An old dream come true: Iris Indigo! - test-drb@ccmp.vtda.org

29 Oct 2007

...
  Perhaps you're referring to the Indigo^2, but the
R4K Indigo has most
 things in common with the R3K Indigo.  Same backplane, same skins, same
 graphics boards and options.  IIRC the little serial PROM is different.
   The PSU is uprated slightly (though some R3Ks used the same PSU) but
 otherwise the same.  Obviously the CPU board is different, and the
 memory on it is standard 72-pin FPM SIMMs instead of the proprietary
 stuff used on the R3K.  In that sense only is it like an Indy.

I think he's referring to the design of the IP20 vs the design of the 
IP12, rather than the design of the rest of the Indigo. The core design 
of IP20 is quite similar to IP22 (but not as close  as Indigo2 and Indy 
are to each other), using a MC1 and DMUX chips coupled with the PM1/PM2 
modules, but the peripherals and their interfaces are slightly 
different- graphics interconnections, audio, etc. (SCSI is the same, 
and enet).