The Peripheral Component Interface [PCI-X] addendum is an enhancement to the current 64 bit 66MHz PCI bus specification. The minimum clock speed for PCI-X is 66MHz [PCI-X 66]. Additional bus speeds include: PCI-X 133, PCI-X 266 and PCI-X 533 providing up to 4.3GBps [PCI-X 1066 in the works]. PCI-X is backwards compatible with PCI; of course none of the enhancements would be available and the best speed would revert to 66MHz. PCI-X provides for the same card form factor, pin-outs, connector, bus widths, and protocols. Many of the changes relate to bus operation, wait states and protocol enhancements, which is not covered here. PCI-X still supports either 32 bit or 64 bit bus widths. The DC switching Specifications look the same for PCI and PCI-X. PCI-X does have a higher output AC driver current than PCI. The PCI-X enhancement was never introduced into the Personal Computer market, although it did show up in the [higher cost] server market.

The PCI spec defines the Electrical requirements for the interface. No bus terminations are specified, the bus relies on signal reflection to active level threshold. The PCI bus operates either synchronously or asynchronously with the "mother Board bus rate. While operating asynchronously the bus will operate at any frequency from 66MHz down to (and including) 0Hz. Flow control is added to allow the bus to operate with slower devices on the bus, allowing the bus to operate at their speed. PCI is an unterminated bus, the signal relay on signal reflections to attain there final value. The PCI specification has been port-ed to a number of other form factors. These include:

PCI: The original specification 'Peripheral Component Interface', @ Rev 2.1
PCI-X: The latest version 64 bits at: PCI-X 66, PCI-X 133, PCI-X 266 and PCI-X 533 [4.3GBps]
cPCI, Compact PCI: PCI in a VME form factor, 3U/6U using 2mm connectors
PC104-Plus: PCI add-on to the PC104 spec, ISA in a square form factor
PISA: PCI add-on with PCAT in the ISA AT form factor
P2CI: PCI on the VME64 P2 connector
PMC: PCI on a Mezzanine Card, 'PMC'
PMCx: PCI-X on a Mezzanine Card 'PMC'
PXI cPCI for Instrumentation
IPCI: Industrial PCI (Another version of cPCI}
Serial PCI: PCI on a serial link
Card Bus: 32 bit PCI on the PC Card (PCMCIA) Format

Each of these additional specifications rely on the PCI spec., normally only the mechanical (form factor) definition changes. Unlike earlier PC buses, the PCI bus is processor independent. The (64bit) PCI bus is made up of the following (major) signals:
Address/Data Bus: 64bit Address; 64bit Data, Time Multiplexed
System Bus: 2bits; Clock/Reset
Interface Control Bus: 7bits; Ready, Acknowledge, Stop.
Parity Bus: 2 bits, 1 for the 32 LSBs and 1 for the 32 MSB bits
Errors Bus: 2 bits, 1 for Parity and 1 for System
Command/Byte Enable: 8 bits (0-3 @ 32bit, and 4-7@ 64bit Bus)
64MHz Control: 6 bits; (2) Enable/Running, (2) Present, (2) Ack/Req
Cache: 2 Bits, Interrupt bus: 4 bits, JTAG Bus: 5 bits
Power: +3.3v, GND. PCI-X does not support +5v [PCI], and I find no data reagrding +12, -12v [used in PCI]

The Time Multiplexed Address and Data bus may exist as either 0 to 31 bits (32bits) or 0 to 63 bits (64bits) using the 64 bit expansion bus. Both the Address and Data line use the same bus, Address first then Data. 32 bit PCI may also use 64 bit addressing by using two address cycles; termed Dual Address Cycles (DAC), the low order address is sent first. Additional control bits are utilized once the bus is increased to 64 bits.
The specification defines both a Reset line and a Clock line. The Clock may be either 33MHz or 66MHz. I believe the 66MHz clock rate is only defined for the 64bit bus width. A number of 'Handshake' lines exist to allow communication, i.e. Ready, and Acknowledge

Two Parity lines are made available, one for the 32 bit bus width (bits 0 to 31) and an additional one for the 64 bit expansion (bits 32 to 63). Two error bits; I assume, 1 for the LSB 32 bits and one for the upper 32 bits.

PCI cards for a personal computer differ from the ISA type by two important factor: Components a mounted on the reverse side of the card, The edge connector is more dense, shorter and the keys reside in different locations.