Could someone running one of these 3COM cards tell me what the
trick is?

I have tried both cards. I have tried hardcoding interupts in 
the kernel config, etc.  I am using the BNC connection so
I should not need *any* flags for ifconfig if I understand
the manpage and recent discussion correctly.

I have used these for quite some time with BSDI and have 
found that we *must* put the 509 in EISA mode before using
it in an EISA slot. Is the same true for NetBSD?

Still, it seems that the EISA (579) version should "just work".

My results:

With the 509 I get SIOCAIFADDR: address already in use
With the 579, it hangs configuring the network.

These cards (if I remember correctly) do some odd things
with port 0x100? The closest thing I have to that address is
a couple of ports on a Hayes ESP.
com2 at isa0 port 0x1e0-0x1e7 irq 9: ns16550a, working fifo
com3 at isa0 port 0x1e8-0x1ef irq 5: ns16550a, working fifo

On a perhaps related note, I get the following messages
booting this machine.
Aug 19 21:53:23 sierra /netbsd: root device eisa not configured

I have found that if I attempt to config the eisa bus option before
building a kernel, I get the following:

SIERRA:86: eisa0: unknown device `eisa'


The hardware is a:
Tyan 1462 PCI/EISA motherboard.
48MB RAM
133mhz P5

Any hints on PCI/Pentium tweaks would be appreciated. This is
my first go with NetBSD on anything beyond a 486 ISA.