Swift Griggs <swiftgriggs%gmail.com@localhost> writes:
> On Fri, 23 Dec 2016, Greg Troxel wrote:
>> Are you saying that the console device itself will refrain from
>> output if either DSR or CD is not asserted? I can see the point of
>> DSR but requiring CD for a console seems non-helpful.
>
> Hmm, out of ignorance, ('cause I wouldn't gainsay you, Greg!) why?
> Carrier detect is pretty modem-ish, but my simple understanding is
> that when using a null modem, you want to connect DCD to DTR and the
> same for DSR. I've even built cables this way and they worked. It's
> all just non-magical 12V low/high. That way you've got a "high"
> signal telling you "Yeah, it's cool to talk. We're connected." For a
> modem, that has the additional meaning "You've got a carrier signal"
> rather than just "The cable got plugged in.".
>
> Straigten me out, guys. Am I wrong?
You are not really wrong in theory.
But, when you connect two DTEs with a null modem, there really is no
carrier detect. And putting a console on an actual modem was odd way
back when (normal would be a DECwriter II on a DL11, and a DZ11 with 8
ports wired to a bunch of modems, on a PDP-11 in 1980 :-). There, getty
waiting for open to succeed until CD was asserted made sense, especially
when the modem/line was shared with outgoing UUCP.
A modem console would be beyond bizarre these days.
My point was really that if the cables are not wired up right and DCD
ends up not asserted (there are a lot of wrong serial cables out there),
then it seems better to just have the console work, rather than not
work.
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