From: utzoo!decvax!harpo!seismo!hao!hplabs!sri-unix!chris.umcp-cs@Udel-Relay
Newsgroups: net.unix-wizards
Title: Re:  Dialing In/Out through same port
Article-I.D.: sri-arpa.904
Posted: Wed Mar 30 17:35:41 1983
Received: Mon Apr 18 08:02:40 1983

From:  Chris Torek <chris.umcp-cs@Udel-Relay>

From:	Mark Callow, Qubix Graphic Systems Inc.
		...decvax!decwrl!qubix!msc
		...ucbvax!ucscc!qubix!msc
	
	I would like arrange to use our dialout port for dialups
	as well.  Does anyone have any ideas how to accomplish this
	feat that would work on 4.1bsd?

	One suggestion I've received requires ther ability to
	dialout without the Carrier Detect line being high, yet
	requires the CD line to function normally on dialups.  I
	can't think of a way to do that.

It's not really all that hard.  We've got a 4.1 vmunix running
right now that does just that.  Basically, you need some extra bits
for each line: an outgoing flag, and a soft-carrier flag.  When
checking for carrier you ask, is either the soft carrier or the
real carrier on.  For dialing out, you set the outgoing flag and
the soft carrier, then open the line.  When you finally close the
outgoing line, you clear the outgoing flag and the soft carrier.
For the incoming line, you wait until there is a real carrier and
no outgoing flag, then return the opened line.

Another change we've made is a "passive" mode.  We have two Vaxen
hooked together with a cable that connects one end's DTR to the
other's CD.  For "passive" mode, you DON'T set DTR until you get
a carrier; for dial mode, you set DTR and the outgoing flag, as
above.  Thus either machine can open the dial line to get to the
other, where the other just tries to get someone to log in all the
time.

Yet a third change is a mode which uses the LMDMBUF bit on one end
and the DTR line on the other to do high-speed flow control.  When
one end wants to stop the other, rather than sending ^S, it drops
DTR.  This makes the other end's CD go off, which in LMDMBUF mode
stops that end.

We'll send complete details once we're sure everything works right.
Currently we've got a bug in passive mode, and the LMDMBUF mode
is acting very strange.
				- Chris