(Oops. Again with no HTML) Std. Serial ports beyond 115K?

["Kevin Traas" <ktraas@netmaster.ca>]

I want to connect an external ISDN to my system using the MB's standard
serial ports.  However, everything's telling me I can only push 115K through
these serial ports - far less than the theoretical max of an ISDN TA that
does V.42bis.

Any comments/suggestions on "overclocking"  ;-)  built-in serial ports to
230 (or 460) Kbps?

Note:  I realize that HS serial ports/boards are available, but I don't want
to shell out the money if I don't have to....
Note2: If you can recommend a good external ISDN TA (North America) or an
internal that supports DialD, I'd love to hear from you.

Thanks,
Kevin