Pci Buses - Silicon Graphics Prism Hardware User's Manual

2: System Overview

PCI Buses

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Each deskside system supports eight 64-bit, PCI-X slots which are divided into four
buses, as shown in Figure 2-10. Note that the top two slots of bus 1 are always occupied
by the static I/O daughter board and the base I/O PCI card. The system's base I/O board
always occupies bus 1, slot 1, and the static I/O daughter board is permanently installed
in the top slot (bus 1, slot 2). The static I/O daughter card provides the USB, L1 USB,
serial console, and ImageSync connectors for the deskside unit.
The remaining six slots are available for other optional PCI/PCI-X boards. Each bus can
support card speeds of up to 133 MHz. The slots are divided into buses, as shown in
Figure 2-10.
• PCI Bus 1 has one usable slot, which always contains the base I/O board.
• PCI Buses 2, 3, and 4 each have two slots.
Follow these general rules when installing PCI and PCI-X boards on the PCI-X buses:
• Avoid placing high and low speed cards on the same bus. This forces the high speed
card to run at the same speed as the slower card. For example, placing a 100-MHz
board on the same bus as a 66-MHz board forces the 100-MHz board to run at the
slower speed.
• Avoid placing storage and network boards on the same bus. These types of boards
use large amounts of bandwidth. Placing both types on the same bus can cause the
system to exceed the bandwidth of the bus.
• Spread the PCI boards across the buses as evenly as possible. For example, if you
have three PCI boards and three available PCI buses, install one board on each bus.
If you must install more than one board on a bus, group the boards by speed.
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