PCI, otherwise known as the PCI Standard, stands for Peripheral Component Interconnect. It is a computer bus that attaches peripheral devices to a computer motherboard. It can take the form of an integrated circuit called a planar device fitted onto the motherboard itself or as an expansion card that fits into a socket.

PCI was developed by Intel and was first introduced to computers in 1993. For several years, both PCI and the ISA bus were used. Eventually, it replaced ISA and VESA and become the standard local computer bus. Nowadays, most computers only have PCI slots and an AVG slot for a display adapter.

It is the most common local I/O bus used today, because it conveniently shares a common data path between the CPU and peripheral controllers in any computer model. Examples of common PCI Cards used today are network cards, sound cards, modems, USB ports, serial ports, TV tuner cards, and Disk controllers.

PCI Specifications specifications include the physical size of the bus such as wire spacing, bus timing, electrical characteristics, and protocols. The specifications can be bought from the PCI Special Interest Group (PCISIG).

PCI Configuration

PCI affords two separate 32-bit or 64-bit address spaces corresponding to the memory and the I/O ports of the x86 group of processors. The PCI Configuration Space is a third address space that allows the software to determine the amount of memory and I/O address space required by each device. Each of these devices can request a maximum of six areas of memory space.

During start-up, the Operating System queries all PCI buses through PCI Configuration Space to identify the devices attached to the computer and the system resources needed by each device. These resources are then determined and allocated to each device. These devices can also have an additional ROM that includes an executable x86 or PA-RISC code and an EFI driver.

Instead of using jumpers on the card, interrupts are often allocated to the device by the Operating System during configuration. These interrupts are assigned to improve their performance. Most PCI devices need special hardware to support sharing. They also need interrupt ports to aid the device in determining whether an interrupt is for itself or for another device sharing the I/O port.

PCI Variants

1. PCI 2.2 allows 66 MHz signaling at 3.3 signal voltage with a peak transfer rate of 533 MB/s. The 5 and 3.3 signal voltages are allowed at 33 MHz.

2. PCI 2.3 allows the use of 3.3 volts and universal keying. However, it does not allow 5-volt keyed add-in cards.

3. PCI 3.0 is the final official standard of the bus. It completely comes without the 5-volt capability.

4. PCI-X amplifies the maximum signaling frequency to 133 MHz with a peak transfer rate of 1066 MB/s.

5. PCI-X 2.0 allows a 266 MHz rate with a peak transfer rate of 2133 MB/s as well as a 533 MHz rate with a peak transfer rate of 4266 MB/s.

6. Mini PCI is the latest form factor of PCI 2.2 which is chiefly used for laptops.

7. CardBus is a PC card form factor for 32-bit 33 MHz PCI.

8. PC/104-Plus is an industrial bus that uses PCI signal lines with different connectors.

PCI Express

PCI Express (formerly known as 3GIO/Arapaho) is the latest interface using PCI programming concepts. It features a serial physical-layer protocol and various connectors. It is projected to replace PCI and PCI derived AGP buses.