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How to build baluns for 10Base5 Ethernet

Newsgroups: comp.dcom.lans,comp.dcom.telecom.tech,sci.electronics
From: oberon@world.std.com (David S Madole)
Subject: Re: Any way to run 10baseT with less than 4 wires?
Organization: The World Public Access UNIX, Brookline, MA
Date: Mon, 28 Mar 1994 12:44:22 GMT

acg@kzin.cen.ufl.edu (Alexandra Griffin) writes:

>As the subject line says, I'm looking for a way to run 10baseT connections
>over fewer than the usual 4 wires.  Specifically, I'd like to provide 10baseT
>connections over existing RJ-11 jacks, where the center two pins are already
>used for voice, and the outer 2 are not used.

>Just thinking about it from a circuit-analysis perspective, I would think
>that by using isolation transformers & maybe some other matching components
>at each end of the connection, it might be possible to convert 10baseT
>lines from differential-mode to single-ended-mode operation.  If this could
>be done, what are the prospects for using one side of the voice pair as a
>"common" terminal between the transmit and receive side of the 10baseT link?

>i.e.:      Wires in jack:
>            O   O   O   O
>            \   \   \   \
>             \   \   \   \___ 10baseT transmit
>              \   \   \______ Voice telephone line signal
>               \   \_________ Common wire ("ground" reference for all signals,
>                \                Ethernet and voice)
>                 \___________ 10baseT receive

Don't bother trying. Your transmit and receive pairs need to be just that - pairs. Single wires won't cut it trying to carry 10 megabit data rates. Particularly with this pinout, since pins 1 and 4 in your jack should be paired together. The crosstalk after just a few feet of wire would wipe the signal out completely.

If you need to run ethernet on one twisted pair, you should use 10base2 (thin coax) equipment with baluns. These convert the unbalanced coax signal into a balanced signal usable on twisted pair. They are available in many of the wiring products type catalogs. I've seen them most recently from DataComm Warehouse for $19/each, which is high.

I've modified a number of IBM3270 type baluns, which are available for $4 or so to work with 10base2. They are probably not as good as the ones built specifically for ethernet, but I've used them through a few hundred feet of good quality (Catagory 5) cable. These contain (at least the cheapest ones, which is what you'd want) nothing more than a small 1:1 toroidal transformer. You need to rewire the leads to connect it as a current balun, like this:

Before:

BNC Jack  (.)-----)||(-----+
           |      )||(     |
           |      )||(     +--->
           +------)||(---------> Modular Jack

After:

BNC Jack  (.)------------+
           |             |
           +------)||(---+
                  )||(
                  )||(
              +---)||(---+
              |          +----->
              +----------------> Modular Jack

This does not provide proper impedance matching of the ~100 ohm twisted pair to the 50 ohm coax, but testing seems to indicate this doesn't matter much.

I'd check out the wiring you are planning to use before you bother with any of this, though. If it's not true twisted pair, and not at least Category 2 (preferably 3 or better), you probably will not get usable results.

E-mail if you decide to you the baluns and need more clarification. Good luck.

oberon@world.std.com


author: Tomi Engdahl