I'm why Herb is telling you to disable Plug & Play (dead) mode.

I had this problem.  The exact problem.  And guess what?

My AMI BIOS is version 1.00.10.AX1.  My 3c509 is a brand new 
AUI/TP.  

Actually, I have two of each (BIOS & 3com).   None of NetBSD, FreeBSD, 
or Linux could talk to my 3c509s until I turned off Plug & Play.

Now all three operating systems talk to the cards just fine.  

Linux just does.  NetBSD will if you ifconfig link0 link1.  
FreeBSD will if you ifconfig link2 and have never ifconfig'ed any 
other link flags.  That last detail cost me a lot of time.

Good luck,

-Dave