Matrox PCI and PCIe Guide
Matrox guide to different types of expansion slots and add-in cards
Matrox makes a variety of graphics cards designed to be inserted into certain types of computer expansion slots. The most common slot types used by graphics cards are PCI™ and PCIe® and for each of these types, there are also several sub-types. The different slot types available are an important consideration when buying a graphics card or computer. This guide describes differences between these slot types and their sub-types.
PCI (Peripheral Component Interconnect) is a type of computer bus for attaching or inserting peripheral devices into a computer. The PCI standard was first proposed by Intel in 1990 and was widely implemented in computers by 1995. Today, the specifications for PCI and its variants are maintained by the PCI-SIG® (PCI Special Interest Group), a consortium of over 700 companies.
PCI is a general-purpose connection standard designed to support multiple devices of various kinds, including graphics hardware, audio hardware, network hardware, and so on. Revisions of the PCI standard have added new features and performance improvements, including different bus speeds and bus widths. Below is a summary of the different potential bandwidths for the most popular variants of the basic PCI standard.
|Bits / connection||MHz||Potential bandwidth
These various types of slots and expansion cards are generally compatible with each other. However, unless a card and slot are designed to use a wider bus (that is, 64 bits) or a faster bus speed (66 MHz) they generally default to the lower setting.
For example, a 64-bit PCI card like Matrox P690 Plus LP PCI has an edge connector that's wider (longer) than for a 32-bit PCI card like Matrox G450x4 MMS. Despite this, a 64-bit PCI card can be inserted into a 32-bit PCI slot. In this case, part of the edge connector simply overhangs the slot and only the first part of the edge connector is used (that is, only 32-bit communication occurs). By the same token, a 32-bit PCI card can be inserted into a 64-bit slot. In this case, the edge connector of the card will only fill part of the slot and the connection will be 32-bit.
Matrox P690 Plus LP PCI (64-bit)
Matrox G450x4 MMS (32-bit)
There's also an extension of the PCI standard referred to as PCI-X (not to be confused with PCI Express). Cards and slots designed for PCI-X are capable of bus speeds higher than 66 MHz. PCI-X slots are commonly available in servers and high-end workstations. A 64-bit, 66 MHz PCI card is compatible with PCI-X slots and can run at 66 MHz in such a slot.
PCI cards and slots are keyed to support different voltages. PCI cards and slots may run at 5 or 3.3 volts. All currently shipping Matrox PCI cards are compatible with either voltage and are keyed accordingly.
PCIe (PCI Express®) is the more recently introduced standard for connecting devices to computers. It's software-compatible with PCI but has higher potential bandwidth and greater flexibility than PCI. The PCIe specification is also maintained by the PCI-SIG.
A connection between a PCIe device and the system is known as a "link" and this link is built around a dedicated, bi-directional, serial (1-bit), point-to-point connection known as a "lane". A lane is capable of simultaneously transferring 250 MB/s of data in each direction. A link can use more than one lane at a time but all links compliant with the PCIe specification must minimally support single-lane connections, referred to as "x1" (pronounced "by-one") links.
For higher potential bandwidth, PCIe devices and systems can optionally support links using multiple simultaneous lanes—for example, a "x16" link uses 16 lanes. To support extra lanes, a PCIe card and slot must be designed to accommodate the extra electrical lines required (2 lines per lane). Card and slot types exist for x1, x4, x8, and x16 links.
Currently, the only devices that use a x16 link are graphics cards. Other devices typically don't require the high potential bandwidths provided by such a connection. Matrox has several PCIe x16 graphics cards, including the six-output C680 PCIe x16, the nine-output C900 PCIe x16, the quad-output M9148 LP PCIe x16, and eight-output M9188 PCIe x16.
Matrox C-Series family of PCIe x16 graphics cards
|PCIe cards will physically fit into slots designed for their lane configuration or higher (up-plugging) but not into slots designed for lower lane configurations (down-plugging). So, for example, a x1 card will fit into x1, x4, x8, and x16 slots but a x16 card will only fit into a x16 slot. A x1 card in any compliant PCIe slot will always run in x1 mode. Matrox introduced the world's first PCIe x1 graphics cards, the Millennium G550 PCIe and Millennium G550 LP PCIe.|
The internal architecture of PCIe is much like a local area network in that each link goes to a central hub in the computer that performs network-like switching. This is in contrast to the PCI architecture, where all devices share the same unidirectional, parallel bus. Because PCIe isn't based on parallel connections that can be hindered by timing issues, PCIe allows data to be more easily and cost-effectively transmitted over longer distances. In fact, the PCI-SIG is developing a cabling specification to allow external devices to be connected to a computer using the PCIe standard.
Potential bandwidths of PCI, and PCIe
The higher potential bandwidth that certain slot types provide don't necessarily result in proportionally higher performance. The bandwidth associated with each slot type is the maximum achievable and is subject to limitations due to software overhead (for example, operating system activity) and whether an application is maximizing usage. For example, a simple 2D application like a spreadsheet or word processing program is less likely to benefit from the advantages of this higher bandwidth. Intensive, real-time, 3D programs are more likely to use such extra bandwidth.
The differences in these bandwidths only affect the speed at which data is transferred between the graphics hardware and the rest of the computer. These bandwidths don't affect the speed of the graphics chip itself and don't directly affect the speed of the rest of the computer.
The following summarizes the differences in potential bandwidth between the various slot types.