I gather the manufacturer wouldn't have put extra traces on the card
    for no reason.

A lot of the inexpensive ethernet cards I've seen have "clever" PCB designs
that allow the same PCB to be used for different transceiever options
depending one which components are populated (ie if FULLY populated, they
do AUI, 10base2, and 10baseT all on one card.)  They might also allow for
the 9V converter to be run from either 5V or 12V inputs, depending on
availability/etc.


    And, if I don't plan on using coax, can I just forget about the 12V
    supply?

I *think* so.  I've not actually designed a PC ethernet card...


    I guess the only real way to find out is to build my circuit
    with only 5V, get it working on the card I have that I know doesn't
    use 12V, then try it with one of these other cards.

Well, you COULD find the actual datasheet(s) for all the components...

BillW

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