Technical Q&A QA1307

PCI/PCI-X slots on the Power Mac G5

Q: How do the slots in the Power Mac G5 cope with PCI/PCI-X cards of different speeds?

A: The Power Mac G5 has 3 slots available for use with PCI/PCI-X cards. These slots are numbered on the rear of the case 2, 3, 4. (Slot 1 is the AGP slot, is generally filled with a graphics card, and is not discussed here.)

In the Power Mac G5 with a 1.6GHz CPU, slots 2, 3, and 4 all run at 33MHz. These slots are compliant with the PCI Local Bus Specification v2.3. In the Power Mac G5 with a 1.8GHz CPU or dual 2.0GHz CPUs slots 2 & 3 run at a maximum of 100MHz and slot 4 runs at a maximum of 133MHz. These slots are compliant with the PCI-X Addendum to the PCI Local Bus Specification, rev. 1.0b.

Since the Power Mac G5 1.6GHz slots only run at 33MHz, the rest of this discussion applies to the Power Mac G5 1.8GHz and dual 2.0GHz.

Slots 2 & 3 share a common bus. Slot 4 is on a separate bus. All cards on a common bus can only run as fast as the slowest card.

For example if there is a 33MHz card in slot 2 and a 66MHz card in slot 3, both cards will run at 33MHz.

If a 33MHz card is put into slot 4, it will not affect the speed of slots 2 & 3.

For example assume there is a 33MHz card in slot 4 and a 100MHz card in each of slots 2 & 3.

The card in slot 4 runs at 33MHz while the cards in slots 2 & 3 run at 100MHz.

More information about the PCI/PCI-X slots in the Power Mac G5 can be found in the Power Mac G5 Developer Note .

[Sep 30, 2003]