What are PCI and PCI Express?

PCI (Peripheral Component Interconnect) is a standard computer bus for connecting peripheral devices directly to your motherboard.

These devices can be ICs that plug into the IC or expansion cards that fit into the connectors. If it stops working, a USB-RJ45 Adapter can be used.

What are PCI and PCI Express Slots on Computer Motherboards & Their History

PCI work began in 1990 at the Intel lab in Atlacomulco, Mexico. Only the component-level specification, version 1.0, was released on June 22, 1992. Version 2.0, the first standard for connectors and motherboard ports, was released in 1993. Version 2.1 was released on June 1, 1995.

It was immediately available for servers, replacing MCA and EISA as an option for the expansion bus.

The replacement of VESA Local Bus on the PC was slower and did not gain enough market penetration with the second generation Pentiums after 1994.

By 1996, VESA was discontinued. Later, companies even replaced the 80486 computer. Apple adopted this slot structure for the Power Macintosh in mid-1995 and Performa in mid-1996.

The new slot versions added features and performance improvements, such as the 66MHz 3.3V standard and the 133MHz standard called PCI-X. Both PCI-X1.0b and PCI-X2.0 are compatible with the previous ones.

With the introduction of the Express series version in 2004, motherboard manufacturers included fewer PCI slots in favor of the new standard. However, it is still common for both interfaces to be implemented.

Automatic Configuration

The x86 processor family has 2 separate 32-bit and 64-bit address spaces corresponding to memory and I/O address ports. The software assigns addressing.

A third address space called the Configuration Space uses a modified addressing scheme that allows the software to determine the amount of memory and I/O address space required for each device.

Each device you connect can request up to six memory or I/O port spaces through the configuration space register.

In a typical system, Firmware consults all PCIs at startup to tell them what devices are available, what resources are available to each device, and what they are.

The configuration space contains a small amount of information about each device that helps the operating system select its drivers or at least have a dialogue about the system configuration.

Devices may have an ROM containing executable code for x86 or PA-RISC processors, an Open Firmware driver, or an EFI driver. These are usually required for devices used during system startup before the operating system loads their drivers.

The bus mastering master has latency timers, which are a mechanism for sharing the bus more fairly. In this case, it means that the devices do not use large portions of the available bus bandwidth, while others are not sufficient and are not needed for the job. Note that this is not the case for PCI-E.

The mode of operation of this is that it can operate in a bus-master mode, which requires each device to apply a clock called a latency clock that limits the amount of time it can occupy the bus.

When the counter reaches 0, the device is asked to leave the bus. If there is no other device waiting for bus ownership, it can take it back and transfer more data.

There is a latency tool available. You can use a search engine for the latest version. This tool can change/adjust any latency.

PCI Hardware Features

This is a typical 32-bit card. The following features represent the most common versions used in PCs;

• 33.33 MHz clock with synchronous transfer.
• 32-bit or 64-bit data bus width.
• 133 MB/s maximum transfer rate on 32-bit data bus (33.33 MHz×32 bits÷8 bits/byte=133 MB/s).
• 266 MB/s maximum transfer rate on 64-bit data bus.
• 32-bit address space (4 GB).
• 32-bit I/O port space (now deprecated).
• 256-byte configuration space.
• 3.3V or 5V, depending on the device.
• Reflected-wave switching.

Variants

• Cardbus is a 32-bit, 33 MHz PCMCIA format.
• Compact-PCI uses Eurocard-size modules connected to a daughterboard.
• Version 2.2 operates at 66 MHz (requires 3.3 volts on signals) (maximum transfer rate of 503 MiB/s (533MB/s).
• Version 2.3 allows 3.3 volts and universal signaling but does not support 5 volts on boards.
• Version 3.0 is the official final standard for the bus, with the 5-volt bracket wholly removed.
• PCI-X slightly changes the protocol and increases the data transfer rate to 133 MHz (maximum transfer rate of 1014 MiB/s).
• PCI-X 2.0 specifies 266 MHz (maximum transfer rate of 2035 MiB/s) and also specifies 533 MHz, expands the configuration space to 4096 bytes, adds 16-bit data path variation, and uses 1 signal, 5 volts.
• MiniPCI is a new version of 2.2 for internal use in laptops.
• PC/104-Plus uses signals with different connectors as an industrial data bus.
• Advanced Telecommunications Computing Architecture (ATCA or AdvancedTCA) is the next generation of data buses for the telecommunications industry.
• PXI is the data bus extension for instrumentation and control.

Full-size Card

The original full-size slot is approximately 107 mm (4.2 in) thick and 312 mm (12.283 in) long. The height includes the card-edge connector. However, the latest cards are half the length or more minor. And many PCs cannot fit a full-size card.

The Backplate Card

In addition to these dimensions, the size of the Backplate is also standardized. The Backplate is the metal part that is used to attach the edge to the chassis and contains the external connectors. The card may be smaller in size. However, the Backplate must be the exact size and properly positioned. Compared to the previous ISA bus, it is located on the opposite side of the card to prevent errors.

The Half-Length Extension Card

In fact, it fits most modern boards and sizes, which are the practical standard today.

