The expansion slots available on motherboards allow for a variety of upgrades to a computer system, but it is necessary to address the combination of the right card with an available slot before making any purchasing decisions. 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 tech tips series, starting with PCI.
The letters “PCI” stand for peripheral component interconnection, and it is the term used to describe a bus that connects components directly to the system memory and to the system processor through the “front end 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 on a computer, and like the PCI bus, they are all responsible for managing communication “traffic” from different devices to the processor.
The front end 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 was built from 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 were clocked at 33 MHz, with a 32-bit bus width and a maximum bandwidth of 132 MB per second. There have been some revisions to the PCI standard that have significantly increased these specifications, taking them to 66 MHz, 64-bit, and 512 MB per second, respectively. The 32-bit and 64-bit versions have different physical characteristics and most motherboards only offer 32-bit connections. The original power specification had PCI devices running on 5VDC, and with the revisions came the ability for the devices to continue to use 5V, as well as being able to now operate on 3.3VDC.
A simple 32-bit and 64-bit explanation can be obtained by continuing the analogy of buses and traffic. Think of each bit as a lane of traffic on 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 road with more lanes, more data can be transferred on a bus with a higher number of bits.
Motherboards can support multiple slots by sharing one 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, you can expect to have one to six PCI slots on a typical motherboard. For example, the mATX format features only two 32-bit PCI slots, while the ATX format features six 32-bit PCI slots.
A 32-bit PCI card has 124 pins to mate with a slot on a system motherboard and will fit in 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 proper slot on a system’s motherboard, but can generally also fit in a 32-bit slot, as long as the characteristics of the motherboard do not 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 reference for comparing 32-bit and 64-bit PCI slots. Four 32-bit PCI slots and two 64-bit PCI slots are shown in the lower left corner of the motherboard.
The next installments in this series of technical tips will discuss AGP and PCI Express, each of which has its own unique physical characteristics. Although different format PCI cards may be interchangeable, PCI, AGP, and PCI Express cards will not work (or fit) in any other type of slot.
Most PCI cards will be 32-bit 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 common items that can be added to a system through the use of a PCI card.
Many of the items listed in the preceding paragraph can be found embedded in modern motherboards, but these embedded devices do not offer upgrade capabilities. PCI devices provide plug and play installation, allowing the user to install (or remove) a device with ease. 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 offers the sound quality they really want. Upgrading is a matter of shutting down the system, swapping cards, rebooting, and installing new software / drivers (okay, maybe a bit more simplified). The great thing about PCI cards is that even if you have a board with a built-in feature (like the built-in sound mentioned above), your motherboard BIOS will generally allow you to disable that feature if you want to add a card (like the sound card Audigy mentioned in the example above), or the card can complement the already built-in function (such as an IDE RAID card).
The only aspect that drove the development of AGP is the performance of PCI-based graphics cards. The demands of fast-paced video games and other graphics-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 the 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 on an AGP or PCI Express graphics card.
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 offered by PCI is more than adequate. Be sure to review the following technical tips in this series to learn the basics of AGP.