Product specifications

The key point in the following diagram is the motherboard is a private
PCI bus (similar to the "cable bus") that connects to two PCI to PCI
Bridge (PPB) chips mounted on the motherboard. Each of the PPB
chips controls a downstream PCI bus. The PPB chips are both attached
to the same upstream bus (the bus controlled by the PCI Expansion
Interface) and are at the same hierarchical bus level within the chassis.
13 Slot Topology
NOTE
The first six PCI cards in the 13 Slot chassis will appear on PCI Bus 2
.
The remaining seven PCI cards will appear on PCI Bus 3.