question about unix & real time

[W. Harry Plantinga]

I am interested in using a smallish computer, like a Sun workstation
or a micro Vax, for a real-time application (controlling a robot arm
with sensors).  I've heard that Unix is not a good operating system 
for real-time applications, but I have a couple of questions:

	-Just what is it about Unix that makes it ill-suited for
		real-time applications?
	-Does anyone know of a "real-time" system on the order of
		a Sun, preferably running Unix?  Preferably with 
		good graphics?

			-Harry Plantinga
			arpa:  planting at wisc-rsch.arpa
			use:   {inhp4, seismo}!uwvax!planting