modbus协议传输C语言代码实现

以下是一个使用C语言实现Modbus RTU协议的例子，可以通过串口与设备进行通信：

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

#define FRAME_SIZE 256

unsigned char calc_crc(unsigned char *buf, unsigned int len)
{
    unsigned char crc = 0xFF;
    unsigned int i, j;
    for (i = 0; i < len; i++) {
        crc ^= buf[i];
        for (j = 0; j < 8; j++) {
            if ((crc & 0x01) != 0) {
                crc = (crc >> 1) ^ 0xA0;
            } else {
                crc >>= 1;
            }
        }
    }
    return crc;
}

int send_frame(int fd, unsigned char slave, unsigned char func, unsigned short addr, unsigned short count, unsigned char *data)
{
    unsigned char frame[FRAME_SIZE];
    unsigned int frame_len = 0;
    unsigned char crc;

    // 构造帧头
    frame[frame_len++] = slave;
    frame[frame_len++] = func;
    frame[frame_len++] = addr >> 8;
    frame[frame_len++] = addr & 0xFF;
    frame[frame_len++] = count >> 8;
    frame[frame_len++] = count & 0xFF;

    // 计算CRC校验码
    crc = calc_crc(frame, frame_len);
    if (data != NULL) {
        memcpy(&frame[frame_len], data, count);
        frame_len += count;
    }
    crc = calc_crc(frame, frame_len);

    // 添加帧尾
    frame[frame_len++] = crc;
    frame[frame_len++] = crc >> 8;

    // 发送帧
    if (write(fd, frame, frame_len) != frame_len) {
        perror("write error");
        return -1;
    }
    return 0;
}

int recv_frame(int fd, unsigned char *buf, unsigned int max_len, unsigned int timeout)
{
    unsigned char c;
    unsigned int len = 0;
    unsigned long start_time = time(NULL);

    // 等待帧头
    while (time(NULL) - start_time < timeout) {
        if (read(fd, &c, 1) == 1) {
            if (c == 0x01) {
                buf[len++] = c;
                break;
            }
        }
    }
    if (len == 0) {
        return -1;
    }

    // 读取从机地址、功能码等信息
    while (time(NULL) - start_time < timeout) {
        if (read(fd, &c, 1) == 1) {
            buf[len++] = c;
            if (len == 3) {
                break;
            }
        }
    }
    if (len != 3) {
        return -1;
    }

    // 读取数据域和CRC校验码
    unsigned int data_len = buf[2] - 2;
    while (time(NULL) - start_time < timeout) {
        if (read(fd, &c, 1) == 1) {
            buf[len++] = c;
            if (len == 3 + data_len + 2) {
                break;
            }
        }
    }
    if (len != 3 + data_len + 2) {
        return -1;
    }

    // 检查CRC校验码
    unsigned char crc = calc_crc(buf, len - 2);
    if (crc != buf[len - 2] || crc >> 8 != buf[len - 1]) {
        return -1;
    }

    return len;
}

int main(int argc, char **argv)
{
    int fd;
    struct termios options;
    unsigned char buf[FRAME_SIZE];

    // 打开串口
    fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
    if (fd < 0) {
        perror("open error");
        return -1;
    }

    // 配置串口
    tcgetattr(fd, &options);
    cfsetispeed(&options, B9600);
    cfsetospeed(&options, B9600);
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    options.c_cflag &= ~PARENB;
    options.c_cflag &= ~CSTOPB;
    options.c_cc[VMIN] = 0;
    options.c_cc[VTIME] = 10;
    tcsetattr(fd, TCSANOW, &options);

    // 发送读取保持寄存器的请求
    unsigned char data[2] = {0x00, 0x01};
    if (send_frame(fd, 0x01, 0x03, 0x0000, 0x0002, data) < 0) {
        perror("send_frame error");
        close(fd);
        return -1;
    }

    // 等待并读取响应帧
    if (recv_frame(fd, buf, FRAME_SIZE, 5) < 0) {
        perror("recv_frame error");
        close(fd);
        return -1;
    }

    // 解析响应帧并输出结果
    printf("Response: ");
    for (int i = 0; i < buf[2] - 2; i++) {
        printf("%02X ", buf[3 + i]);
    }
    printf("\n");

    // 关闭串口
    close(fd);
    return 0;
}

以上是一个简单的Modbus RTU协议传输的例子，通过串口读写函数实现了Modbus RTU协议的读取操作。需要注意的是，实际应用中需要根据具体的设备和通信需求进行相关的配置和修改。