Most 32-bit PCI cards will function properly in 64-bit PCI-X slots, but the bus clock rate will be limited to the clock frequency of the slowest card, an inherent limitation of PCI's shared bus topology.
For example, when a PCI 2.3, 66-MHz peripheral is installed into a PCI-X bus capable of 133 MHz, the entire bus backplane will be limited to 66 MHz.
To get around this limitation, many motherboards have two or more PCI/PCI-X buses, with one bus intended for use with high-speed PCI-X peripherals, and the other bus intended for general-purpose peripherals.

Many 64-bit PCI-X cards are designed to work in 32-bit mode if inserted in shorter 32-bit connectors, with some loss of performance.
An example of this is the Adaptec 29160 64-bit SCSI interface card.
However, some 64-bit PCI-X cards do not work in standard 32-bit PCI slots.


Installing a 64-bit PCI-X card in a 32-bit slot will leave the 64-bit portion of the card edge connector not connected and overhanging.
This requires that there be no motherboard components positioned so as to mechanically obstruct the overhanging portion of the card edge connector.
Most 32-bit PCI cards will function properly in 64-bit PCI-X slots, but the bus clock rate will be limited to the clock frequency of the slowest card, an inherent limitation of PCI's shared bus topology. For example, when a PCI 2.3, 66-MHz peripheral is installed into a PCI-X bus capable of 133 MHz, the entire bus backplane will be limited to 66 MHz. To get around this limitation, many motherboards have two or more PCI/PCI-X buses, with one bus intended for use with high-speed PCI-X peripherals, and the other bus intended for general-purpose peripherals.
Many 64-bit PCI-X cards are designed to work in 32-bit mode if inserted in shorter 32-bit connectors, with some loss of performance. An example of this is the Adaptec 29160 64-bit SCSI interface card. However, some 64-bit PCI-X cards do not work in standard 32-bit PCI slots.

Installing a 64-bit PCI-X card in a 32-bit slot will leave the 64-bit portion of the card edge connector not connected and overhanging. This requires that there be no motherboard components positioned so as to mechanically obstruct the overhanging portion of the card edge connector.
Peripheral Component Interconnect