The 3Com 3C905C-TX-M network card is no longer being
manufactured. The card that has replaced it, the
3C905CX-TX-M, does not work with RT-TCP/IP 1.1.
VenturCom is aware of this problem and plans to address
it in RT-TCP/IP 2.0, which they say should be ready for
release around February. In the meantime, they offer
no fix.
If you have purchased the RT-TCP/IP SDK, you might be
able to modify the source code to work with this NIC
and then recompile the TCP/IP stack. Otherwise, until
version 2.0 is released, the only way around this
problem is to try to acquire an older version of the
NIC, or to use an NE2000-compatible ISA card.