Expansion Slot

First you need to examine your motherboard for the video expansion slot. Most common expansion slots used for video are AGP, PCI and PCI Express (or PCI-e). In modern computers PCI slots are not used any longer for video cards so we will concentrate on AGP and PCI-e (don’t confuse this with PCI-x).

AGP slots come in variations:
  • AGP 1x - 32-bit channel operating at 66 MHz. Its bandwidth is 266 MB/s, which is twice higher than original PCI's bandwidth of 133MB/s.
  • AGP 2x - 32-bit channel operating at the doubled bandwidth of 533 MB/s at the same 66 MHz owing to dual-way data transfers similar to DDR memory (only directed to a video card.)
  • AGP 4x - the same 32-bit channel at 66 MHz, but with effective frequency quadrupled to 266 MHz, maximum bandwidth exceeding 1 GB/s.
  • AGP 8x - additional modifications allowed achieving 2.1 GB/s bandwidth. AGP video cards require 1.5 V (older) or 3.3 V (newer) AGP slots

The graphics card voltage specification needs to match the motherboard AGP slot voltage specification, so don’t expect a 3.3 V graphics card to work in your old motherboard with 1.5 V AGP slot. Most new graphics cards have a special key that will not even let you insert the card into the wrong AGP slot.

Some motherboards have what’s called an AGP Express slot. Stay away from the motherboards with this slot as they tend to perform much slower.

PCI Express is the latest bus standard. Unlike the regular PCI and AGP buses, the PCI Express uses serial interface and not parallel and operates in a full-duplex mode. It has less PINs and more bandwidth. Other examples of serial bus interfaces are SerialATA, HyperTransport, USB and Firewire.

Data is transmitted in this bus through two pairs of wires called a lane. Each lane allows a maximum transfer rate of 250 MB/s in each direction. The PCI Express bus can be built by combining several lanes in order to achieve higher performance. We can find PCI Express systems with 1, 2, 4, 8, 16 and even 32 lanes. For example, the transfer rate of a PCI Express system with 16 lanes (x16) is 4 GB/s (250 * 16).

Bus

Maximum Transfer Rate

PCI

133 MB/s

AGP 2x

533 MB/s

AGP 4x

1,066 MB/s

AGP 8x

2,133 MB/s

PCI Express x1

250 MB/s

PCI Express x2

500 MB/s

PCI Express x4

1,000 MB/s

PCI Express x16

4,000 MB/s

PCI Express x32

8,000 MB/s


The second version of PCIe (PCIe 2.0) doubles the transfer rates of the PCIe 1.0, but supports both specifications for backwards compatibility with previous versions of PCIe (1.0 and 1.1). Backward compatibility of the new PCI Express 2.0 allows to use of old solutions with 2.5 Gbps in 5.0 Gbps slots, which will just work slower.

Most current video cards are PCI Express. Make sure you pay close attention to this when matching video card with the motherboard to make sure not only that the card physically fits into your motherboard expansion slot, not only that it works, but also that it performs at its highest capacity. For example, don’t buy an expensive PCI Express x16 card and a cheap PCI Express x1 motherboard. Make sure your motherboard expansion slot will also support x16 PCIe bus.

Some motherboards have built-in video card, which means that the Graphics Processing Unit (GPU) or the graphic chip is already included on the motherboard. In most cases you can still add a video card and disable the on-board video in the BIOS.


Next: Output Connectors