On Sun, 14 May 2000, Carl Lowenstein wrote:

> That "all" needs a lot of qualification.
> 
> It is exactly true of all DEC MMJ terminal cables.  It is logically
> true of older DEC DB25 RS232 cables connecting from DEC computers to
> DEC terminals.  (i.e. they are null-modem cables)
> 
> But there are a lot more computers and terminals out in the world that
> weren't made that way.  :-)

Yes....

*But* there is actually a standard here, which few adheres to. DEC and HP
were good at following it.
So, DEC cables and connectors follow that standard, and does things the
right way. When you find something that behaves differently, they are
doing things wrong.

To put it simple here, once and for all:

DTEs (that is, terminals, computers and such...)

DB25 *male*

1 Common gnd
2 TxD
3 RxD
4 RTS
5 CTS
6 DSR
7 Signal GND
8 DCD
20 DTR
22 RI

(I skipped the uninteresting signals here)

A DCE (modems and such)

DB25 *female*

All wires run straight from the DTE, and all the pins actually have the
same names, which means that for a DCE pin 2 is TxD which is where the
modem *recieves* data.

Between the two modems, all signals crosses over (obviously) so that TxD
on one modem goes to RxD on the other modem.

A NULL-modem cable does this cross-over physically in the cable, which
also means that a NULL-modem cable have two DB25 female connectors, while
normal modem cables are straight and have one male and one female
connector.

All this is so very simple and basic that I'm very surprised how companies
(and people) even can manage to get in wrong once. :-)

If everyone would just have sticked to this, we would only have two kind
of cables, and everything would work with evrything else. You'd have modem
cables and null-modem cables. One is straight, with two different genders
at each end, and can only fit one way into things. The other would be a
null-modem cable, which had the same gender at both ends, was crossed, and
could only be used for this purpose. You'd never have problems extending a
cable, you'd just add more modem cables in between. They are the only ones
that fits, and they just runs straight through.

Sorry for the long post, I just had to get this off my chest once and for
all. Now I'll be quiet again. :-)

Oh, RS-232 isn't defined on a DB9 anyway, so technically IBM didn't break
anything, but to me it seems like they did go the extra mile just to
try...

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt@update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol