MicroVAX 3100 Questions.

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Sun Dec 5 17:19:54 2004

On Dec 5 2004, 17:48, Scott Stevens wrote:
> On Sun, 5 Dec 2004 13:01:29 -0500, Pat wrote:
> >On Sunday 05 December 2004 06:43, Pete Turnbull wrote:
> >> Better than hacking RJ45s would be to hack RJ11 (6P/6C) plugs.
> >> They're very common, about a quarter of the price of MMJs, and
> >> are low-cost crimp tools available.
> >
> >Agreed, the only difference between an RJ12/RJ11 and MMJ is the tab
> >location, which can be easily sliced off (though that usually makes
> >a loose fitting connector).
> >
> >> For anyone in the UK who needs *a few*, I believe I still have a
> >> of adapter cables for Emulex terminal servers. These are short
> >> (about 8-10 inches) flat cables with an MMJ on one end and an RJ45
> >> the other. You can have a few for the cost of postage.
> >
> >And if anyone in the US wants one, I can do up to, say, 25ft
> >MMJ->RJ(12/45) cables for $5 + shipping. I had a few takers last
> >I did this, and they seemed to be happy with my handiwork. : )
> >
> If you made something like this, would the RJ-45 end plug into one of
> the serial adaptors that I got from the back of a rackmount US
> Robotics modem box? It's one of those plugs that adapts from RJ-45
> jack to a DB-25 male plug.

Pat can tell you how his cables are wired, but the easiest way,
especially with flat cable, is pin-to-opposite-pin (ie, bearing in mind
that an MMJ has 6 pins and an RJ45 has 8, it goes MMJ-1 to RJ45-7,
...., MMJ-6 to RJ45-2). This happens to be the way DEC made most of
their MMJ cables. DEC also used a standard signal arrangement, with
the two centre pins being ground, the two next to those being Rx and
Tx, and the outermost two being the handshakes. That meant a
null-modem was just a question of plugging in the cable between console
port and terminal.

The reason I mention this is that *some* RJ45-DE9/DB25 adapters and
*some* serial-on-RJ45 ports use a similar convention; Suns and Cisco
kit tend to do so, and some terminal servers, but not all. Take a look
at the YOST page: http://yost.com/Computers/RJ45-serial/ and also at
Celeste Stokely's serial port pages:

There is one other quite common method, though; we use it at York, and
I've seen it in several other places. Based on the assumption that we
only need Rx and Tx, no handshaking, we wire Tx to RJ45-1, Rx to
RJ45-3, and ground to RJ45-2 and RJ45-6. Why? Because 1+2 and 3+6 are
twisted pairs in Cat5 and they are the pins used by Ethernet cables, so
it means we can use standard Ethernet Cat5 cables for straight-through
connections, and standard Ethernet crossover cables as, well,
crossovers (I couldn't really call them null-modem as they don't carry
handshake signals).

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Sun Dec 05 2004 - 17:19:54 GMT

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