On Mon, 29 Jul 2002, M L Riechers wrote:

>
> On Fri, 26 Jul 2002 10:14:02 -0700 (PDT) Bill Studenmund
> <wrstuden@netbsd.org> sez:
>
> A nifty idea, but RxD to DTR?
>
> Sorry to nitpick, but IIRC, the RS-232 transmit line (or receive --
> all point of view), is normally held to low (i.e. < -3.0 Volts, or,
> these days, nominally at GND), while DTR asserted will be at > +3.0
> Volts.  With your setup, when disconnected, his receive would see a
> _constant_ break condition -- as long as DTR were and continued to be
> asserted.
>
> If DTR were negated, of course, it would work perfectly well.
> Possibly some tty's do that as a matter of course, particularly in
> local mode.

Doh, yes. Actually TxD would probably be better.

> 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.

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

Take care,

Bill