The following reply was made to PR docs/85355; it has been noted by GNATS.
From: garys@opusnet.com (Gary W. Swearingen)
To: Yar Tikhiy <yar@comp.chem.msu.su>
Cc: jpeg@thilelli.net, docs@freebsd.org
Subject: Re: docs/85355: [patch] Error in the pin numbers of the described
connector in the Handbook (serial).
Date: Mon, 29 Aug 2005 07:52:16 -0700
Yar Tikhiy <yar@comp.chem.msu.su> writes:
> It might be reasonable to tell that in general, a null-modem cable
> is for connecting a DTE directly to another DTE. AFAIK, null-modem
> cables can be constructed for synchronous comms, too. Then, the
> topic can be narrowed down to async comms.
I doubt if Julien wants to do more rewrites on the section. But I
can't resist replying anyway; maybe Yar wants to rewrite it later.
It probably should have some of the cable stuff from the "Serial
Ports" section which says that a null-modem is AKA DTE-to-DTE cable
and what DTE is, eg, a computer. And terminals traditionally have
included teletypes and printers. (Once there were no CRTs or LCDs.)
> To me, www.hardwarebook.net doen't seem the definite resource.
> IMHO, if the topic is rather wide, the reader should better be
> hinted to do a (re)search on the Net instead of pointed to a single
> resource, which is likely to become incomplete, outdated, or down.
I was thinking the same things.
> Apropos, has there ever been a DTE printer? I think that printers
> or sync comms shouldn't belong there if it were told above that we
> would deal with async DTE-DTE comms only in this section.
Serial printers were once common (I have one) and I think few, if any,
were configured as DCE (eg, modems); the bulk were DTEs. But sync
comms don't need to be mentioned; I don't know if FreeBSD can even
handle it. I just wanted some note about the large number of
null-modem designs for different purposes, for folks raised on USB.
> We may show two or three different designs in the handbook if we
> can tell the reader about their merits. The problem with the design
> currently in the handbook is that it is erroneous *and* bogus. I'd
> suggest adding another row to the table so that it becomes evident
> that DTR on this side is connected to DSR+DCD on the other side
> while DTR on the other side is connected to DSR+DCD on this side.
The Note below the pin-out is supposed to make it evident, and with
the Note the design is symmetric.
> An RS-232 null-modem cable should be symmetric, to my mind.
A "typical async null-modem cable", yes. But few of the many RS-232
null-modem cable designs shown in the book are symmetric, owing to the
variety of designs of much DTE. (Less true today than yesteryear.)
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