unable to read Through linux Serial port

Question

Hi I have written a program in c to communicate with /dev/ttyS0 to a device . when i do write i am able to do that but i get ERROR: Resource temporarily unavailable, -1 as return from read(). can you please check what i am doing wrong . the port is supposed to be a half duplex ,9600bps,8N1 , and a inter byte delay of 4ms . also can anyone tell me how to set inter byte delay in milliseconds. Below is my code.

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


    #define BAUDRATE B9600
    #define PORT "/dev/ttyS0"

    int main()
    {
        struct termios new_tio,old_new_tio;
        int fd;
        unsigned char msgS[5]="",msgR[10]="";

        fd = open(PORT,O_RDWR|O_NOCTTY|O_NDELAY);
        if(fd == -1)
            printf("Failed to open PORT: %s  \n\n",PORT);
        perror("Error:");
        tcgetattr(fd,&old_new_tio);
        memset(&new_tio,0,sizeof(new_tio));

        new_tio.c_cflag = BAUDRATE|CS8|CLOCAL|CREAD|CSIZE|PARENB;
        new_tio.c_oflag = 0;
        new_tio.c_iflag = 0;
        new_tio.c_cc[VMIN] = 10;
        new_tio.c_cc[VTIME] = 0.004;
        cfsetispeed(&new_tio,BAUDRATE);
        cfsetospeed(&new_tio,BAUDRATE);
        new_tio.c_cc[VINTR]    = 0;    
            new_tio.c_cc[VQUIT]    = 0;  
            new_tio.c_cc[VERASE]   = 0;  
            new_tio.c_cc[VKILL]    = 0;  
            new_tio.c_cc[VEOF]     = 4;  
        new_tio.c_cc[VSWTC]    = 0;     /* '\0' */
            new_tio.c_cc[VSTART]   = 0;     /* Ctrl-q */ 
            new_tio.c_cc[VSTOP]    = 0;     /* Ctrl-s */
            new_tio.c_cc[VSUSP]    = 0;     /* Ctrl-z */
            new_tio.c_cc[VEOL]     = 0;     /* '\0' */
            new_tio.c_cc[VREPRINT] = 0;     /* Ctrl-r */
            new_tio.c_cc[VDISCARD] = 0;     /* Ctrl-u */
            new_tio.c_cc[VWERASE]  = 0;     /* Ctrl-w */
            new_tio.c_cc[VLNEXT]   = 0;     /* Ctrl-v */
            new_tio.c_cc[VEOL2]    = 0;  

        tcflush(fd,TCIFLUSH);
        tcsetattr(fd,TCSANOW,&new_tio);
        msgS[0] = '*';
        msgS[1] = 'G';
        msgS[2] = 'U';
        msgS[3] = 'S';
        msgS[4] = '\r';

        printf("Message to be sent : %s \n\n",msgS);
        int i = write(fd,&msgS,5);
        if(i != 5)
            printf("error while writing to the port\n\n");

        int j = read(fd,&msgR,10);
        if(j != 10)
            printf("error while reading from port\n\n");

        printf("Message Recieved : %s\n\n",msgR);

        return 0;
    }

If you're using non blocking I/O, use select() to inform you when data is available (for reading) or there's room for output data (for writing). — Jayesh Bhoi

dragonroot dragonroot · Accepted Answer · 2014-03-26T04:16:22

You open the port in non-blocking mode (using O_NDELAY). Therefore your read would never block and only read what's already arrived. You should either use select to wait for the data first, or use a blocking mode.

I doubt you can fine-tune inter-byte delay. To increase it, you can enable using two stop bits instead of one (CSTOPB).

unable to read Through linux Serial port

1 Answers