I'm having troubles with high-speed input on the Sparc serial

ports. What should I do?

Try using UUCP. The Solaris 2.x sparc serial driver has
trouble receiving data at or above 9600 bps. Symptoms include
sluggish response, `NOTICE: zs0: silo overflow' console
messages, sending spurious control-Gs to the serial port, and
applications that cannot be killed even with `kill -9'. This
problem surfaces in many applications, including Kermit and
tip. UUCP seems immune, though, because its protocol throttles
input sufficiently.

People have reported success in later releases of Solaris (2.3+).

Solaris 2.5 supports much higher baudrates and hardware flowcontrol
in two directions. The latter is also available as a patch for
2.3 (102028) and 2.4 (102845, note that this patch conflicts with
patch 102062-08, which should be installed first if at all).
The zs device can be set to 38400bps in 2.4 and earlier
and 76800 in 2.5 and later.

The latest UltraSPARC systems with PCI have a newer serial chip,
with the "se" device driver. It can sustain speeds of upto 400k bps.

ASGAUR