21 Aug
2011
21 Aug
'11
1:22 p.m.
On 08/21/2011 12:49 PM, Tony Duell wrote:
Hmmm.. I find rather too many devices expect some handshake inputs to be
asserted before they will do anything. I guess it depends on what you
work on. DEC machines almost never use hardware flow control, but a lot
of test instruemtns do, for example.
Yes, I can buy that. I almost never interface to test equipment via
RS-232. I do lots of HP-IB comm, but I have very little test equipment
that uses RS-232. I'm almost always interfacing either old computers
(and my focus there is on DEC), old networking equipment, or new
networking equipment. All of those will almost always work with no
handshaking lines at all.
Same here, and same here. I've found, however, that in the context
of RS-232, "why it works" is usually preceded by "why doesn't it
work",
and the answer to that is almost always either "the standard just sucks"
or "creative interpretation of the standard".
In the context of networking equipment, it's usually a matter of the
customer standing there sweating, begging me to get something working
because half of the network is down. I could sit there for ten minutes
with a breakout box turning it into a lesson on how that particular
engineer decided to implement RS-232, or I can whip out my "make
anything work" bundle of cables and get the network back up.
I was probably in a rush; my apologies.
It's odd... I would give the opposite advice.
Since I started using a
breakout box (or similar) to see just what the devices were doing I find
I can wirte a serial cable and get it to work first time (at least most
of the time :-)).
Yes, I agree in principle, but in the wild there are really very few
machines that require any of the handshaking lines to be used, and if you're
trying to get a console port talking as quickly as possible (say,
at a customer site) then my approach really is the way to go. Plug,
doesn't work, re-plug, works. I can do that in less time than it takes
to fish the breakout box out of the tool bag.
As ever I like to know why it works. I've been bittne too many times. I
personally use a slightly different procedure in practice. I use
one of the quick-checkers that you describe, though, the one with seven
Ah, so why didn't you say this tyhe first time? Those testers are very useful for finding wheter a
device is a DTE or
DCE, and are certainly the first thing I grab. Can you still get them I
wonder?
Yes, I've seen them new as recently as a year ago. I think Radio
Shack (over here) even still carries them. They're available via eBay
(new) all day long.
There are a couple of other things that I find solve
an awful ot of RS232
problems. The firsti s the 'universal gender cable'. I've never seen
this commercially, but it only takes a few minutes to make. It's a length
of 25-wire IDC ribbon cable with a DB25 plug and a DB25 socket at each
end (say put the socket about 3" along the cable and the plug at the very
end), of course with corresponding pins connected.
Yes, those are very handy. I have a similar one that's
commercially-made; it has both DB-25 and DE-9 connectors on each end.
That was a neat trick; I grabbed it as soon as I saw it!
Incidentally, I've yet to find a male DB25 on a
piece of equipment wired
as a DCE. Male is always DTE. Femal should eb DCE, but in practice it can
be either. HP wre particularly bad about this!. Oh, and computers are
normally DTEs, but there's at least one Apple ][ serial card that is
wired as a DCE only (!).
Since the very first time I fought with an RS-232 interface
(in-yer-face) decades ago, I've (as have you) seen so many violations of
DCE vs. DTE, and of course that distinction was never useful to begin
with, that I've decided (for me anyway) the best way to deal with that
distinction is to ignore it. After all, is that PC on your desk a DTE
or a DCE? Which role it plays depends on what you're doing with it. So
now, what's the "correct" pinout?
That's not the way I generally work, and I know full well that it's
not the way you EVER work, but in this instance, in my experience, this
is the best way to approach RS-232.fucker working
The RS-232 standard is a piece of crap in this regard, but it isn't
going away anytime soon, so we deal with it.
-Dave
--
Dave McGuire
Port Charlotte, FL