Hello James!

  JWB> On the 3/50 and 3/60, the connector is used for
     > nothing but power, but it is compatible with the
     > power connections on the P3 connector in the 9U VME
     > chassis...

That's good to know, thanks!

  JWB> ...in other words, you can shove a bunch of 3/50 and
     > 3/60 motherboards into a VME chassis and they'll
     > work.

That thought had occurred to me!  I could build a fairly
compact, yet underpowered and current-thirsty cluster of
NetBSD machines for experiments in distributed processing!
<very wide grin> It may not be as silly as it first sounds
though, 3/60s are selling for peanuts on ebay, and for the
right application (9600 BpS remote access server?) It might
make sense...the AUI<->10Base-T transciever may cost more
than the 3/60 board though!  =o)

  JWB> On actual VME boards, the connectors are labelled P1,
     > P2, and P3 from left to right.  P1 and P3 are VME
     > proper...

That's wierd!  As I understand it, VME only normally covers
3U with one DIN connector and 6U cards with one or two.  If
P1 and P3 are VME, the fact they're further apart than is
'normal' for VME presumably prevents the use of standard 32
-bit VME cards... I wonder if that was the intention?

  JWB> P1 mostly address and data, P3 mostly power.

I've just dug out some VME information, which shows row B of
(VME) P2 containing extra data and address lines, and some
power and rows A and C as being user defined.

  JWB> P2 is Sun's private bus which is used primarily for
     > communications between CPU boards and memory boards,
     > but there are a few other groups of boards (primarily
     > framebuffers and the GP* accelerators) which use it
     > too.

Hmm... I wonder if row B of SUN's P2 is like that of
standard 6U VME with the private bus living on rows A and C.
Or is SUN's P2 entirely proprietary, with 'VME P2' moved out
onto Sun's P3 connector?  You can tell I'm confused can't
you?  %-)

  JWB> On all VME chassis of six slots or more, the P2 bus
     > is segmented to allow multiple groups of boards to
     > use it without conflicting -- CPU and RAM boards on
     > one segment, framebuffer and accelerator on another,
     > etc.

Can this be used to house two 'separate' machines on the
same backplane?  Or would signals from P1 and P3 conflict?
I understand VME can be a multi-processor bus, but I'm not
sure if the SUN processor cards are built with this in mind.

Regards,
  - Andy Ball.