Re: Trouble reading from my serial device (a microcontroller)

[Chuck Messenger <chuckm@rochester.rr.com>]

OK, I figured it out.  Yeah, I'm having a conversation with myself.  But 
maybe it'll help someone else with the same problem who finds this 
message on a search.

The answer turned out to be: set DCD high.  DCD is pin 1 on a 9-pin 
serial connector.   That was all there was to it.  The program I 
included below works fine.  But if DCD isn't high (in my case, it wasn't 
connected), then read() will hang indefinitely.

I had read here and there that open() would hang if DCD wasn't high, 
depending on the flags (that is, unless O_NONBLOCKING was used). 
However, I found that open() didn't hang.

To recap:  If you're trying to connect to a simple serial device, and 
all you connect is Tx, Rx and ground, then you may find (as I did) that 
read() will hang.  You also must connect DCD (pin 1) to +12 V (I put it 
through a 2.2k resistor, just in case...).

     - Happy Chuck

Chuck Messenger wrote:
> I'm unable to read from a serial device under Linux (Mandrake 9.1, 
> 2.4.21-0.13mdk kernel), but the device works find in Windows.  I'm 
> hoping someone can offer me suggestions.
> 
> My device is a Basic Stamp microcontroller, which I've programmed to 
> spit out a certain byte continuously -- 0x40 -- at 38400 baud,n,8,1. The 
> device loops RTS back to DSR (i.e. it connects pins 6 and 7 of the 9-pin 
> serial connector -- DSR and RTS).  Other than that, only Rx and Tx are 
> connected (and ground, of course).
> 
> I know the hardware works (both controller and PC), because I can boot 
> Win2k on the same machine and run a short program I wrote which 
> continuously dumps to the console whatever comes in.
> 
> Also, I know that things are _basically_ configured right, in my Linux 
> boot, since I can run "statserial /dev/tts/0", which shows, in real 
> time, the status of the DSR line.  When I unplug from my device, DSR 
> goes low; when I plug it back in, DSR goes high.
> 
> I tried writing a simple port-dumping program on Linux, based on Peter 
> Baumann's sample in his Serial Programming Howto:
> 
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <termios.h>
> #include <stdio.h>
> 
> #define BAUDRATE B38400
> #define MODEMDEVICE "/dev/tts/0"
> #define _POSIX_SOURCE 1 /* POSIX compliant source */
> 
> int
> main()
> {
>     int fd,c, res;
>     struct termios oldtio,newtio;
> 
>     fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );
> 
>     if (fd < 0) {
>         perror(MODEMDEVICE);
>         exit(-1);
>     }
> 
>     tcgetattr(fd, &oldtio); /* save current port settings */
> 
>     memset(&newtio, 0, sizeof(newtio));
> 
>     newtio.c_cflag = BAUDRATE | /*CRTSCTS |*/ CS8 | CLOCAL | CREAD;
>     newtio.c_iflag = IGNPAR   // ignore parity errors
>                    | IGNBRK;  // ignore BREAK characters
>     newtio.c_oflag = 0;
> 
>     /* set input mode (non-canonical, no echo,...) */
> 
>     newtio.c_lflag = 0;
> 
>     newtio.c_cc[VTIME]    = 0;   /* inter-character timer unused */
>     newtio.c_cc[VMIN]     = 1;   /* blocking read until 1 chars received */
> 
>     tcflush(fd, TCIFLUSH);       // Flush an data received by not read
>     tcsetattr(fd,TCSANOW,&newtio);
> 
>     while (1) {       /* loop for input */
>         char buf[1];
>         res = read(fd,buf,1);   /* returns after 1 chars have been input */
>         printf("%02x ", buf[0]);
>         fflush(stdout);
>     }
> 
>     tcsetattr(fd,TCSANOW,&oldtio);
> 
>     return 0;
> }
> 
> 
> However, the program fails to read any bytes -- it hangs at the read().
> 
> I've tried running serlook, but that, too, failed to see any bytes.
> 
> Any suggestions?
> 
> I've tried 9600 baud instead of 38400 -- still no luck (although it 
> works under Win2k).
> 
> Poking around, I've seen mention of the DCD control line -- could that 
> be the problem (i.e. does the Linux serial driver require it, somehow? I 
> don't have it hooked up.)
> 
> 
>     - Chuck Messenger
> 
> 
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