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PCI and PCIe Tutorial

PCI SlotsPCI (Peripheral Component Interconnect) expansion slots came on the scene in 1993. As I stated on another page, expansion slots are a means by which you can add different types of expansion cards, such as a sound or video card, to enhance a PCs functionality. Before PCI, there were the now obsolete ISA slots. PCI is still in use today but has basically been replaced by PCI Express. This describes a brief overview of PCI vs PCIe. Most PCI slots are white but can be other colors.

PCI utilizes a 32-bit bus, meaning data is transmitted 32 bits at a time, that is shared among all the PCI devices attached to it. Because of this, the bus has to have some way of deciding which component gains access to it. Also, more cards connected results in more interference from each device, which makes it more difficult for a signal to be detected. That's why PCI allows no more than five devices. Below is a simple diagram of PCI design.

PCI Diagram

PCI Express:
In 2004 another standard was introduced called PCI Express, also written as PCIe or PCIe. PCIe is extremely high-speed. The reason is that unlike traditional PCI, PCI Express does not use a shared bus. Each device has a dedicated path to a single chip called a switch, therefore resulting in faster data transmission.

PCI Slots

The individual paths are termed lanes. The number of lanes in a slot determines the type of PCI-E, and there are several: x1, x2, x4, x8, x16, and x32 (powers of 2). If there's a single lane, then it's x1. If there are two lanes, it's x2, and so forth. x1 and x16 are the most popular. The more lanes, the faster the data transfer, just like a highway. With these incredible speeds, PCIe is an excellent fit for today's video and audio technology. The connectors come in different lengths and unlike PCI, PCI Expess supports hot swapping, meaning expansion cards can be added or removed without turning off the machine. PCI and PCIe are not compatible. You cannot use a regular PCI card in a PCIe slot and vice versa. The architecture is different. A PCIe diagram is shown below.

PCIe Diagram

Types & Compatibility:
Presently we have PCIe 1.x and 2.0. The first version carries data at 250 MB/s (2.5GB/s) per lane. Version2.0, which arrived in 2007, doubles the rate of the previous bus at 500MB/s (5GB/s) and is backward compatible, so plugging a version 1.0 card into a 2.0 slot works, but will still run at 1.0 speed. It was also designed for forward compatibility - sticking a 2.0 card in a 1.0 slot. But this defeats the purpose of using PCIe2, since it will run at the slower speed.

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