Interesting Apple ][ card

From: Peter C. Wallace <>
Date: Fri Jul 12 12:44:01 2002

On Fri, 12 Jul 2002, Tothwolf wrote:

> On Thu, 11 Jul 2002, Eric Smith wrote:
> > Strange, a 10baseT/100baseT Ethernet interface should be able
> > withstand telephone voltages (even ringing), even if the jack has them
> > wired to the data pairs. Even if it sustained damage, there should
> > not have been any sparks.
> Many of those commercial/digital phone systems often have 24VAC or
> something similar available to power the phone on one of the pairs. Most
> of them also have quite a bit of current available. I've seen a few cable
> pair testers loose their magic smoke due to those types of phone systems.
> -Toth

        100BT Ethernet coupling transformers should easily be able to
withstand most faults from a telephone line, since they typically have less
than .2 Ohms dc resistance. That makes it hard to dissipate much power in the
transformer unless you have much more than 1A available. 10BT transformers
with a common mode choke are around 1 ohm so your 24 vac power would probably
damage them, though normal telephone line current would not...

        As far as damaging the rest of the circuitry goes that depends on
whether the PHY can take the 48V ~1 usec spikes resulting from connection the
isolation transformer input to one (or many) 48V steps as the connectors are

Peter Wallace
Received on Fri Jul 12 2002 - 12:44:01 BST

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