Phone Talk / Apple Talk

From: CRC <technobug_at_comcast.net>
Date: Tue Aug 31 13:38:48 2004

On Mon, 30 Aug 2004 17:53:08 -0700 Ron Hudson
<ron.hudson_at_sbcglobal.net> asked:
>
> Anyone know how these are wired inside? could I make a dedicated 3 node
> "wire" out of just
> wire and connectors? perhaps an odd resistor or two?

First some definitions: Appletalk is the protocol that Apple used for
connecting to printers and other computers. Localtalk is the hardware
specification. Looking around for the latter will give you a lot more
information.

The problem Apple (and others) solved is how to connect multiple RS232
(RS422) devices together without blowing everything to bits. Their
solution was to use a transformer with two independent windings on the
primary and one on the secondary (along with a few resistors). The
secondary windings are paralleled among the various devices. The
primaries are connected to the Tx and Rx of the RS232. This makes for a
simple two-wire network. Normally, the ends of the secondary bus are
terminated in 110 Ohm resistors.

Apple's implementation used proprietary cables. Third party boxes
(Farallon, etc.) used RS11 phone cables. I've seen these boxes at
various charity outlets in the recent past going for next to nothing.
If you have an old phone modem laying around (parts - not
collectable...) you can use the output coupling transformer to make an
adaptor. They generally have the two-primary/one-secondary format and
have the correct ratio. The receiver side of the primary should be
terminated with a couple hundred Ohm resistor.

As a side note, there are a lot of Localtalk printers and the like
showing up for cheap. Connection to Windoze and Linux boxes are
generally a problem. There are cards that talk Appletalk, but they are
fairly scarce. However, there are a number of bridges coming available
that connect ethernet to Localtalk and the Appletalk drivers are
available for both OSs.

        CRC
Received on Tue Aug 31 2004 - 13:38:48 BST