When HP-UX was first released on PA-RISC hardware, it supported ... or claimed to ... "real time" capability.

I remember seeing a demo of the HP 9000/840 at the Interex Madrid Conference, March 1986, where the 840 was monitoring a light beam being 'broken' by ball of a Newton's Cradle.

Here is the technical data sheet for HP-UX 7.0, mentioning real time capability, as late as 1989:

hp_ux_7_0_os_technical_data.pdf (3 MBs)