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PCI Express serves as a fast serial interface that functions more like a network than a traditional bus. Explore how PCI Express boosts computer performance and replaces AGP while viewing PCI Express visuals.
Image courtesy of Consumer Guide ProductsPeripheral Component Interconnect Express, or PCIe, is a cornerstone in computer hardware. It offers a rapid interface for linking peripheral devices (like keyboards, sound cards, or external hard drives) to a computer’s motherboard. From its inception to its modern-day versions, PCIe has continuously advanced to meet the ever-growing needs of computing.
Whether you're a tech enthusiast building your own system or simply curious about your device's inner workings, this article will help. We'll explore the differences between PCI and PCIe, how PCI Express enhances computer speed, its ability to improve graphics performance, and its role in replacing the Accelerated Graphics Port (AGP) slot.
The Importance of a PCIe Interface
Peripheral Component Interconnect (PCI) slots have become so essential in computer design that their presence is often taken for granted. For many years, PCI has been a reliable way to connect sound cards, video cards, and network devices to the motherboard.
However, PCI has its limitations. As processors, video cards, sound cards, and networks have become faster and more powerful, PCI has remained unchanged. It has a fixed 32 bit width and can support only five devices simultaneously. While the newer 64-bit PCI-X bus offers more bandwidth, its larger width introduces new challenges that PCI already faces.
Thankfully, PCI Express (PCIe) overcomes many of these limitations, offering increased bandwidth and maintaining compatibility with existing operating systems.
Fast Serial Connection
In the early days of computing, data was primarily transmitted over serial connections. Computers would divide the data into packets and send them one by one. Although reliable, serial connections were slow, prompting manufacturers to adopt parallel connections to transmit multiple data pieces at the same time.
However, parallel connections introduced their own issues as speeds increased—such as electromagnetic interference between wires. As a result, the trend is shifting back toward highly-optimized serial connections. Enhancements in hardware and the techniques for segmenting, labeling, and reassembling packets have resulted in faster serial connections like USB 2.0 and FireWire.
The Benefits of PCIe Devices
PCI Express is a serial connection that operates more like a network than a traditional bus. Rather than using a single bus to handle data from multiple sources, PCIe utilizes a switch that manages several point-to-point serial connections. (For more information, refer to How LAN Switches Work.)
These connections branch out from the switch, providing direct links to the devices that need the data. Each device gets its own dedicated connection, eliminating the need for shared bandwidth like in a standard bus system. We'll dive deeper into this process in the following section.
Special thanks to Joshua Senecal for his invaluable help with this article.
The Data Transfer Speed of PCIe Lanes
Mytour.comWhen the computer powers on, the PCI Express slots detect which devices are connected to the motherboard. It then maps the links between the devices, establishing a path for data flow and negotiating the width of each connection. This device and connection recognition follows the same protocol as PCI, meaning PCIe does not require any changes to software or operating systems.
Each PCIe lane consists of two pairs of wires—one for transmitting and one for receiving. Data packets travel across the lane at a rate of one bit per cycle. A x1 connection, the smallest PCIe link, contains one lane with four wires and carries one bit per cycle in each direction. A x2 link has eight wires and transmits two bits at once, a x4 link sends four bits, and so on. Other configurations include x12, x16, and x32.
Peripheral Component Interconnect Express is available for both desktop and laptop PCs. It could reduce motherboard production costs as its connections have fewer pins compared to PCI connections. PCIe also supports a wide range of devices, including Ethernet cards, USB 2, and video cards.
But how can a single serial connection outperform the 32 wires of PCI or the 64 wires of PCI-X? In the upcoming section, we will explore how PCIe achieves high bandwidth through its serial configuration.
The "x" in an "x16" connection stands for "by." PCIe connections are scalable by one, two, four, and so on.
PCI Express Data Transfer Rates
Devices connected via PCI share a common bus, whereas each device using PCI Express benefits from its own individual connection to the switch.
