串口（UART）c ++读取

Question

我在c ++中有一个读入函数的小问题。我正在尝试读取UART的RxD端口上的数据。

运行此代码时，我得到结果（读取＆lt; 0）因此它在读取时显示错误消息错误。应该怎么做才能读入并显示读取的数据。我确信RxD端口上有可用的数据我已经用示波器检查了它。

#include <iostream>
#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <stdlib.h>
#include <sys/ioctl.h>

#define BAUDRATE B19200
#define PORT "/dev/ttyO4"
#define _POSIX_SOURCE 1

#define FALSE 0
#define TRUE 1
volatile int STOP=FALSE;

void signal_handler_IO(int status);
int wait_flag = TRUE;



main ()
{
    int fd=0, res=0, result=0;


    char SYNC  [] = {0x55};
    char PID [] = {0x6A};

    struct termios oldtio, newtio;
    struct sigaction saio;
    char buff[255];

    fd = open(PORT, O_RDWR | O_NOCTTY | O_NONBLOCK);
    if (fd<0) {perror(PORT); exit(-1);}

    saio.sa_handler=signal_handler_IO;
    saio.sa_flags=0;
    saio.sa_restorer = NULL;
    sigaction(SIGIO, &saio,NULL);
    fcntl(fd, F_SETFL, FASYNC);
    tcgetattr(fd, &oldtio);

    newtio.c_cflag = BAUDRATE | CS8 | CLOCAL | CREAD;
    newtio.c_iflag = IGNPAR;
    newtio.c_oflag = 0;
    newtio.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    //newtio.c_lflag = 0;
    newtio.c_cc[VMIN]=1;
    newtio.c_cc[VTIME]=0;
    tcflush(fd, TCIFLUSH);
    tcsetattr(fd, TCSANOW, &newtio);

    ioctl(fd, TIOCSBRK);
    usleep(1300);

    ioctl(fd,TIOCCBRK);
    usleep(200);

    write(fd, SYNC, sizeof(SYNC));
    write(fd,PID, sizeof(PID));

    res = read(fd, buff,255);

    if (res < 0)
    {
        printf("read error\n");
    }
    else if (res ==0)
    {
        printf("read = 0 \n");
    }
   else
    {
        sprintf(result, "%x",res);
        buff[res]=0;
        printf(": %s :%d :%d\n", buff,result,res);

    }
    close (fd);

}

void signal_handler_IO(int status)
{
    printf("received the signal\n");
wait_flag=FALSE;
}