I'm tempted to try booting via an Exa 8200 right now...
> 
> Oh ... you can switch the 400/425/433 ROM back to "HPUX" mode from Domain 
> mode... I forget the magic sequence, but it's possible... anyone know it?
> 

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...

> 
> ...which reminds me, I've found some more bugs in COMPAT_HPUX that I need 
> to fix... :-)

and 1.2beta apparently doesn't like fixed block Exabyte writing anymore:
dd if=SYS_INST of=/dev/rst32 conv=osync
st0: I/O not block aligned 1024/512
st0: ILLEGAL REQUEST
dd: /dev/rst32: Input/output error
1+0 records in
0+0 records out
0 bytes transferred in 1 secs (0 bytes/sec)
Sep 18 00:44:43 hp400 /netbsd: st0: I/O not block aligned 1024/512

... not to mention that conv=osync with bs=20b doesn't work with 1.2beta's
'dd'. Tried, just for fun, to boot it via an Exabyte, but it doesn't work.
Perhaps it needs fixed mode blocks, who knows. Does HP prefer fixed blocks with
Exabytes like SGI does, or variable blocks like Sun (and NetBSD) ??

Regards, Michael