So... just out of curiosity, is your AAUI transceiver one of those ones that has two RJ-45 ports on it? Those essentially have a two port hub built into them. (They were designed so "Mac People" who were used to LocalTalk-style daisy chaining could sub Ethernet in the same geometry. It's actually a pretty stupid idea and violates a couple rules of Ethernet network design but it did work after a fashion. The drawback of them is they by definition mean your onboard Ethernet chip behind the AAUI port can't operate in full duplex mode even if it's capable.)