This works on RBEM100-56G 

Forum: Xircom PCMCIA Ethernet Adapter Issues
Re: CBEM56G sleeps 'til a little nudge thru tcpdump(?!) (Kai Krebber)
Re: Maybe just set promiscuous mode? (David Hinds)
Date: 1999, Dec 10
From: Chris Hanson <cph@zurich.ai.mit.edu>

I had the same problem with an RBEM100-56G, which appears to be fixed by the "ifconfig eth0 promisc" workaround. So I think Dave's supposition is right.

to: "This works on RBEM100-56G"