n < 0)
    {
        perror("read");
        return -1;
    }

    crc = calculate_crc(response, n - 2);
    if (crc != ((response[n - 2] << 8) | response[n - 1]))
    {
        return -1;
    }

    return n;
}

int write_single_coil(int fd, unsigned char address, unsigned short address, unsigned short value)
{
    unsigned char request[8];
    unsigned short crc;
    int n;

    request[0] = address;
    request[1] = FUNCTION_WRITE_SINGLE_COIL;
    request[2] = address >> 8;
    request[3] = address & 0xFF;
    request[4] = value ? 0xFF : 0x00;
    request[5] = 0x00;
    crc = calculate_crc(request, 6);
    request[6] = crc & 0xFF;
    request[7] = crc >> 8;

    n = write(fd, request, 8);
    if (n < 0)
    {
        perror("write");
        return -1;
    }

    usleep(50000);

    n = read(fd, request, 8);
    if (n < 0)
    {
        perror("read");
        return -1;
    }

    crc = calculate_crc(request, n - 2);
    if (crc != ((request[n - 2] << 8) | request[n - 1]))
    {
        return -1;
    }

    return n;
}

int write_single_register(int fd, unsigned char address, unsigned short address, unsigned short value)
{
    unsigned char request[8];
    unsigned short crc;
    int n;

    request[0] = address;
    request[1] = FUNCTION_WRITE_SINGLE_REGISTER;
    request[2] = address >> 8;
    request[3] = address & 0xFF;
    request[4] = value >> 8;
    request[5] = value & 0xFF;
    crc = calculate_crc(request, 6);
    request[6] = crc & 0xFF;
    request[7] = crc >> 8;

    n = write(fd, request, 8);
    if (n < 0)
    {
        perror("write");
        return -1;
    }

    usleep(50000);

    n = read(fd, request, 8);
    if (n < 0)
    {
        perror("read");
        return -1;
    }

    crc = calculate_crc(request, n - 2);
    if (crc != ((request[n - 2] << 8) | request[n - 1]))
    {
        return -1;
    }

    return n;
}

int main(int argc, char **argv)
{
    int fd;
    unsigned char response[1024];

    fd = open_port();
    set_port(fd);

    read_coils(fd, SLAVE_ADDRESS, 0, 8, response);
    read_holding_registers(fd, SLAVE_ADDRESS, 0, 8, response);
    write_single_coil(fd, SLAVE_ADDRESS, 0, 1);
    write_single_register(fd, SLAVE_ADDRESS, 0, 1234);

    close(fd);
    return 0;
}

在上面的示例中，我们使用了Linux系统调用来打开串口设备文件，并配置串口参数。然后，我们使用read_coils，read_holding_registers，write_single_coil和write_single_register函数来实现Modbus RTU通信。这些函数接受不同的参数，以便读取或写入数据。

希望这个示例可以帮助你理解如何使用C语言编写Modbus RTU主站程序。