• Width: 15.24mm (0.6 inches)
• Depth: 175.26mm (6.9 inches)
• Height: 106.68 mm (4.2 inches)

Low Profile Card (Medium Size)

It defines a standard for low-profile boards that basically fit in the following ranges:

• Height: 36.07 mm (1.42 in) – 64.41 mm (2.536 in)
• Depth: 119.91 mm (4.721 in) – 167.64 mm (6.6 in)

The height of the rack is also reduced to a standard of 79.2 mm (3.118 in). The smallest rack will not fit a standard PC. Many manufacturers solve this by providing both types of racks (the racks are usually screwed to the board, so swapping them is not difficult).

Mini-PCI

This slot structure was added to PCI version 2.2 for use in notebook computers. It uses a 32-bit, 33-MHz bus with driven connections and bus raid and DMA support. The standard size for Mini PCI cards is about 1/4 of the life-size.

Since the cards do not have external access like on the desktop, Mini PCI is generally limited in the functions they can perform.

Many Mini PCI devices have been developed with Wi-Fi Technology, Fast Ethernet, Bluetooth, Modems, sound cards, encryption accelerators, SCSI, IDE/ATA, SATA combination cards, and regulators.

Regular PCI cards can be used with Mini PCI hardware. Instead, Mini PCI to PCI and PCI to Mini PCI converters can be used. PCI Express Mini Card has replaced mini PCI.

Technical Details of Mini PCI Cards

These cards have a maximum consumption of 2W. This limits the functionality that can be implemented in this form factor. They also require support for CLKRUN#, which is the signal used to start and stop the clock for power management purposes.

There are three card form factors: Type I, Type II, and Type III. The card connector used for each type uses a 100-pin docking connector in Type I and II. Type III uses a 124-pin edge connector. For example, the Type I and II connectors differ from Type III in that the connector is on the edge of the card, as in an SO-DIMM.

The additional 24 pins provide the additional signals needed for the rear input path of the connector system. Type II cards are equipped with RJ11 and RJ45 connectors. These cards must be placed on the edge of the docked computer or station so that the RJ11 and RJ45 ports can be installed for external access.

Other Physical Variations

Typical consumer systems specify “NxPCI slots” without specifying the actual dimensions of the usable space. In some small form factor systems, there is not yet enough room for half-length cards to fit into this slot. Despite this limitation, these systems are still usable because many modern cards are considerably smaller than half-length cards.

Typical cards have one or two key notches, depending on their nominal voltage. Cards requiring 3.3 volts have a 56.21 mm notch on the front of the card. Cards requiring 5 volts have a 104.47 mm notch on the front of the card. Universal cards have both key notches and accept both signal types.

What is PCI Express?

PCI Express (PCI-E or PCIe) is the latest evolution of the PCI bus. It is a native serial bus, unlike PCI, which is parallel. It was also developed by Intel, who first introduced it to the market in 2004 on the 915P chipset.

This new bus is used to connect expansion cards to the motherboard. Plus, it is intended to replace all of a computer’s internal expansion buses, including PCI and AGP.

Functions

It is connected in series with PCIe peripherals. Each serial connection operates at 250MB/s and 0.8V with 75W dedicated power for the entire data path. It also requires fewer cables.

In this case, we are talking about the PCIe lane. The rails take up less space on motherboards, and the connectors are more miniature. Its advantages are especially noticeable in laptops and high-end motherboards.

The PCIe bus can be created by combining multiple lanes to achieve higher performance. We can find the bus in various versions; there are X1, x2, x4, x8, x12, x16 and x32 lane versions.

For example, a card with 8 lanes (x8) has a transfer rate of 2 GB/s (250 x 8). In version 1.1, it allows transfer rates from 250 Mb/s to 8 Gb/s.

PCI Express Buses

1) 1x

Express 1x offers a transfer speed of 250 Mb/s. All existing motherboards have 1 or two.

2) 2x

Express 2x offers a transfer speed of 500 Mb/s. This bus is not very common and is reserved for servers.

3) 4x

Express 4x is reserved for servers, offering a transfer rate of 1000 Mb/s.

4) 16x

Express 16x offers transfer speeds of 4000 Mb/s, is widely used, found on all modern motherboards, and is the standard format for graphics cards.

5) 32x

Express 32x offers a transfer speed of 8000 Mb/s. It has the same format as PCI Express 16x. It is usually used in high-end motherboards to power the SLI or Crossfire bus. References for these motherboards are usually called 32.

This allows you to have two 16-format express ports on 16 lanes, wired in 2×8 lanes or, in the basic Crossfire, 1×16 + 1×4 lanes, unlike classic SLIs. These motherboards are also characterized by the presence of an additional south bridge dedicated exclusively to this 32x bus.

New Generation

PCI-SIG, responsible for defining the specifications of the express bus, has launched the PCIe 2.0 bus. The latest Intel chipset and the latest generation of graphics cards support it.

The PCIe 2.0 bus features double bandwidth for each lane, 500Mb/s against 250Mb/s, while the power supply of the bus is between 75 Watt and 150 Watt due to the increased power of the graphics cards on the board.

Also, in the PCIe 2.0 specifications, there is a new 8-pin power connector for the latest generation of graphics cards.

PCI Express 3.0 has a bandwidth of 7.877 Gbit/s (984.625 MB/s), while version 4.0 has speeds of 15.752 Gbit/s (1969 MB/s).