Preliminary documentation of DC(IV) and DN (IV).

Minimal information regarding the dc and dn interfaces is available in the
UNIX manual under DC(IV) and DN(IV). In brief, the two work together to
connect the PDP/11 to external systems. To dial a number, one must first
open /dev/dn, and then write the telephone number to be dialed by it. For
example,

	i = open ("/dev/dn", 1);
	write (i, "3940291",7);

will dial the 370 from inside a C program.

The statement

	j = open ("/dev/dc", 2);

opens /dev/dc for reading and writing.  Thus the three statements will
allow reading from and writing to the 370 as if it were a file with
descriptor j. The mode of this file  must be set correctly for
communication to work; see stty(II) in the UNIX manual.


The DC-11 and DN-11 (two pieces of DEC hardware) are connected
to a CCU-2 computer calling unit manufactured Timeplex.
Two modem cards are available for the DC-11: a 300 baud full
duplex card and a 1200 baud half duplex card. Different drivers are
required for the different cards.  The standard systems runs with the
driver for the 1200 baud card, which is intended to be used with the New York
Times Information Bank as part of the Intelligent Terminal Project.
We expect to combine the two drivers so that a change of cards will
will be effectively all that is required to change speeds.
