I can recommend MAGMA products ( http://www.magma.com/ ) -
I use an LC 2+1 Sp ( http://www.magma.com/sbus/sbus_lc2+1sp.html ) with
an IPX.
NetBSD has drivers but I am currently using it with SunOS 4.

NetBSD 1.4/1.5 handle a 56k on built-in serial ports without problems in
my experience on SS1/SS1+  with ipf & ipnat (at full throughput,
interrupt load seems to be 30-50% according to systat but no evidence of
overruns, unlike SunOS 4).

Toby

Chris.Smith@raytheon.co.uk wrote:
> 
> Hi,
> 
> Anyone know of a good `fast' serial port SBus card for
> a Sun Classic workstation?  i.e. one that wil support
> a 56k modem without falling over like the internal
> ports do :P  I assume that putting one of these in it
> would make it good for a 56k internet router compared
> to a doorstop (it's current task :).
> 
> Obviously, it's got to work nicely with NetBSD 1.5!
> 
> Cheers,
> 
> - Chris.