Real time unix?

[gregory.p.kochanski]

Is there a real time unix which might allow me to read or write to a device
in 50 microseconds or less?  To be more specific, I do experimental physics,
and am basically poking around, looking at atoms on surfaces with a tunneling
microscope.  This is an experiment where the software and hardware need to
be closely coupled.  Basically, you read a word from a A-D converter,
do a few computations, and output a word or two to a D-A converter.

The computations are not stylized enough or simple enough to convenientlyy
put in a device driver (and I'm not that masochistic, anyway).
Are there unixes where this is possible?  Perhaps allowing a user
program direct access to some hardware, and locking processes in the CPU?