rs485通讯协议代码详解

RS485通讯协议是一种串行通讯协议，常用于工业自动化、智能家居等领域。以下是一个基于C语言的RS485通讯协议代码示例，详解如下：

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

/* 定义串口设备 */
#define SERIAL_DEV "/dev/ttyUSB0"

/* 定义通讯参数 */
#define BAUD_RATE B9600
#define DATA_BITS CS8
#define PARITY_NONE 0
#define STOP_BITS 1

/* 定义RS485控制信号 */
#define RS485_ENABLE 1
#define RS485_DISABLE 0

int serial_fd;

/* 打开串口设备函数 */
int open_serial_device()
{
    struct termios options;

    serial_fd = open(SERIAL_DEV, O_RDWR | O_NOCTTY | O_NDELAY);

    if (serial_fd < 0)
    {
        perror("open_serial_device(): Unable to open serial device\n");
        return -1;
    }

    fcntl(serial_fd, F_SETFL, 0);

    tcgetattr(serial_fd, &options);

    cfsetispeed(&options, BAUD_RATE);
    cfsetospeed(&options, BAUD_RATE);

    options.c_cflag &= ~CSIZE;
    options.c_cflag |= DATA_BITS;

    options.c_cflag &= ~PARENB;
    options.c_iflag &= ~INPCK;

    options.c_cflag &= ~CRTSCTS;
    options.c_cc[VMIN] = 1;
    options.c_cc[VTIME] = 0;

    options.c_cflag &= ~CSTOPB;

    tcsetattr(serial_fd, TCSANOW, &options);

    return 0;
}

/* 设置RS485控制信号函数 */
void set_rs485_control(int enable)
{
    int fd;
    struct termios termios_options;

    fd = open(SERIAL_DEV, O_RDWR | O_NOCTTY);

    if (fd < 0)
    {
        perror("set_rs485_control(): Unable to open serial device\n");
        return;
    }

    tcgetattr(fd, &termios_options);

    if (enable)
    {
        termios_options.c_cflag |= (CLOCAL | CREAD);
        termios_options.c_cflag &= ~CSIZE;
        termios_options.c_cflag |= CS8;
        termios_options.c_cflag &= ~PARENB;
        termios_options.c_cflag &= ~CSTOPB;
        termios_options.c_cflag &= ~CRTSCTS;

        termios_options.c_iflag &= ~(IXON | IXOFF | IXANY);
        termios_options.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);

        termios_options.c_oflag &= ~OPOST;

        termios_options.c_cc[VMIN] = 1;
        termios_options.c_cc[VTIME] = 0;

        ioctl(fd, TIOCMGET, &termios_options.c_cflag);
        termios_options.c_cflag |= TIOCM_RTS;
        termios_options.c_cflag &= ~TIOCM_DTR;

        tcsetattr(fd, TCSANOW, &termios_options);
    }
    else
    {
        termios_options.c_cflag &= ~(CLOCAL | CREAD);

        termios_options.c_iflag &= ~(IXON | IXOFF | IXANY);
        termios_options.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);

        tcsetattr(fd, TCSANOW, &termios_options);
    }

    close(fd);
}

/* 发送数据函数 */
int send_data(unsigned char *data, int len)
{
    int ret;

    set_rs485_control(RS485_ENABLE);

    ret = write(serial_fd, data, len);

    set_rs485_control(RS485_DISABLE);

    return ret;
}

/* 接收数据函数 */
int receive_data(unsigned char *data, int len, int timeout)
{
    int ret;
    fd_set rfds;
    struct timeval tv;

    FD_ZERO(&rfds);
    FD_SET(serial_fd, &rfds);

    tv.tv_sec = timeout / 1000;
    tv.tv_usec = (timeout % 1000) * 1000;

    set_rs485_control(RS485_ENABLE);

    ret = select(serial_fd + 1, &rfds, NULL, NULL, &tv);

    if (ret < 0)
    {
        perror("receive_data(): select() failed\n");
        return -1;
    }
    else if (ret == 0)
    {
        perror("receive_data(): Timeout occurred\n");
        return -1;
    }

    ret = read(serial_fd, data, len);

    set_rs485_control(RS485_DISABLE);

    return ret;
}

int main()
{
    unsigned char cmd[] = {0x01, 0x03, 0x00, 0x00, 0x00, 0x02, 0xC4, 0x0B};

    open_serial_device();

    send_data(cmd, sizeof(cmd));

    sleep(1);

    unsigned char response[1024];

    receive_data(response, sizeof(response), 1000);

    return 0;
}

代码中，`open_serial_device()`函数用于打开串口设备并设置通讯参数；`set_rs485_control()`函数用于设置RS485控制信号，使串口设备处于发送或接收状态；`send_data()`函数用于发送数据；`receive_data()`函数用于接收数据。其中，`send_data()`和`receive_data()`函数均会在发送或接收完成后自动将串口设备设置为接收状态。

以上代码仅供参考，实际使用时还需根据具体情况进行修改。