22 Aug
2011
22 Aug
'11
12:19 p.m.
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
DEC stuff almoast always ignores the handshake lines...
networking equipment. All of those will almost always
work with no
handshaking lines at all.
Right,...
I forgot to metnionthat when I am sorting out an unknown device, I link
it ot a terminal which I know the behaviour of (for excample, I know
it'll ignroe handshake lines). Only having one unknown device helps a lot.
Of coure sometimes I am asked to link up 2 devices I know little about.
In this case, I take along a terminal of my own (often my trust HP95LX)
and get ach device working seaprately if I can. It's quicker in the end.
or "creative interpretation of the
standard".
In my experience, it's the devices that interpret the standard to the
letter that cause the most problems. mainly becasue they'll use RTS and
CTS as half-duplex turn-awround signals and confuse the hell out of
anything else...
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
Well, it was useful for the original application (linking a terminal to a
modem, and no other purpose). The fact that RS232 has been used for all
sorts of other applications is not really the standard's fault.
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
I think it's dififuclt to justify that a computer should be a DCE.
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.
The thing that annoys me is that (as commonly implemented), it's not
symmetical, which means that null-modem cables are at best a kludge. OK,
you swap TxD with RxD, RTS with CTS and DSR with DTR, but what do you wap
DCD with (and yse, some devices do insist on it). Most of the time you
can strap DCD input on a DTE to whatever is also driving the DSR signal,
but not always.
-tony