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PNC_003 Phonenet vs. FOCUS Turbonet

Phonenets are better that FOCUS Turbonet

Problem:

FOCUS keeps saying there stuff is better

Truth:

The greatest difference between the two connectors is in the input noise reducti on circuit. The PhoneNET circuit consists of a resistive attenuator which effect ively increases the DC impedance of the isolation transformer's secondary as see n by the receiver circuitry in the Macintosh. This prevents noise from being int erpreted as valid signals to which a sending node should defer. In the ST, the c ircuit is a "self-terminating" network consisting of two 120 ohm resistors in se ries with back to back LEDs. In effect, this circuit draws current from the netw ork and turns it into light. The amount of current drawn, and therefore the amou nt of effective resistance the connector presents to the network, varies with th e level of the applied input voltage. At a differential voltage of 3.4 volts, th e ST draws 6.25 mA which is equivalent to 540 ohms of termination resistance. At 2.6 volts, it draws 3.33 mA which equals 780 ohms. Below 1.7 volts, the ST draw s negligible current and is equivalent to an unterminated PhoneNET. The ST's sel f-termination circuit is never equal to 120 ohms of termination.

Nuvotech claims this circuit eliminates the need for external termination. Beyon d the obvious flaw that termination in the connector does no good for a network from which the connector has been removed, the claim is unfounded because the te rmination value presented by the connector does not match the line impedance. At high signal levels the ST does sink some current, which dampens the line more t han no termination but less than 120 ohms. This loading causes signal levels to be reduced faster than they would by the higher impedance PhoneNET connector. In long networks with many connectors, this phenomenon could degrade performance, although the clamping action of the circuit mitigates the effect as signal level s approach 1.7 volts. At the low levels found in larger networks, there is no da mping, and the long test networks described below demonstrate the effects of no termination under these conditions.

It should also be noted that as the signal level into an ST connector decreases, the light output of the LEDs decreases. Signal levels too low to excite the LED s are more than adequate for correct operation of a network. Signals transmitted from a device directly attached to an ST connector are always at high levels, l ighting the LEDs brightly. Therefore, a blinking LED on an ST connector does not ensure network connectivity nor does the absence of light indicate a failure.

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