Seeking modem pinouts (3b1)

[Greg Andrews]

In article <1991Feb20.023929.24345 at blilly.UUCP> bruce at balilly.UUCP (Bruce Lilly) writes:
>>
>>I disagree with that wiring of DTR to CD at the 3B1 end.  It is MOST
>>important for the 3B1 to be able to detect when the modem:
>>
>>1] connects, via its CD signal going true, and
>>
>>2] disconnects, via its CD signal going false.
>
>Unless CD is asserted, a write to/read from a tty port will fail. So
>something like:
>
>echo ATZ >/dev/tty001
>
>will not work, if a modem connected to the port has not asserted CD. The
>connection I gave will work, as DTR is asserted when the port is
>open(2)'ed.
>
>>[Thad's comments about HDB-induced anger deleted]
>
>Nothing in any of the HDB UUCP files will affect the example given above.
>I've been using the connection above with no problems whatsoever.
>
>1)	What will break given the connection I posted?
>
>2)	How would you be able to write to the modem (simply) from the shell?

What will break is detecting that the other modem has disconnected on
you, or that you've lost the connection due to phone line trouble.  Also
your system will not notice when a dial-in user disconnects without logging 
off - your system won't kill their processes and restart getty because
it doesn't know they're gone -- DCD is still on!  Not very secure...

>Sidebar: many modems have a hardware (and possibly software) option to
>continuously assert CD so that writing to the modem is possible when
>carrier is not present (e.g. refer to page 2-4 of the Digicom Systems,
>Inc. 9624LE User's Manual).

Heck, that's the DEFAULT setting for most modems. 

The REAL solution to your Question #2 above and the problem as a whole 
is to change the modem's DCD handling.  What you want is for DCD to be on
always (allowing writes and dial-out access) until a modem connection is
broken.  Then DCD should turn off for a short period of time and come back
up.  You get access to the port and carrier sensing all at the same time.

Telebit modems can control DCD this way by setting S131=3.  If you have an 
older revision with S53 instead of S130/S131, then set S53=4 and wire the 
modem's DSR (pin 6) to the system's DCD (pin 8).  The amount of time DCD
stays off is controlled by S47 (normally 200 milliseconds).

-- 
.-------------------------------------------.
| Greg Andrews      |   gandrews at netcom.COM |
`-------------------------------------------'