To: M L Riechers <mlr@rse.com>
cc: <port-macppc@netbsd.org>
Subject: Re: db> message after some hours (using Serial Console only)
In-Reply-To: <200207292225.SAA22994@nnwest.rse.com>
Message-ID: <Pine.NEB.4.33.0207311003260.4062-100000@vespasia.home-net.internetconnect.net>
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On Mon, 29 Jul 2002, M L Riechers wrote:



On Wed, 31 Jul 2002 10:07:13 -0700 (PDT) Bill Studenmund
<wrstuden@netbsd.org> averred:

> ...

> Doh, yes. Actually TxD would probably be better.

Hmmmmmm.  (Mumble, mumble, mumble, etc, etc. as he mulls "good
positive feedback").  Perhaps you're right about that -- if the
ne'redowells are _really_ grounding the RD+ differential inputs.

>> Perhaps a better idea would be to use GND instead of DTR?
>
> No. The spec says (for RS-232) a space (0) is > +3 V, and a mark is < -3
> V. Ground is not a valid voltage. Using ground relies on the hysteresis of
> the receiver.

You're right, of course.  The specs really do say that. However, most
RS-232 receivers (and all of those I've used and/or run across) are
perfectly happy receiving data switched between ground and, say +5
Volts. So you can run your 5 Volt only system into a terminal or
modem, for instance.

> For a mac it'd be real bad because the mac really has differential
> data input. Typical serial cables tie RS-232 data in to RD-, and ground
> RD+....

Aaaarg, Aaaarg, and double Aaaarg.  That's most incommodious, and
shame on them.  The trick above is to set your RD+ to ~ 3.0 Volts,
(say, actually about 3 diode drops from ground towards +5 (or whatever
hi) ~2.4V) simulating a reasonable 3.0 Volt decision point that RS-232
would use.  (I know, I know, -3 to +3 (supposed) hysteresis, but it
works. I suspect our modern chips are a bit more sophisticated than
the spec developed (can it be?) some 40 years ago).

Serial cables ground the RD+ input, huh?  Can't afford $0.001(US)
diodes and resistors, maybe?  (More likely, can't find a good source
of hi?)  Maybe that's a source of some of the troubles and confusion
people have had hooking up to macs.  Maybe me, for one.

> .... If you also ground RD-, then the data input is determined by a battle
> of the grounds. That would be VERY noisy.

Amen, brother.  For one, see above.

> Take care,
>
> Bill

-Mike