in my PIC&Ethernet quest, I have aquired a bunch of NE2000
    cards based on the RTL8019AS chip.  Some of them clearly have no
    connection to the +12V pin on the ISA bus - a bonus as then all I need
    is +5V. I do have a few cards that the +12V line is connected to though.
    However, they go to a component that I can't identify. I tried chipdir
    and a google search, and although there were some matches, there was no
    ID of what it might be. The closest thing was a product table for
    transformers. Question is, why would a transformer need +12? Anyway, the
    part number seems to be CFUS0509-C. It's a 24 pin wide DIP, and it's a
    big tall block, not an IC.

This is very likely to be an isolated 5V to 9V DC/DC converter.  I believe
that cards with an AUI (15 pin) connector will require 12V to power the
transceiver at the other end of the AUI cable (the requirement for 12V is
part of the AUI specification.)  Cards with "thinnet" connectors (10base2:
BNC connectors) use the 5V/9V converter to power the section of the circuit
that actually connects to the coax (and need to be at least somewhat
isolated from the PC's power supply.)  A card that does ONLY twisted pair
shouldn't need 12V OR the DC/DC converter.

You might look for some thinnet schematics/appnotes to confirm this; check
for vendors who actually make the tranceiver internals.

BillW

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