In my collection of computer junk I have several items of equipment that only have an Ethernet AUI(the 15 pin 'D') connector. To connect these to my 10base2 (thin-net) lan I needed a tranciever/AUI. A bit of thought led me to have a look at some old Ethernet cards that have the AUI connector and the BNC. Tracing out the circuit reveals something like this ...
When the jumpers are in the AUI position the signals from the board go straight out the AUI connector. When the jumpers are in the BNC position the signals go to the on board tranciever. BUT! if we jumper the pins (by soldering on the back of the board) as marked in the drawing (from one side of the block to the other, not via the centre row of pins) we have the AUI connector connected to the 10base2 internal tranciever. A male to male 15 pin D cable gives us an AUI/10base 2 tranciever!
Note that the signal lead will go through the header but some cards don't have the +12 going through the header. This may not be a problem, but it will leave you with the rest of the +12 on the card energized from your AUI connection.
Even worse are cards that use a +5 to -5 volt converter for the on-board tranciever. To use these you would have to put a regulator on board to drop the +12 to +5, more effort than its worth when you can just rummage through the junk-box to find a card that is more usable.
Note also that modern (ie probably useful as Ethernet cards in their own right) Ethernet cards don't use a header to switch between interfaces and are useless for converting this way.
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