It's under 'boot' in the undocumented 'C' menu (hit space during boot, hit C <ret> instead of picking boot media -- the same way you pick the serial console (C menu). As I'm still not lucky enough to spot that d*n location of the %^$^%& utility chip on the 400t under my desk, I'm really trained at that:
Fresh from a kermit session with a 400t:
Press the 'SERVICE' switch (green LED is on), press RESET.
ROM will boot, box will chirp once, then twice.
Now press RETURN on keyboard:
MD12 REV 1.2 1990/08/07.14:27:08
BOOTROM Series 400 Rev. 1.1
>
enter 'cf':
>CF
Configuration Control
keys Control Class
---------------------------
1 I/O Configuration
2 Boot Mode Selection
A Abort without changes
---------------------------
Type [key] RETURN ?
Enter '2' + RETURN:
Boot Mode Selection
keys Mode Status
---------------------------
1 Domain Compatible P
2 HP-UX Compatible
3 undefined
C Clear Temporary
E Execute
A Abort without changes
---------------------------
Type [key] RETURN ?
Enter '2' + RETURN again:
2 HP-UX Compatible
Temporary or Permanent
Type T or P RETURN ?
Now you can choose if you will give it just one try or fix the HPUX mode for
after the next reset. Let's say we want to play:
Enter 'T' + RETURN:
Boot Mode Selection
keys Mode Status
---------------------------
1 Domain Compatible P
2 HP-UX Compatible T
3 undefined
C Clear Temporary
E Execute
A Abort without changes
---------------------------
Type [key] RETURN ?
And now hit 'E' + RETURN and the beast will boot in HPUX mode.
The opposite way, from HPUX mode to Domain/OS, goes as this:
wait till 'System Search Mode' appears, hit SPACE and then enter 'C' +
RETURN:
(takes some time, 10+ secs or so):
Configuration Control
keys Control Class
---------------------------
1 I/O Configuration
2 Auto System Selection
3 Boot Mode Selection
A Abort without changes
---------------------------
Type [key] RETURN ?
Pretty much the same again...