Mytour.comThe 32-bit PCI bus operates at a maximum speed of 33 MHz, allowing for a data transfer rate of up to 133 MB per second. In contrast, the 64-bit PCI-X bus doubles the width of the PCI bus. Various PCI-X standards support data transfer rates ranging from 512 MB to 1 GB per second.
A single PCIe lane, on the other hand, can manage 200 MB of data in each direction per second. A x16 PCIe connection can handle an impressive 6.4 GB of data per second in each direction. At these speeds, a x1 connection is more than sufficient for a gigabit Ethernet connection, as well as audio and storage applications. Meanwhile, a x16 connection can easily support high-performance graphics cards.
How is this possible? A few key innovations have led to this remarkable leap in the speed of serial connections:
- Data prioritization, which ensures that the most crucial data is processed first, preventing bottlenecks.
- Time-sensitive (real-time) data transfers.
- Enhancements in the materials used to construct the connections.
- Improved handshaking and error detection techniques.
- Better methods for splitting data into packets and reassembling them. Additionally, since each device has its own dedicated, point-to-point connection to the switch, signals from multiple sources no longer need to navigate the same bus.
In parallel connections, interference and signal degradation are common. Inferior materials and crosstalk between nearby wires introduce noise, which slows down the connection. The added bandwidth of the PCI-X bus allows it to carry more data, but this increased data traffic also generates more noise. PCI protocol lacks data prioritization, causing important data to get stuck in the bottleneck. While the Accelerated Graphics Port (AGP) slot offloads some of the traffic from video cards, it isn't enough to keep up with faster processors and I/O devices.
PCIe Card and Advanced Graphics
NVIDIA SLI link card
Photo courtesy NVIDIAWe've established that the data link layer of PCIe can remove the necessity for an AGP connection. A x16 PCIe slot has the capacity to handle much more data per second than the current AGP 8x connections.
Additionally, a x16 PCIe slot can provide 75 watts of power to the video card, compared to the 25-watt/42-watt limitation of the AGP 8x connection. But PCIe offers even greater potential for the future of graphics technology.
Manufacturers Leveraging External PCI Express
With the appropriate hardware, a motherboard equipped with two x16 PCIe slots can support two graphics adapters simultaneously. Various manufacturers are creating and releasing systems that exploit this feature:
NVIDIA Scalable Link Interface (SLI): With an SLI-compatible motherboard, two SLI-certified graphics cards, and an SLI connector, users can install two video cards in the same system. The cards cooperate by dividing the screen in half, with each card managing one half, and the connector ensures synchronization between them.
ATI CrossFire: Two ATI Radeon video cards, one equipped with a "compositing engine" chip, connect to a compatible motherboard. ATI's approach emphasizes image quality and does not mandate the use of identical video cards, though high-performance systems require matching cards. Crossfire divides the rendering task in one of three ways:
- splitting the screen into two halves and assigning one half to each card (known as "scissoring")
- dividing the screen into tiles (similar to a checkerboard), with one card rendering the "white" tiles and the other handling the "black" tiles
- alternating between the cards to render every other frame
Alienware Video Array: Two standard video cards are combined with a Video Merger Hub and custom software. This system incorporates specialized cooling and power solutions to manage the additional heat and energy demands from the video cards. Alienware's setup may eventually support up to four video cards.
The PCI Express Standard
Since PCI, PCI-X, and PCI Express are all compatible with one another, they can coexist indefinitely. So far, video cards have transitioned the most quickly to the PCIe format.
Network and audio adapters, along with other peripherals, have evolved more slowly. However, as PCIe is fully compatible with existing operating systems and supports faster data transmission to multiple devices, it is expected to eventually replace PCI as the standard for PCs. Over time, PCI-based cards will become outdated.
Smaller PCIe cards can fit into larger PCIe slots, with the system simply ignoring the extra connections. For instance, a x4 card can be inserted into a x16 slot. On the other hand, a x16 card would be too large to fit into a x4 slot.
