and have two key notches to indicate that, there are still a large number of 5 V-only cards on the market. The M66EN pin is an additional ground on 5 V PCI buses found in most PC motherboards. Cards and motherboards that do not support 66 MHz operation also ground this pin. If all participants support 66 MHz operation, a pull-up resistor on the motherboard raises this signal high and 66 MHz operation is enabled. The pin is still connected to ground via coupling capacitors on each card to preserve its AC shielding function. The PCIXCAP pin is an additional ground on PCI buses and cards. If all cards and the motherboard support the PCI-X protocol, a pull-up resistor on the motherboard raises this signal high and PCI-X operation is enabled. The pin is still connected to ground via coupling capacitors on each card to preserve its AC shielding function. At least one of PRSNT1# and PRSNT2# must be grounded by the card. The combination chosen indicates the total power requirements of the card (25 W, 15 W, or 7.5 W). SBO# and SDONE are signals from a cache controller to the current target. They are not initiator outputs, but are colored that way because they are target inputs. PME# () Power management event (optional) which is supported in PCI and higher. It is a , open drain, active low signal. PCI cards may use this signal to send and receive PME via the PCI socket directly, which eliminates the need for a special Wake-on-LAN cable. Mixing of 32-bit and 64-bit PCI cards in different width slots 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