Last Winter I played a bit with interrupt-driven serial
communications and found that even a 4.7 MHZ XT board will flawlessly
receive serial data at 9600 baud if the receive buffer is several
Kbytes and one uses interrupt-driven routines.  A 486 or better which
is not hobbled by the resource-hogging overhead of Windows should
positively scream through just about any serial speed up to tens of
Kbaud.  Just the scaling factor alone should put it up around T1
capability.

        I will never understand why IBM did not include a
interrupt-driven serial communication routines in their ROM BIOS when
the P.C. first came out.  There must have been a good reason, but
going interrupt-driven makes all the difference in the world.  The
MSKermit terminal emulator and file transfer program uses
interrupt-driven routines and I have used that on old cast-off XT's
for years at 9600 baud.

        The 16-550 UARTS have a built-in 16-byte buffer which helps
still more, but your software must know how to enable this buffer or
the chip acts just like the 8250.  MSKermit does make use of the
buffer.  If you write your own receiver routine using interrupts, give
yourself about an 8-K buffer and you can drag out that old P.C. that
still works.  I wouldn't put any more than 9600-baud through those
oldies, though.  That is pushing things.

Martin McCormick