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Message-ID: <3AA5BFD1.43809E15@beamreachnetworks.com>
Date: Tue, 06 Mar 2001 20:57:53 -0800
From: "Eric M. Monsler" <emonsler AT beamreachnetworks DOT com>
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To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Subject: Serial port interface under cygwin.  Lossy?
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All,

I have a small application that communicates with embedded devices
across the PC's serial port.

I have a trivial header and trailer defining a message packet across the
interface.

I have been testing it by running my application on both ports, with a
cable between them, and sending messages.

When I run at 115200 baud, everything works great.

When I run at 9600 baud, I lose a significant fraction of the messages. 
Like, half of them.  Some lost because the header was not received
properly, but more often because the 1-byte trailer was not the correct
value (or, was not in the correct location).

Has anyone seen this type of behavior with cygwin's serial port
interface?

I have no particular reason to blame cygwin vs. NT's serial drivers, or
the PC's hardware, but I do see the same effect on two different PC's,
different motherboards.  So, I thought I'd ask.

A snip of the code I use to setup the port is below.

Eric Monsler


/************* Port open and setup code ****************************/

  /* 
     Now we know we have the correct parameters to use, proceed to
     open port and write 
  */

  fd = open(serialdevice, O_RDWR | O_NOCTTY | O_NDELAY);
  if (fd <0) {perror(serialdevice); exit(-1); }

#if DEBUG
  printf("Opening serial port %s returned: %d\n",serialdevice,fd);
#endif
  
  tcgetattr(fd,&oldtio); /* save current port settings */
  
  /* Start with old settings, then we'll modify */
  newtio = oldtio;

  /* 
     Now do the port setup to match what the EPLD is expecting 
  */

  /* Set baudrate */
  cfsetispeed(&newtio,baud_enum);
  cfsetospeed(&newtio,baud_enum);

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

  if(strcmp(parity,"8N1") == 0)
    {
      /* Set 8 bit characters, no parity, 1 stop (8N1) */
      newtio.c_cflag &= ~PARENB;	/* Clear previous parity bits values */
      newtio.c_cflag &= ~CSTOPB;	/* Clear previous stop bits values */
      newtio.c_cflag &= ~CSIZE;	/* Clear previous charsize bits values
*/
      newtio.c_cflag |= CS8;	/* Set for 8 bits. */
    }
  else
    {
      printf("Unsupported parity selection, using 8N1.\n");
      /* FIXME */
      printf("Actually, only 8N1 supported at this time \n");
      /* Set 8 bit characters, no parity, 1 stop (8N1) */
      newtio.c_cflag &= ~PARENB;	/* Clear previous parity bits values */
      newtio.c_cflag &= ~CSTOPB;	/* Clear previous stop bits values */
      newtio.c_cflag &= ~CSIZE;	/* Clear previous charsize bits values
*/
      newtio.c_cflag |= CS8;	/* Set for 8 bits. */
    }

  /* I think we want to disable hardware flow control.  FIXME  */
  newtio.c_cflag &= ~CRTSCTS;

  /* When we read, we want completely raw access  */
  newtio.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);

  /* I think we want to disable software flow control.  FIXME  */
  newtio.c_iflag &= ~(IXON | IXOFF | IXANY);
   
  /* Set output filtering to raw. */
  newtio.c_oflag &= ~OPOST;

  /* Set readsize and timeout to some reasonable values, just to be safe
*/
  newtio.c_cc[VTIME]    = 0;   /* inter-character timer unused */
  newtio.c_cc[VMIN]     = 0;   /* nonblocking read */
  

  /* 
     Now we can flush, and then make the control changes!
  */
  tcflush(fd, TCIFLUSH);
  tcsetattr(fd,TCSANOW,&newtio);

--
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