PCI
Available expansion slots on motherboards allow for a variety of upgrades to a computer system, but matching the right card to an available slot should be addressed before any purchasing decision is made. The most common types of expansion cards for modern computer systems can be divided into three formats: PCI, AGP, and PCI Express. Each of these formats will be covered separately in this three-part series of Tech Tips, starting with PCI.
The letters “PCI” stand for Peripheral Component Interconnect and is the term used to describe a bus that connects components directly to system memory and the system processor via the “front side bus.” When talking about communications on a motherboard, the term “bus” has nothing to do with the big yellow thing that takes kids to school. There can be multiple buses in a computer, and like the PCI bus, they are all responsible for managing communication “traffic” from different devices to the processor.
The front side bus is a high-speed connection that manages the processor’s communication with things like hard drives, memory, and PCI devices, without burdening the processor with all management responsibilities.
First developed by Intel in the early 1990s, PCI grew out of even earlier (and slower) bus architectures like ISA (Industry Standard Architecture) and VL-Bus (VESA Local), which were common in the 1980s and 1990s.
The original specifications for the PCI bus had a speed of 33 MHz, with a bus width of 32 bits and a maximum bandwidth of 132 MB per second. There have been a few revisions to the PCI standard that have increased these specifications significantly, bringing it up to 66 MHz, 64-bit, and 512 MB per second, respectively. The 32-bit and 64-bit versions have different physical features, and most motherboards only offer 32-bit connections. The original power specification had PCI devices operating on 5V DC, and with revisions came the ability for devices to continue to use 5V, as well as now being able to operate on 3.3V DC.
A simple explanation of 32-bit and 64-bit can be obtained by continuing the analogy of buses and traffic. Think of each bit as a lane of traffic in the communication path. Think of a 32-bit bus with 32 lanes of traffic and a 64-bit bus with 64 lanes of traffic. Just as more cars can travel simultaneously on a highway with more lanes, more data can be transferred on a bus with more bits.
Motherboards can support multiple slots that share a PCI bus, and while not particularly common, they can include more than one PCI bus. Depending on the size of the motherboard form factor and other features that may be taking up space on the board, one to six PCI slots can be expected on a typical motherboard. For example, the mATX form factor has only two 32-bit PCI slots, while the ATX form factor has six 32-bit PCI slots.
A 32-bit PCI card has 124 pins to mate with a slot on a system’s motherboard and will fit into a 32-bit or 64-bit slot (although data transfer will be 32-bit in either type of slot). .
A 64-bit PCI card has 184 pins to mate with the appropriate slot on a system’s motherboard, but can generally fit into a 32-bit slot as well, as long as the motherboard’s features don’t interfere. When installed in a 32-bit slot, data transfer on a 64-bit card will be limited to 32-bit.
The Intel STL2 Dual Socket 370 Server Board with VRM The Intel STL2 Dual Socket 370 Server Board is a good benchmark for comparing 32-bit and 64-bit PCI slots. Looking at the lower left corner of the motherboard, it shows four 32-bit PCI slots and two 64-bit PCI slots.
Subsequent installments in this Tech Tip series will discuss AGP and PCI Express, each of which has its own unique physical characteristics. Although PCI cards of different formats can be interchangeable, PCI, AGP, and PCI Express cards will not work (or fit) in any other type of slot.
Most PCI cards will be of the 32-bit variety, and the selection of items available is quite wide. Graphics cards, sound cards, network cards, RAID controllers, TV tuners, modems, and USB/Firewire controllers are all common items that can be added to a system using a PCI card.
Many of the items listed in the previous paragraph can be found built into modern motherboards, but these embedded devices do not offer upgradability. PCI devices provide plug and play installation, allowing the user to easily install (or remove) a device. For example, a cheap 2-channel sound card may be good enough for someone initially, but in the future they may decide that something like the 7.1-channel Sound Blaster Audigy 2 delivers the sound quality they really want. Upgrading is a matter of shutting down the system, swapping cards, rebooting, and installing the new software/drivers (okay, maybe a bit more streamlined). The nice thing about PCI cards is that even if you have a board with built-in functionality (like the built-in sound mentioned above), your motherboard BIOS will usually allow you to disable that feature if you want to add an upgraded version. (such as the Audigy sound card mentioned in the example above), or the card can complement the already built-in function (such as an IDE RAID card).
The area that drove the development of AGP is the performance of PCI-based graphics cards. The demands of fast-paced video games and other graphic-intensive applications require a large amount of bandwidth, which was simply not available on the PCI bus. Considering that all devices on the PCI bus share the available bandwidth, an even faster dedicated bus was required to handle only graphics data. However, PCI graphics cards are still available and are an easy way to add a second display to a system that currently runs an AGP or PCI Express graphics card.
Last words
The PCI slot has been around for a while and seems to have a place at least in the near future of computer architecture. AGP and PCI Express offer performance benefits that the PCI standard cannot match, but for many applications, the performance that PCI offers is more than adequate. Be sure to check out the following tech tips in this series to learn the basics of AGP.