The problem is a line in the term.c file.  The lines that have
to be changed for rn to work are:

	Before change:

		[...]
    		devtty = open("/dev/tty",O_RDONLY);
    		if (devtty < 0) {
			printf(cantopen,"/dev/tty") FLUSH;
			finalize(1);
    		}
    		fcntl(devtty,F_SETFL,O_NDELAY);
		[...]

	After change:

		[...]
    		devtty = open("/dev/tty",O_RDWR);    /* changed for UNIX PC */
    		if (devtty < 0) {
			printf(cantopen,"/dev/tty") FLUSH;
			finalize(1);
    		}
    		/* fcntl(devtty,F_SETFL,O_NDELAY);   /* changed for UNIX PC */
		[...]

	This change is required because the /dev/ph* devices require
DATA connections to be opened with O_RDWR, so changing the O_RDONLY
(or, in some versions of rn, the constant 0) to O_RDWR will solve your
problems.  Note that these changes are also required for rn-derived
programs like trn.