LAN Product Overview

Fast Ethernet

Hubs & Switches

Desktop Connectivity

Mobile Communications

Timbuktu Pro

Netopia Internet Routers

Dr. Farallon Performance Clinic

Free Eval Software

Upgrades

Where to Buy

On-line Store

Support

Corporate Info

Search

Site Map

Product Registration

Job Board

PNC_004: Balanced Signaling or "Why Polarity doesn't matter"

Why are PhoneNET connectors polarity insensitive?

Problem:

How come polarity of the two wires used by our PhoneNET connectors doesn't matte r - it works no matter which way you attach the two wires?

Situation:

This is just in case some noid maniac out there really wants to know what balanc ed signaling is all about.

Solution:

Well, binary signals are always represented by a measurable electrical quantity (like voltage or current) which is compared to a reference like 0 for example.< p>

In unbalanced systems, such as the digital signals within a Mac, the binary sign als are most often represented by voltages and the reference is derived from som e common voltage levels such as ground and the +5 volts used to supply power to the ICs.

In balanced systems, the reference accompanies the data signals, thereby increas ing the number of conductors required to send a signal from one point to another .

In an RS-422 system (or Local Talk) the signal transmitted is represented by the voltage DIFFERENCE, between two conductors. So if one wire of the two in a pair is more positive than the other by more than 0.5 volts or so, one binary value is represented. If the same wire is more negative than the other by more than 0. 5 volts or so, the opposite binary value is represented. The receivers in RS-422 /LocalTalk pay attention to the voltage difference between the two wires, not th e absolute value of those voltages with respect to a local ground reference. Vol tages imposed on BOTH wires of a pair are called common mode signals and they ar e ignored by the receivers since the same voltage added to both wires does not a ffect the difference voltage.

Noise from electrical wiring, radio stations, lightening, etc. generate magnetic or electrical fields that induce voltages on both wires of a pair equally, so t hey are ignored. If, instead, the signals were transmitted on a single wire and the voltage on that wire were compared to, say, the green wire ground in the ele ctrical plug of each receiving machine, the noise would not affect the reference level but would affect the signal, and errors would occur.

Click to send questions, comments or problems regarding Farallon products and services or our Website.