I should clearly have been more specific.

My NetBSD machine has no problems doing SLIP or PPP with plenty of other
terminal servers, including other Xylogics Annexes configured damn' near the
same right down to the IP addresses on both ends of the link.  The Annex I 
*can't* connect to has no trouble connecting to anything _else_.  It's not
a routing problem; trying to get things going immediately after a "route flush",
with either a default route added by hand or with the pppd defaultroute option,
doesn't work -- though this works fine with other Annexes, as I said above.

It really really looks like some kind of weird protocol incompatibility.  I
have no idea if it's NetBSD's fault or Xylogics' fault; all I know is that the
NetBSD machine sends stuff, and gets nothing back.  (As the modem lights are my
witness. :-)  Next step, I guess, is to upgrade the Annexes to _yet_another_
unofficial_Xylogics_patch_release, and call Xylogics tech support and get them
in on the act.

Still, if this Annex can talk to anything _but_ my NetBSD machine, it seems 
likely that there's something different about what the NetBSD machine's sending
it.  So I'll ask again:  What's "different" about NetBSD slip/ppp?