ArcNet and the Pursuit of Multiple Topologies

From: Pete Turnbull <>
Date: Sat Oct 13 05:34:34 2001

On Oct 12, 13:46, Eric J. Korpela wrote:

> Just wired the house with Cat-5 this week. 10bT has the advantage that
> also 100bT with a change of equipment. (ISTR that you can use the
> unused pair in the cable for LocalTalk, but I haven't yet tried it).

That's true, you can run all sorts of things over Cat5 "structured wiring",
in fact that's the point of structured wiring. 10baseT uses two pairs
(pins 1+2, 3+6), the phone uses the centre pair (pins 4+5). ISDN uses all
four pairs.

Serial lines use either the middle pairs (for Tx/Rx) or all four (DTR/DSR,
Tx, Rx, RTS/CTS) if they follow the DEC (and other) system. Some use the
same pairs as Ethernet (which means a crossover cable for Ethernet also
works as a null-modem for Tx/Rx on serial lines).

100baseTX uses the same pairs as 10baseT.

You're not supposed to run a phone line (pins 4+5) on the same cable as an
Ethernet (pins 1+2, 3+6); it breaks the spec. However, you'll normally get
away with it with 10baseT, and it *may* work for 100baseTX over short
distances. It certainly won't work for 100baseT4 (does anybody still have
any of that?) or 1000baseTX (Gigabit) because they use all four pairs.
 Nevertheless, you can buy little boxes which are essentially a Y-piece for
phone+10baseT -- you need one at each end of the cable. They consist of a
small box with two RJ45 sockets and a short Cat 5 lead ending in an RJ45
plug. You can also get similar boxes to combine/separate two 10baseT
signals (they move the second signal from pins 1+2 and 3+6 onto 4+5 and
7+8) and ones to simply double up the sockets (pin 1 to pin 1 to pin 1; ...
; pin 8 to pin 8 to pin 8) which are sometimes used for ISDN. For some
reason they tend to be different prices, and all overpriced (over here they
cost ?8 - ?15, though you can make one for about ?3 including the cost of
the box to put the PCB in).

I'd be wary of running Localtalk over a cable carying a phone or Ethernet
signal. Again, it might work, but it will probably generate interference
which might be hard to track down -- I write from experience[1]. Better
to use one cable for one service at a time. The real reason for using
different pairs for different services is partly serendipity, and partly to
avoid damage if you plug the wrong device into a cable which is already
connected to something incompatible at the other end.

In my workshop and study I have quite a lot of Cat5 (actually Cat5e, the
Cat5 standard is officially obsolete) which I use for 10baseT, 100baseTX,
serial lines, ISDN, a phone line, and even the audio to the speakers. I'm
about to add a couple of Econet adapters, and I'll probably use it for some
LocalTalk (PhoneTalk) when I get round to sorting out the non-Ethernet

[1] More than one bad experience, but exemplified by the occasion when I
had to make a trip to Northern Ireland with a couple of technical staff
from Acorn to visit some schools who had networking problems. The school
staff insisted everything was done to the standard, but it turned out that
their erratic network problems were due to putting LocalTalk down the
existing network cables. It had worked for a while, but when they extended
the networks, both suffered.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Sat Oct 13 2001 - 05:34:34 BST

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