Reading serial port with C

[flyeng4]

I have two usb2serial adapters connected to my PC with a null modem (I am doing some testing to learn serial comm for a particular application). I am writing to /dev/ttyUSB0 a trying to read /dev/ttyUSB1. I have used screen (screen /dev/ttyUSB1 115200) and am able to see the characters being written. I am also able to read and write with Python with the following code

import serial,time,sys
ser1 = serial.Serial('/dev/ttyUSB0',baudrate=115200)
ser2 = serial.Serial('/dev/ttyUSB1',timeout=2,baudrate=115200)
ser1.open()
ser2.open()
ser1.flush()
ser2.flush()
ser1.write('a')
read = ser2.read(1)
ser1.close()
ser2.close()

I need to be able to do this in C. I am able to do the writing and have confirmed with screen as described in the first paragraph. I cannot however read. My C code is as follows

#include <stdio.h>  
#include <fcntl.h>
#include <termios.h>
int main()
{
  int fd1, fd2, n,i,maxfd,res; 
  char buffer[1];
  char *bufptr; 
  fd_set readfs;
  struct timeval timeout;
  struct termios options1;
  struct termios options2;
  bufptr = buffer;
  fd1 = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
  if (fd1 < 0) printf("Not connected to ttyUSB0\n");
  fcntl(fd1, F_SETFL, 0);
  tcflush(fd1, TCIFLUSH);
  tcgetattr(fd1, &options1);
  cfsetispeed(&options1, B115200);
  cfsetospeed(&options1, B115200);
  options1.c_cflag &= ~PARENB;    // set no parity, stop bits, data bits
  options1.c_cflag &= ~CSTOPB;
  options1.c_cflag &= ~CSIZE;
  options1.c_cflag |= CS8;
  tcsetattr(fd1, TCSANOW, &options1);
  fd2 = open("/dev/ttyUSB1", O_RDWR | O_NOCTTY | O_NDELAY);
  if (fd2 < 0) printf("Not connected to ttyUSB1\n");
  fcntl(fd2, F_SETFL, 0);
  tcflush(fd2, TCIFLUSH);
  tcgetattr(fd2, &options2);
  cfsetispeed(&options2, B115200);
  cfsetospeed(&options2, B115200);
  options2.c_cflag &= ~PARENB;    // set no parity, stop bits, data bits
  options2.c_cflag &= ~CSTOPB;
  options2.c_cflag &= ~CSIZE;
  options2.c_cflag |= CS8;
  tcsetattr(fd2, TCSANOW, &options2);
  if (fd1 > fd2) maxfd = fd1+1;
  else maxfd = fd2+1;
  maxfd = fd2+1;
  n = write(fd1,"a\n",1);
  printf("Wrote %d bytes.\n",n);
  close(fd1);
  FD_SET(fd2, &readfs);
  timeout.tv_usec = 0;
  timeout.tv_sec  = 1;
  res = select(maxfd, &readfs, NULL, NULL, &timeout);
  if (res == 0) printf("Serial not ready to read.\n");
  i = read(fd2,buffer,1);
  if (i != 1)  printf("Unable to read.\n");
  close(fd2);
  return(0);
}

What am I doing wrong?

Thanks,
Bill

[Rubberman]

I'll have to look at this closer later. I have to go teach a class tonight, so it may be a day or so before I have an answer for you, unless someone else gets to it first.

[flyeng4]

Rubberman,

Thanks for replying. I was able to get the above code to work. I change the write command from

n = write(fd1,"a\n",1);

to

n = write(fd1,"a\r",1);

I was able to run the code just fine. The end goal is to try to do faster than normal serial communication. I think I am running into a OS interrupt frequency limitation. My kernel is compiled at 1000 hz timer interrupt frequency. I need to be able to communicate a little faster than 1000 hz across the serial line. I have no idea of how to do this but would love to hear any thoughts on the issue.

Bill

[Rubberman]

It depends upon how the serial port was configured (see the stty command). The way it was configured was to use just a carriage-return (\r) to empty the buffer. The new-line (\n) character won't do that. Python is probably a bit smarter in dealing with that cruft than you on C are...