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From: fair@dual.UUCP (Erik E. Fair)
Newsgroups: net.micro
Subject: Re: Hayes Smartmodem carrier detect
Message-ID: <664@dual.UUCP>
Date: Tue, 10-Jul-84 21:23:28 EDT
Article-I.D.: dual.664
Posted: Tue Jul 10 21:23:28 1984
Date-Received: Thu, 12-Jul-84 01:14:52 EDT
References: <492@trwspp.UUCP>
Organization: Dual Systems, Berkeley, CA
Lines: 106

>> From: brahms@trwspp.UUCP
>> Newsgroups: net.micro
>> Subject: Hayes Smartmodem carrier detect
>> Date: Mon, 9-Jul-84 18:28:46 PDT
>> Organization: T R W, Redondo Beach, CA
>> 
>> Is there any way to make the Hayes Smartmodem drop carrier detect (pin 8)
>> when it detects loss of signal?  Even a hardware hack would be acceptable.
>> 
>> On page 7-12 of the Hayes manual it states:
>> 
>> 	"The Smartmodem 1200 will always act as if a carrier is present".
>> 
>> This makes it impossible for the system to know if the remote modem has
>> hung up.  Thanx for any help you can give.
>> 
>> 			-- Brad Brahms
>> 			   usenet: {decvax,ucbvax}!trwrb!trwspp!brahms
>> 			   arpa:   Brahms@USC-ECLC

Time for lecture #1 on autodial vs. answer only modems and computer
modem control. First some background on modem control:

Some Definitions:

RI  -> Ring Indicator (Someone is calling us on the phone!)
CD  -> Carrier Detect (Someone is actually there, complete with carrier)
DSR -> Data Set Ready (Hello computer, I'm ready (modem))
DTR -> Data Terminal Ready (Hello modem, I'm ready (computer))

The proper way to do modem control on answering (the electrical signals):

MODEM					COMPUTER
					(periodically scans for RI)
RI (I'm calling yooouooou!)
					DTR (OK, answer the phone)
DSR (OK, I'm answering the phone)
					(wait for CD)
CD (Someone is giving us a carrier)
					(print login banner and converse)
					``Frobozz Inc 4.2 BSD UNIX''

This sequence gives maximum control of the modem and the conversation
to the computer. Among other advantages, the modem will not answer the
phone when the computer is down or if the software has been told not to
answer.  Most computers on the low end of the price spectrum, however,
do things this way:

MODEM					COMPUTER
					DTR (``always answer the phone'')
					(periodically scans for DSR)
RI (I'm calling yooouooou!)
DSR (OK, I'm answering the phone)
CD (Someone is giving us a carrier)
					(print login banner and converse)
					``Frobozz Inc 4.2 BSD UNIX''

Note that the computer doesn't care about CD. It watches for
transitions in DSR signal instead. Since answer-only modems of recent
manufacture seem to raise and lower DSR along with CD, this is not a
problem. It does tend to make the hardware in the serial interface and
the software a little easier to deal with, since all you do is tweak
DTR and watch DSR.

What does this have to do with autodialing modems and such? Well hold on,
I'm getting to that...

With new autodial modems you have a paradox. You have to converse with
the modem before there is actually someone there (remotely, that is) in
order to get it to dial. In order to fool the computer (particularly
one that follows the poor man's modem control as detailed above), the
modem must assert DSR whenever DTR is asserted, in order to let the
computer talk to it. With the rich man's modem control, and depending
upon the sophistication of the software, you should be able to tell the
difference between talking to the modem (DSR only) and someone remote
(DSR and CD both asserted).

HOWEVER! Should you desire that your extremely intelligent autodialing
modem should only answer the phone (and that you should be able to tell
when someone has hung up), what you need to cause the modem to do
(particularly with poor man's modem control) is to raise and lower DSR
along with CD. Note, however, that this effectively prevents
autodialing, because you will only be able to talk to the modem when
someone is there on the other end. I believe that the DC Hayes
Smartmodem 1200 has this as a configuration option somewhere on the
back dip switches if you read the manual very carefully. If not, I
suggest that your login process have a 5 min timeout after which the
computer will drop DTR to get the modem to hang up the phone. If your
modem won't do that, then you're really out of luck...

To reconsider this statement:

>> On page 7-12 of the Hayes manual it states:
>> 
>> 	"The Smartmodem 1200 will always act as if a carrier is present".

I interpret this to mean that DSR will always be raised when DTR is asserted
by the computer in order that you can talk to the modem to get it to dial.

	another eddy in the muddy waters...

	Erik E. Fair	ucbvax!fair	fair@ucb-arpa.ARPA

	dual!fair@BERKELEY.ARPA
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