docs/85355: [patch] Error in the pin numbers of the described connector in the Handbook (serial).

[Gary W. Swearingen]

Yar Tikhiy <yar at 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 resource, which is likely to become incomplete, outdated, or down.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.)