Thursday, July 17, 2008

Graphics Card Interface


This is the location of the graphics card interface, which nowadays can be AGP or PCI Express.

This is the part of the video card that plugs into your computer's motherboard. It is through this slot, or 'interface', that your graphics card and computer hand each other information. Since most motherboards only have a single type of graphics card slot, it is very important to buy a graphics card that matches the slot on your motherboard. For example, a PCI Express graphics card will not work in an AGP card slot. Not only will it not fit physically but the protocols for data transmission are different.

The most important aspect of a graphics card interface is the bandwidth. The term "bandwidth" refers to the amount of information that can pass through the interface in a given time.. The more bandwidth the interface offers, the faster a graphics card can perform - in theory. In practice, however, the interface is far less important than what the industry claims.


ISA stands for Industry Standard Architecture

Displayed here only for reference, this is the oldest PC card standard interface. Graphics cards with this interface became obsolete a long time ago. In fact, you won't even be able to buy a motherboard today with an ISA slot.

There are 8 bit and 16 bit ISA cards; only the latter uses both connector blocks (see image). EISA or Extended ISA cards were introduced for motherboards with higher bandwidths at 32 bits wide, and they offered bus mastering. However, they were expensive and were phased out when newer interfaces were commercialized.


The 32 bit classic PCI bus. Still today it is used for all sorts of expansion cards.

PCI stands for Peripheral Components Interconnect. It is a 32 bit wide bus that runs at 33 MHz, delivering a bandwidth of 133 MB/s. The PCI interface replaced ISA and its extensions (VL - Vesa Local Bus) in the 1990s, with the benefit of much higher bandwidth. PCI is the current standard for most computer add-in cards, but contemporary graphics cards no longer use PCI, as they have long since moved on to the AGP (and PCI Express) interface.

In many cases, however, computers from large manufacturers will come without an AGP or PCI Express port for future graphics expansion. In order to upgrade the graphics card on these machines, the only option is a video card with a PCI interface, but these are scarce, overpriced and have low performance.


(Accelerated Graphics Port)

AGP is a high-bandwidth interface designed specifically for graphics cards. It was based on the PCI rev. 2.1 specification. Unlike PCI, which is a shared bus, AGP is dedicated to one device. This allowed AGP to have numerous advantages over PCI such as directly read/write capabilities with the system memory, de-multiplexing or simplification in the organization and transfer of data, and increase clock speeds.

AGP has gone through three major revisions, with the newest being AGP 8x at 2,1 GB/s, meaning that it is eight times faster than the initial AGP standard at 266 MB/s (32 bit, 66 MHz). AGP is being replaced by the PCI Express interface on new motherboards, but AGP 8x (and even AGP 4x) still offer sufficient bandwidth for contemporary video cards. All AGP 8x cards will work in both AGP 4x and AGP 8x slots


PCI-X stands for 'Peripheral Component Interconnect - Extended', which can be taken literally: Its 64 bit wide interface delivers up to 4,266 MB/s, depending on the bus clock speed. PCI-X (not to be confused with PCI Express!) was first a speed upgrade to the PCI bus, but was upgraded with certain features that are required in the server space. It is not very common in ordinary PCs, and PCI-X graphics cards are very rare. You can use a PCI-X card in a regular PCI slot as long as it is a current revision (PCI 2.2 or higher), but you cannot add them to your motherboard with a PCI Express slot.

PCI Express

In contrast to ISA, PCI and AGP, PCI Express is a serial interface subsystem. Thanks to this, it runs with very few connections. Different from parallel buses, the total bandwidth is available for every device, while e.g. several PCI cards have to share the total available bandwidth.

PCI Express works on the basis of multiplying as many single links (or lanes) as required to lineup the desired bandwidth. PCI Express x1 slots are short and compact, and their connection offers 250 MB/s both ways (upstream to the system and downstream to the device). PCI Express x16 (16 links) offers a bandwidth of 4 GB/s up and down or 8 GB/s total. The inferior slot options (x8, x4, x1) are not used for graphics. A mechanical x16 slot does not necessarily have to run at 16 connected PCI Express lanes, though. There are many motherboards available that are capable of running two PCI Express x16 slots at x8 bandwidth each in order to support twin-graphics cards.

Although increased bandwidth is a welcome innovation, a more pressing need was facing the industry: power consumption. The AGP 3.0 standard (AGP 8x) could only deliver a maximum of 41.8 W (6 A from 3.3 V, 2 A from 5 V, 1 A from 12 V = 41.8 W and an additional 1.24 W could come from the 3.3 V auxiliary at 0.375 A). Video cards were introduced with dual 4 pin power sockets such as the ATI Radeon X850XT PE, which has one socket while the Nvidia GeForce 6800 Ultra has two.

By adding the four-pin connections, manufacturers extended the life of AGP cards as each supplied 6.5A or 110.5 W from these right angle connections (12 V + 5 V or 17 V x 6.5 A = 110.5 W). Overall, PCI Express is a much simpler solution as it can deliver 75 W through the x16 connector and an additional 75 W per six-pin connection for a total of 150 W. PCI Express solved the concern for future bandwidth and the power consumption need.

No comments: