Well, I'm still pursuing this one.  Some persistent reading of various
things leads me to this :

 - There are 4 sio ports in the 'utility chip'.
    0 : apollo keyboard port
    1 : disabled in HP mode in favour of the onboard dca port
    2 : goes to 232 port
    3 : goes to 232 port

 - The register model is that of the 8250, with 8-bits registers on
   longword (32-bit) boundaries.

 - There is a bytewide control register at 0x2021c0e0 which contains
   control bits (interrupt enables perhaps?) for these ports, the 4
   MSB correspond to the 4 ports.

I still haven't been able to find the absolute base address of the ports;
if someone with an HP/UX system can send me the /etc/master details for
the apci driver I'd be very grateful, as I think that's the key.

-- 
]] Mike Smith, Software Engineer        msmith@gsoft.com.au             [[
]] Genesis Software                     genesis@gsoft.com.au            [[
]] High-speed data acquisition and      (GSM mobile)     0411-222-496   [[
]] realtime instrument control.         (ph)          +61-8-8267-3493   [[
]] Unix hardware collector.             "Where are your PEZ?" The Tick  [[