Raspberry pi, communicate with Serial Device

Question

I try to communicate from Raspberry pi to the a serial device with out success.

I connect, write data but never read data back.

The serial device works fine in windows with the following settings on the com port - Baud Rate: 9600, Parity: None, Bit: 8, Stop bit: 1.

For communication I use a USB to Serial FTDI adaptor.

My working c code is :

int set_interface_attribs(int serialPort)
{
  struct termios tty;
  if (tcgetattr(serialPort, &tty) < 0) {
    printf("Error from tcgetattr: %s\n", strerror(errno));
    return -1;
  }
  printf("Old Serial port flags, i:%d, o:%d, c:%d\n", (int)tty.c_iflag, (int)tty.c_oflag, (int)tty.c_cflag);
  int speed = B9600;
  cfsetospeed(&tty, (speed_t)speed);
  cfsetispeed(&tty, (speed_t)speed);
  tty.c_cflag = CS8 |  CLOCAL | CREAD;
  tty.c_iflag = IGNPAR;
  tty.c_oflag = 0;
  tty.c_lflag = 0;
  tcflush(serialPort, TCIFLUSH);
  if (tcsetattr(serialPort, TCSANOW, &tty) != 0) {
    printf("Error from tcsetattr: %s\n", strerror(errno));
    return -1;
  }
  printf("New Serial port flags, i:%d, o:%d, c:%d\n", (int)tty.c_iflag, (int)tty.c_oflag, (int)tty.c_cflag);
  return 0;
}

int main( int argc, char *argv[] ) {
  printf("Start program\n");
  int serialPort=0;
  //if ((serialPort=open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NONBLOCK))<0) {
  if ((serialPort=open("/dev/ttyUSB0", O_RDWR | O_NOCTTY))<0) {
    printf("Error opening port: %s\n", strerror(errno));
    return 1;
  }
  set_interface_attribs(serialPort);
  long n;
  char strSend[20];
  sprintf(strSend,"98000001%c%c", 0x0D,0x0A);
   if ((n = write(serialPort, strSend, strlen(strSend)) ) < 0 ){
    perror("Error write to serialPort");
    return 0;
  }
  printf("Write (%d) chars (%s) to serialPort\n", (int)n, strSend);
  sleep(1);
  printf("Start reading from serialPort\n");
  char wData[20];
  memset(wData, '\0', sizeof(wData));
  if ( (n = read(serialPort, wData, 20) ) <= 0 ){
    printf("Error %d read from serialPort\n", (int)n);
  } else{
    wData[n] = 0;
    printf("Read:%d, %s\n", (int)n, wData);
  }
  printf("Close serialPort\n");
  close(serialPort);
  return 0;
}

and the out put is :

Start program
Old Serial port flags, i:4, o:0, c:2224
New Serial port flags, i:4, o:0, c:2224
Write (10) chars (98000001
) to serialPort
Start reading from serialPort
Error 0 read from serialPort
Close serialPort

any iden or suggestion ;

If I had to guess, if there is nothing to be read you're not going to have anything in the read value. It's 0, so it goes into your error message. I'm not super familiar with the serial libs, but I had a very similar problem over TCP. I was reading data from a socket that didn't have anything on it yet and I had to wait for data. — Bmo
@Bmo similar test on windows 7 PC with exactly same cables and serial device, return this string 9900033200000839. (Read:16, 9900033200000839) — Giannis

Jnavero Jnavero · Accepted Answer · 2016-12-01T18:06:46

The reading should be at the same time that the write.

You should do an infinite loop. This bucle should always be reading the port. It is recomended to make a buffer into a thread (you could use lpthread.h).

The life cycle:

Configure serial port
Open serial port
Create thread to read
Write (to test it)
Close serial port.

On the other hand, the best option (if you want to test your code) is make a jumper on RX and TX pinouts.

So all you write you will be able to read it.

Raspberry pi, communicate with Serial Device

1 Answers