Brian D Chase <brianc@carpediem.com> wrote:

>DB9                   DB25
>-----                 -----
>pin 5   is wired to   pin 7 
>pin 2   is wired to   pin 3
>pin 3   is wired to   pin 2

>This works really well for me... but I have no idea if it will work for
>anyone else as RS232 specs seem to be either fundamentally random or a
>function of your location in the time-space space continuum modulo the
>number of M&Ms you've eaten in your lifetime.

That's a PC cable.

(Ok, what happened was, the DB25 being massive luxury overkill
when not talking to a modem, everybody subsetted it their own way.
But you know that...)

All pins below are as seen from a terminal/computer/printer.

When Digital went to 9 pins to save cost/space, (VT220 printer port, 
microvax console, maybe even earlier?) they did it the minimalist
way, just moving DTR and RI up, and keeping the basic stuff in place.
2-transmit, 3-receive, 5-DTR, 6-DSR, 7-signal ground, (forgotten the rest).
A console would probably only use 2,3 and 7.
(The shorting together of 8 and 9 in a console cable has significance at
least for a VS2000, which does console on the workstation screen otherwise)

IBM, when they did it (on the AT?), were more 'innovative', so:
1-DCD, 2-receive, 3-transmit, 4-DTR, 5-ground, 6-DSR, 7-RTS, 8-CTS, 9-RI
Serial ports on DEC Alphastations follow this convention.

#flame on
One would have thought that the lesson would be painful enough to do it
better next time, but no, the modular connector was so cheap that
it was soon used for all the wrong things, and so we still have to have
straight AND twisted ports and/or cables for TP ethernet.
There is a perfectly good 4-pin hermaphrodite plug on my telephone headset 
cable, which would have made it impossible to connect wrong, and made every
cable an extension cable. Sigh.
#flame off

Btw, for terminology snobbery, it's actually not a DB-9 but a DE-9;
the *B* is the size of the plug body, which is clearly not the same
as that of the DB-25... 

Thus:
DA-15 (ethernet Xceiver connector)
DB-25
DC-37
DD-50 (triple row)
DE-9
and there is also a D?-23

For high pin density we get:
DA-26
DB-44
DC-62
DD-78
DE-15 (VGA video connector)

Then there are the coaxial variants and the pin/coax combos and...
Oh, were you sleeping? sorry :)

Stellan