读取可变长度和频率的串行数据

Question

我正在尝试以不同的频率读取 2 个可变长度的串行数据缓冲区。

1. 第一个缓冲区每 400 毫秒发送一次，缓冲区大小为 187。
2. 每 500 毫秒发送第二个缓冲区，缓冲区大小为 216。

使用以下实现的代码，即使接收到的缓冲区长度是 187 或 216 字节，接收到的缓冲区大小也始终为 230 字节。

数据字节读取调用等待最多 230 个字节被接收，因此接收到的数据格式不符合预期的数据格式。

下面是实现的代码：

#define DEVICEPORT "/dev/ttyUSB4"
#define SLEEP_TIMEOUT 400
#define MAX_BUFF_SIZE 230

int main () {

    int ret = 0;
    struct termios options;

    int fd = open(DEVICEPORT, O_RDWR | O_NOCTTY | O_NDELAY );
    
    if(fd == -1)
        cout << "Error in opening the port" << endl;
    else
        cout << "port opened successfully" << endl;

    tcgetattr(fd, &options);

    cfmakeraw(&options);
    cfsetispeed(&options, B19200);
    cfsetospeed(&options, B19200);

    options.c_cflag |= PARENB;
    options.c_cflag |= PARODD;
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS7;
    options.c_cflag &= ~CRTSCTS;
    options.c_cflag |= CREAD | CLOCAL;//enable receiver
    
    options.c_iflag &= ~(IXON | IXOFF | IXANY );
    options.c_iflag |= (INPCK | ISTRIP);

    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);

    options.c_cc[VMIN] = 10;
    options.c_cc[VTIME] = 1;

    tcflush(fd, TCIFLUSH);
    if((tcsetattr(fd,TCSANOW,&options)) != 0) /* Set the attributes to the termios structure*/
    {
        cout << "ERROR ! in Setting attributes" << endl;
    }
    else
    {   cout << "couldn't set options .. " << endl;
        tcflush(fd, TCIFLUSH);
    }

    ioctl(fd, TCFLSH, 2);

    char buff[MAX_BUFF_SIZE];

    while(true) {
        cout << "Reading the data : "<< endl;

        int n = read(fd, buff, sizeof(buff));

        if (n < 0) { 
            cout << "Error while reading .. " << endl;
        } else {
            cout << "Read data size :" << n << "\n" ;  
            for (size_t i = 0; i < n; i++ ) {
                cout << buff[i];
            } cout << endl;
        }
        
        std::this_thread::sleep_for(std::chrono::milliseconds(SLEEP_TIMEOUT));
    }
    return 0;
}

我应该设置什么串口设置来读取可变长度和频率的缓冲区？

Answer 1

将int n = read(fd, buff, sizeof(buff)); 替换为逐字节读取，至少在您读取协议标头和大小之前。也许类似于此伪代码的内容：

int n = read(fd, header, sizeof(header));
if(n != sizeof(header))
{ /* handle errors */ }

int expected_size = header.size;

n = read(fd, buff, expected_size)
if(n != expected_size)
{ /* handle errors */ }

/* now the first n bytes of buff contain the protocol */

你还应该在某个地方进行 CRC 检查，因为这么长的串行数据是不可信的。