Another word to clarify the operation of a PhoneNet adaptor: it allows to connect several Macs and other LocalTalk capable peripheral devices at a time. It allows to set up reliable mid-distance connections of up to several hundred metres, using the cheapest possible kind of wiring. The probably most important feature of the tiny electronics inside the adaptor is to maintain an electrical band pass operation: it must block out low freqency current (DC), like driven from a grounding offset in the line power supply of distant located Macs. It should not allow to pick up high frequency noise from the environment.

High quality adaptors use transformers, there ist also a cheap alternative called CapNet, which uses just a capacitor.

P.S.: some LocalTalk capable devices for the Macintosh decide which protocol to use by an electrical check of the serial connection (like the HP DeskWriter C). It uses a standard serial protocol if connected with a standard printer cable, and the LocalTalk protocol if connected with a LocalTalk adaptor. In case you need a single local connected device of this kind to behave as a network device (and there are cases, where this is necessary), it is possible to spoof a LocalTalk connection by preparing the standard printer cable with a Resistor of about 1000 Ohms, connected from Tx(-), Pin 5, To Tx(+), Pin 4 on the Printer's side of the cable. For details look at:

http://www.fortunecity.de/wolkenkratzer/486/60/cap3.htm