modular null modem pinout?

[maurice.r.baker]

In article <205 at bongo.UUCP>, julian at bongo.UUCP (julian macassey) writes:
> In article <433 at bigbroth.UUCP>, rk at bigbroth.UUCP (rohan kelley) writes:
> > I'm trying to make a direct connect between a bell tech 386/25 and
> > a 3b2/400.  The port on the 386 is an ace intelligent port board
> > with 8 modular ports and on the 3b2 an enhanced port board.
> > 
> > I'm trying to construct a modular null modem cable.  This is the
> > wiring diagram I've come up with, but haven't tried it yet.  Does
> > anybody know if this is correct?
> > 
> > What I'm trying to say is:
> > 
> > G       1    to   1  G
> > CTS     2 tie to  8 (plug)
> > TDXO    3    to   5  RXDO
> > DTR     4    to   6  DCD
> > RXDO    5    to   3  TDXO
> > DCD     6    to   4  DTR
> > G       7    to   7  G
> > RTS     8 tie to  2  (plug)
> > 
> > Neither the 386 or 3b2 have DSR (RS232 pin 6) which is usually tied to
> > DTR.  Is it needed?
> 
>     What you have is not recognisable by me. Here is my RS-232 memory jogger 
> file.
> 
> This may help some people.
> 
      
           ( Handy listing of DB9 and DB25 pinouts deleted)   

It does not appear recognizable because both of these (ACE and 3B2) use
8-pin modular connectors........similar to the ones on a telephone cord,
but with 8 pins.  Besides being cheaper than DB-25 hardware (less labor
intensive and material costs), they take up much less "real estate" in
the machine or on I/O boards.  A lot of RS-232 interfacing these days
seems to be 3-wire "Hayes Smartmodem" kind of stuff anyhow, and they
were able to boil 99% of all RS-232 interfacing down to 8 basic leads.
DSR went by the wayside for modular plugs......according to the Bell Tech
manual:  "DSR is the modem equivalent of DTR.  In common usage, it is
redundant since modern modems are assumed to be functional when they are
turned on".  Likewise, RI was excluded in lieu of the Hayes-like response
codes available for digestion in software.  Now, I'm NOT saying that these
RS-232 lines don't have any uses....quite the contrary, just sharing
an explanation with you.  Please don't flame me --- I miss them too, sometimes,
when confined to the 8-pin modular connectors and/or I/O ports which ignore
them!

For the ACE side, here are the "official pinouts" (from the ACE manual):

	SHIELD		1
	CTS		2
	TXD		3
	DTR		4
	RXD		5
	DCD		6
	GND		7
	RTS		8

For the 3B2 EPORTS side, the list is the same.  (Incidentally, the 3B2
Expanded I/O board is identical, too, except that it does not support
RTS/CTS).   Reference sources are AT&T 3B2 Computer Enhanced Ports Manual,
305-441 ---and--- AT&T 3B2 Computer Expanded Input/Output Capability Manual,
305-530.

When viewing the socket on the I/O board(s), pin numbering as indicated below:
                 ^^^^^^

            Pin 1 --->  ------
                        |:   |
			|:    --|
			|:    --|
			|:   |
            Pin 8 --->	------

I thought that this information would be a useful addition to Julian's
list of RS-232 interfacing facts.

As to the original question, yes......it looks like your null modem cable
should work fine.  Don't forget the little "flying lead" with lug is protective
ground (Pin 1) attached to the post on the 3B2 frame by each I/O jack.  I
suppose a precise interpretation of "Protective (Frame) Ground" in the RS-232
standard required AT&T to not rely on ground continuity through pin 1 and
the I/O board connections into the mainframe. 

One thing to watch out for, though, when working with these little 8-pin
beasties:  there are 2 kinds of cables.  Both have 8-pin modular plugs
on the ends, but one variety has the pins wired identically (Pin 1 to 1,
Pin 2 to 2, etc.) and the other has the pins flipped (Pin 1 to 8, Pin 2 to 7,
and so forth).  The former is known as a D8W distribution cord, 
and has uses in the telephone industry.  The latter seems to be more commonly
available (it's the kind Bell Tech includes with the ACE card, for example) and
is meant for use with pre-assembled modular jack/DB-25 adaptors.

>From the ACE book again, here's the pinouts for these common adaptors:

		"Standard Terminal Adaptor" (Attaches to DTE)

	Modular					DB-25 Female
	-------					------------
	SHIELD	1				SHIELD	1
	CTS	2				RTS	4
	TXD	3				RXD	3
	DTR	4				DCD	8
	RXD	5				TXD	2
	DCD	6				DTR	20
	GND	7				GND	7
	RTS	8				CTS	5

.........................................................................

		"Standard Modem Adaptor" (Attaches to DCE)

	Modular					DB-25 Male
	-------					----------
	SHIELD	1				SHIELD	1
	CTS	2				CTS	5
	TXD	3				TXD	2
	DTR	4				DTR	20 
	RXD	5				RXD	3
	DCD	6				DCD	8
	GND	7				GND	7
	RTS	8				RTS	4

An easier way to make your null modem cable might be to conenct
a terminal adaptor to a modem adaptor, and then plug in modular
cables to the 3B2 and ACE cards.  I might suggest including an
inexpensive DB-25 equipped RS-232 LED "tester" in between the
two adaptors ---- it's cheap (less than $10) and makes trouble-shooting
much easier.  Of course, a protocol analyzer at this point is the
ultimate debugging tool, but costs a bit more :-) 

Our stockroom, for example, contains all sorts of externally-identical
modular-to-DB25 adaptors (none marked in any distinguishable fashion).
As always, the final test is to pop the thing open (screws or press
fit) and see what's inside.
	
A final question for all the net-readers out there:

	Who chose these pinouts?  Why did the make these specific
		assignments?  In particular, why didn't they make
		the physical placement symmetrical with the electrical
		signals (i.e., SHIELD/GND on 1/8, CTS/RTS on 2/7,
		TXD/RXD on 3/6, and DTR/DCD on 4/5 for example) so
		that a "flip cable" would act like a null modem and
		a D8W type cable would connect stuff 1:1 ???
		Seems like that would have been really handy.

Hope this helps with your RS-232 puzzles..............and I'll be
watching for more discussion on the subject.

M. Baker
homxc!mrb1