Talking to device over Serial: Write works, Read doesn't

This is a discussion on Talking to device over Serial: Write works, Read doesn't within the C Programming forums, part of the General Programming Boards category; I'm trying to talk to a device over a USB serial device (/dev/ttyUSB0). I can send commands to it with ...

  1. #1
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    Talking to device over Serial: Write works, Read doesn't

    I'm trying to talk to a device over a USB serial device (/dev/ttyUSB0). I can send commands to it with write(), but I'm having problems with read().

    When I set VMIN and VTIME, read fails with "Resource Temporarily Unavailable". Otherwise, it "succeeds" with 0 Bytes returned.

    I have configured the serial device as such:
    Code:
    int fd, ret;
    struct termios options;
    
    /*fd = open(device, O_RDWR | O_NOCTTY | O_NDELAY);*/
    fd = open(device, O_RDWR | O_NOCTTY);
    
    cfsetispeed(&options, B1200);
    cfsetospeed(&options, B1200);
    options.c_cflag |= (CREAD | CLOCAL);
    options.c_cflag &= ~CSIZE;
    options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    options.c_cflag |= CS8;
    options.c_oflag &= ~OPOST;
    options.c_iflag &= ~(IXON | IXOFF | IXANY);
    options.c_iflag |= (IGNPAR);
    
    /*
    options.c_cc[VMIN] = 0;
    options.c_cc[VTIME] = 1;
    */
    
    ret = tcsetattr(fd, TCSANOW, &options);
    Is this sufficient to read() input? Someone suggested I use poll(), but I haven't tried it yet.

  2. #2
    Kernel hacker
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    Have you checked that there isn't a getty process also trying to use that device.

    If you use "cat /dev/ttyUSB0", can you read from it that way?

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
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    Quote Originally Posted by matsp View Post
    Have you checked that there isn't a getty process also trying to use that device.

    If you use "cat /dev/ttyUSB0", can you read from it that way?

    --
    Mats
    How can I check? I doubt any other process is trying to use it unless a previous instance of my own app didn't close it properly.

    I don't see any input from cat, but it does close after a few seconds.

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