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

   On actual VME boards, the connectors are labelled P1, P2, and P3 from
left to right. P1 and P3 are VME proper -- P1 mostly address and data,
P3 mostly power. 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. 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.

   --James B.