Well, I pulled out my copy of H&H, and, while I don't
have Wagner's experience (It was but a mere fifteen
years ago that I built my first modem cable), I can
say that Wagner is right and <gasp> H&H is wrong
(although I must say that H&H to me never seemed the
place to go for a reference on comm protocols).
Doing a quick google search, I found the following
page, that seems to explain the signals pretty well:

   http://www.sangoma.com/signal.htm

The odd thing to me is that in Table 10.4, at the top of
page 724, H&H has it right. The table calls RTS "Request
to Send", while the text right below calls it "Ready to
Send". If you buy into how the text describes things,
then there is no functional difference either between RTS
and DTR or between CTS and DSR; the text claims that these
pairs are always asserted exactly in lock-step.  If this
was true it would be an enormous waste of a control line.

Still, there is the old aphorism, "The great thing about
the RS-232 standard is that there are so many of them".
People have been abusing RS-232 for as long as it has
existed, and probably before. My guess is that this is
because RS-232 is so over-engineered; there's all those
control lines that sound too much the same, and there's
this inexplicable *appearance* of symmetry where there
shouldn't be any (why on Earth, for example, is the
DTE given a CTS pin?). Look at I2C, RS-485 and LVD, for
example -- people have learned a great deal over the past
few decades about making interfaces. Perhaps if we are real
lucky, USB will finally kill off RS-232 once and for all,
with RS-422, RS-485 or LVD being used where USB is simply
not practical.

--Bob

On Wed, Apr 07, 1999 at 08:47:06PM -0400, Wagner Lipnharski wrote:
> Dear Ian, excuse me if I am bold, but I worked 19 years at IBM and 14 at
> teleprocessing dealing with all kinds of telecommunication systems and
> ...

--
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Bob Drzyzgula                             It's not a problem
bob@drzyzgula.org                until something bad happens
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