Thread: Write to serial port

Hello,

I am trying to write to device connected via a serial port. It uses 8 data bits, 1 stop bit, even parity and a communication rate of 2400bps. I am developing in Linux.

I need to send message 0x80007F23FF.

But "it doesn't work" (I know that's vague, but I don't know how to check what's wrong - for example, what message was received (and if it was received). I just don't get the desired output (which would be to turn a light on on the device).

Below is the relevant code I am using.

Thanks in advance

Code:

int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
fcntl(fd, F_SETFL, 0);

// Set options for the serial port
struct termios options;
tcgetattr(fd, &options);

// Set the baud rate to 2400
cfsetispeed(&options, B2400);
cfsetospeed(&options, B2400);

// Set even parity checking
options.c_cflag |= PARENB;
options.c_cflag &= ~PARODD;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
options.c_iflag |= (INPCK | ISTRIP);

// Enable the receiver and set local mode
options.c_cflag |= (CLOCAL | CREAD);

// Choose raw input
options.c_lflag &= ~(ICANON | ECHO | ECHOE | IEXTEN | ISIG);

// Choose BRKINT and ICRNL options
options.c_iflag &= ~(BRKINT | ICRNL | IMAXBEL);

// Choose raw output
options.c_oflag &= ~(OPOST | ONLCR);

// Set the new options for the port
tcsetattr(fd, TCSANOW, &options);

// Try to send a message
unsigned char msg[5] = {0x80, 0x00, 0x7F, 0x23, 0xFF};
write(fd, &msg, 8);

is it possible you're sending the bytes in reverse order?

Post the code showing how you are trying to do multiple reads/writes.

Jim

I don't use any flow control.

This is what I am doing:

Code:

char aux[8];
unsigned char readRequest[5] = {0x80, 0x00, 0x7F, 0x6F, 0xFF};
while (1) 
{   
    write(fd, &readRequest, sizeof (readRequest));
    read(fd, aux, 8); 
    for (int i = 0; i < 8; i++)
        printf("%x ", aux[i]);
    printf("\n");
    sleep(2);
}

After I looked more carefully, I realized I'm not getting such weird results afterall. The result I am expecting is

Code:

80 00 22 60 00 0B 78 7F

But I am getting

Code:

00 00 22 60 00 0B 78 7F

Notice that the difference is in the most significant bit on the first and the last byte (it should be 1 and it is 0). By the device's design, only those bytes can have a MSB=1 (and it should always be 1).

Thanks!

I used CuteCom and saw that it got the communication working nicely, so I saw what options it used (with stty -a -F /dev/ttyS0) and set them accordingly. What I miss-setting were INPCK, ISTRIP and IXON.

Just one more thing. I am using the same write/read/printf as before, and I am now getting this output:

Code:

ffffff80 0 22 60 0 b 71 ffffffff

instead of

Code:

80 00 22 60 00 0b 71 ff

Is that just a printing problem (wrong %x configuration) or am I still getting the wrong data?

%02x did the trick for the 0, but I'm still intrigued on why 0x80 get's printed ffffff80 - is that just a printing problem or is there something deeper? If it's just printing I don't care, as I will perform some calculations on them and convert to decimal.

Thanks for all the help!

It worked! Thanks a lot!