Thread: Serial Programming, Write-Read Issue

  1. #1
    Registered User
    Join Date
    Jul 2010

    Serial Programming, Write-Read Issue


    I am still new to serial programming in linux. So to communicate with an A to D device I found some code online and modified it for my purpose. The problem is I can never perform "Write then Read".

    Whereas when I perform "just Read" and "Read then Write and then Read", the code works with no issues. But if I write a command to the port first and then try to read the response, the program just waits for the response indefinitely.

    Another interesting thing I noticed is while the program is waiting for the response, if I open a connection to the port using "CuteCom" then the program starts reading the response.

    Also if I perform the write operation using a different binary, then I can read the responses with no issues.

    I really don't understand why this is happening. Any form of help on this regard would really be appreciated. My system details and code are below.

    Thanks and Regards,


    System Details:
    Ubuntu 10.04 & Connected the RS-232 cable to my laptop by using RS-232 to USB converter.

    #include <termios.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <sys/signal.h>
    #include <sys/stat.h>
    #include <sys/types.h>
    #include <sys/time.h>
    #define BAUDRATE B115200
    #define PORT "/dev/ttyUSB0"
    #define _POSIX_SOURCE 1 /* POSIX compliant source */
    #define FALSE 0
    #define TRUE 1
    void signal_handler_IO ( int status );   // definition of signal handler        
    volatile int STOP = FALSE;         
    int wait_flag = TRUE;                  // TRUE while no signal received
    int main()
        int fd, res, sen;
        int i;
        struct termios oldtio, newtio;
        struct sigaction saio; /* definition of signal action */     
        char comm[7], resp[10];
    	for ( i = 0; i < 10; i++ ) { resp[i] = 0x00; }
        /* open the device to be non-blocking (read will return immediatly) */
        fd = open( PORT, O_RDWR | O_NOCTTY | O_NONBLOCK );
        if (fd < 0) { perror(PORT); return 1; }
    	/* install the signal handler before making the device asynchronous */
        saio.sa_handler = signal_handler_IO;
        //saio.sa_mask = 0;
        saio.sa_flags = 0;
        saio.sa_restorer = NULL;
        sigaction( SIGIO, &saio, NULL );
        /* allow the process to receive SIGIO */
        fcntl( fd, F_SETOWN, getpid() );
        /* Make the file descriptor asynchronous (the manual page says only 
           O_APPEND and O_NONBLOCK, will work with F_SETFL...) */
        fcntl( fd, F_SETFL, FASYNC );
        //tcgetattr(fd,&oldtio); /* save current port settings */
        /* set new port settings for canonical input processing */
        newtio.c_cflag = BAUDRATE | CRTSCTS | CLOCAL | CREAD;
        newtio.c_iflag = IGNPAR | ICRNL;
        //newtio.c_iflag = IGNPAR;
        newtio.c_oflag = 0;
        newtio.c_lflag = ICANON;
        //newtio.c_lflag = 0;
        newtio.c_cc[VMIN] = 0;
        newtio.c_cc[VTIME] = 0;
        tcflush(fd, TCIFLUSH);
        tcsetattr( fd, TCSANOW, &newtio );
    	comm[0] = 'V'; comm[1] = '\r';
    	printf("Write start...\n");
    	sen = write( fd, comm, 2 );
    	printf("Write stop...\nsen = %d\n", sen);
    	printf("Read start...\n");
        res = read( fd, resp, 4 );
    	printf("Read stop...\n");
        resp[res] = '\0';
        printf("Response = %s\n", resp );
        /* restore old port settings */
    	tcsetattr( fd, TCSANOW, &oldtio );
      return 0;
     * signal handler. sets wait_flag to FALSE, to indicate above loop that     *
     * characters have been received.                                           *
    void signal_handler_IO (int status)
    	printf("received SIGIO signal.\n");
    	wait_flag = FALSE;

  2. #2
    Registered User
    Join Date
    May 2010
    A couple of things you can try:
    1. flush both in/out buffers
     tcflush(fd, TCIOFLUSH);
    2. Many embedded systems look for the cr/lf pair so you might try adding '\n' to your send string.

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