From  : Irv Hoff

Date  : 01 June 87

Topic : IMP and DTR


	Nearly all modems permit using an external DTR voltage on the
RS-232 DB-25 connector pin 20.	This is the method by which the operator
can intentionally disconnect the modem when finished.  By either shutting
off that DTR voltage (or momentarily dropping it), the modem disconnects.

	Some computers (such as the Osborne OS-1) do not have this ex-
ternal voltage without making an internal change to the wiring.  A few
modems (notably the Anchor Mark XII) do not permit using this external
voltage.  They do, however, use a software feature that can disconnect
the modem:  Some serial I/O devices (such as the Motorola 6850 ACIA used
in many of the Apple serial interface boards) do not have a DTR pin, but
use the RTS option instead.  That usually comes out to the RS232 DB-25
connector, pin 4.

	IMP now has an option has an option called "NODTR" at address
011E.  If set to 00 ('NO') this assumes you do have DTR wired from the
computer to the modem, with any selector switch on the modem set to
"remote use".  When using ESC N in the terminal mode, or either DSC or
BYE in the command mode, IMP DTR to disconnect the modem from the phone
line.  This takes place almost immediately.

	If you have a modem that does not support DTR (or are using a
computer that does not have DTR or RTS either one), or you prefer not to
connect the necessary wire from this purpose, you can say "YES" to that
NODTR option.  This causes the IMP program to use the command:

	   (1 second pause) +++ (1 second pause) ATH0

to disconnect the modem.  This normally works just as well as dropping
DTR other than it takes 3-4 seconds from the request until the line is
disconnected.

	   NOTE:  This only works for modems that
		  use the Hayes 'AT' protocol.

					- Irv Hoff
					  Los Altos Hills, CA 94022
					  (415) 948-2513 PRACSA RCPM
					  (415) 948-2166 voice