Re: very low cost ethernet hub with monitor port?

Robert Redelmeier wrote:
glen herrmannsfeldt <gah@xxxxxxxxxxxxxxxx> wrote in part:

If you can remove the terminating resistor on a dongle
receive port, it should be possible to connect it to a pair
in the middle of a cable, such that the cable can be used
to pass traffic. You won't be using the transmit port for
snooping, so don't even connect that.

Only if the isolation transformer is 10base2-style high resistance.
But what if the isoXfo is low resistance (50ohm)? Why would a
terminating resistor exist? Or is there too much inductance?

The line should be terminated somewhere close to 100 ohms.
10baseT is pretty different, the whole transceiver is isolated
from the AUI cable, but not from the ethernet cable. Collision
detect is based on the DC (more or less) voltage on the cable,
so it can't have a transformer there.

For 10baseT and 100baseTX the line should be terminated at 100
ohms for megahertz range frequencies, the low frequency resistance
of the transformer might be lower. It might be that newer
ones can terminate from inside the transceiver IC, which would
make it harder to remove.

-- glen

.

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