C语言实现MODBUS协议

MODBUS协议是一种串行通信协议，通常用于连接工业自动化设备。在C语言中实现MODBUS协议需要使用串口通信库和MODBUS协议的相关函数库。下面是一个简单的示例：

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

#define MODBUS_READ_HOLDING_REGISTERS 0x03

int open_serial_port(const char *device)
{
    int fd = open(device, O_RDWR | O_NOCTTY | O_NDELAY);
    if (fd == -1) {
        perror("open");
        exit(-1);
    }
    fcntl(fd, F_SETFL, 0);
    return fd;
}

void set_serial_port(int fd, int speed)
{
    struct termios options;
    tcgetattr(fd, &options);
    cfsetispeed(&options, speed);
    cfsetospeed(&options, speed);
    options.c_cflag |= (CLOCAL | CREAD);
    options.c_cflag &= ~PARENB;
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    options.c_oflag &= ~OPOST;
    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    options.c_cc[VMIN] = 0;
    options.c_cc[VTIME] = 5;
    tcsetattr(fd, TCSANOW, &options);
}

int modbus_read_holding_registers(int fd, unsigned char *buffer, int address, int count)
{
    unsigned char request[8];
    request[0] = address;
    request[1] = MODBUS_READ_HOLDING_REGISTERS;
    request[2] = (count >> 8) & 0xff;
    request[3] = count & 0xff;
    request[4] = (address >> 8) & 0xff;
    request[5] = address & 0xff;
    int length = write(fd, request, 6);
    if (length == -1) {
        perror("write");
        return -1;
    }
    length = read(fd, buffer, count * 2 + 5);
    if (length == -1) {
        perror("read");
        return -1;
    }
    return length;
}

int main(int argc, char **argv)
{
    if (argc != 2) {
        fprintf(stderr, "Usage: modbus /dev/ttyUSB0\n");
        exit(-1);
    }

    int fd = open_serial_port(argv[1]);
    set_serial_port(fd, B9600);

    unsigned char buffer[1024];
    int length = modbus_read_holding_registers(fd, buffer, 0, 10);
    if (length == -1) {
        fprintf(stderr, "Failed to read holding registers\n");
        exit(-1);
    }

    for (int i = 0; i < length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");

    close(fd);
    return 0;
}

上面的示例中，我们使用了Linux系统的串口库来打开和配置串口，然后实现了一个读取Holding Registers的函数。最后，我们在main函数中调用了modbus_read_holding_registers函数来读取寄存器数据，并输出到终端。注意，这里的寄存器地址和寄存器数量是固定的，需要根据实际情况进行修改。