21 Aug
2011
21 Aug
'11
9:49 a.m.
On 08/20/2011 02:04 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.
[RS232]
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 Ignore the standard, and leave the break-out
box in the drawer.
After years of practically memorizing the (horrible) standard and trying
to do things "by the book" analytically, etc, I refer to it as "RS-232
in-your-face" rather than "interface".
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 :-)). 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?
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.
The 2 adapters I use a lot are a null-modem including the handshakes
(crossed TxD and RxD, crossed RTS and CTS, crossed DTR with DSR strapped
to CD) and a null modem with looped handshakes (crossed TxD and RxD, then
RTS linked to CTS at each end and DTR linked to DSR and CD at each end).
99% of problems can bve solvd with those.
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 (!).
-tony