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Quoted from <417@ukecc.UUCP> ["Re: 3B2/300 uucp ports & Permissions"], by edward@ukecc.UUCP (Edward C. Bennett)...
+---------------
| All this raises the more useful question:
| 
|         1. Can the 3B2 control its outgoing DTR signal?
+---------------

$ stty 0 > /dev/ttyxx	# turn off DTR
$ # turning it on is more difficult, you have to open the port without
$ # DCD and ioctl() the baud rate to non-zero (see below)

+---------------
|         2. Can a tty port be opened without the incoming CD signal
|            being high?
+---------------

#include <fcntl.h>
#include <termio.h>
#include <sys/ioctl.h>

#define TTY	"/dev/ttyxx"

main() {
	int fd;
	struct termio ttydat;
	
	if ((fd = open(TTY, O_RDONLY|O_NDELAY)) < 0)
		exit(1);
	ioctl(fd, TCGETA, &ttydat);
	ttydat.c_cflag &= ~CBAUD;
	ttydat.c_cflag |= Bmybaud;	/* substitute your baud rate */
	ioctl(fd, TCSETA, &ttydat);
	exit(0);
}

The above program uses the fact that you can open for O_NDELAY a tty despite
neither DTR nor DCD being asserted; you can't read or write, but you CAN
ioctl.  These are tested under System III (I have a much more complex version
of the above C program, run as ``dtr +tty22'' or similar, ``+'' for on, ``-''
for off), but should work under sys5 because the tty structure and code hasn't
changed (modulo sxt's).

If you want my ``dtr'' program, let me know.

--Brandon
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