MicroVAX 3100 Questions. - test-drb@ccmp.vtda.org

5 Dec 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 there
...
  >> 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 for
...
  >a loose fitting connector).
 >
 >> For anyone in the UK who needs *a few*, I believe I still have a box
...
  >> 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 on
...
  >> 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->MMJ or
...
  >MMJ->RJ(12/45) cables for $5 + shipping.  I had
a few takers last time
...
  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:
http://www.stokely.com/unix.serial.port.resources/A-B-Ycablepinout.html

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