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132 pin EDGE (AGP bus) connector layout
132 pin EDGE (AGP bus) connector
at the computer motherboard (key NOT shown)
AGP (Accelerated Graphics port) is a modified version of PCI bus designed to speed up transfers to video cards.

The Accelerated Graphics Port (also called Advanced Graphics Port) is a high-speed point-to-point channel for attaching a single device (generally a graphics card) to a computers motherboard, primarily to assist in the acceleration of 3D computer graphics. Many classify AGP as a type of computer bus, but this is something of a misnomer since buses generally allow multiple devices to be connected, while AGP does not. AGP originated from Intel, and it was first built into a chipset for the Pentium II microprocessor. AGP cards generally slightly exceed PCI cards in length and can be recognized by a typical hook at the inner end of the connector, which does not exist on PCI cards. Nowdays AGP is almost replaced by PCI-Express.

AGP versions:

In addition, in the world of workstations different AGP Pro cards exist with extra connectors which allow card to draw more power. In order to make life easier, the AGP standard defines some backward compatibilty. The AGP 1.0 specification requires that all implementations support the 1x speed multiplier at 3.3 volts. By default, when the AGP 1.0 machine powers up it selects the fastest speed multiplier supported by both the video card and the motherboard. If they both support 2x then they will run at 2x. Otherwise they run at 1x which is always implemented by all AGP 1.0 video cards and motherboards. The AGP 2.0 specification has a similar requirement. 2x and 1x support at 1.5 volts are required and 4x support is optional. The AGP 3.0 specification requires support for 8x. The 3.0 specification isn't as clear as the 1.0 and 2.0 specifications on the subject of requiring the lower multiplier but all AGP 3.0 almost all implementations support both 8x and 4x. As a result, you can completely ignore speed multipliers when you're checking for compatibility between an AGP video card and an AGP motherboard. If the video card and motherboard both support the same signaling voltage then there is always at least one common speed multiplier supported by both at that voltage. You only need to make sure that the video card and motherboard have at least one signaling voltage in common.

AGP cards and slots
Graphics Card Types Connector Type* Description
AGP 3.3V Card 3.3V slot Supports only 3.3V signaling. Available speeds 1x, 2x.
AGP 1.5V Card 1.5V slot Supports only 1.5V signaling. Available speeds 1x, 2x, 4x.
Universal AGP Card Double slotted Supports 3.3V and 1.5V signaling. Available speeds 1x, 2x at 3.3V and 1x, 2x, 4x at 1.5V.
AGP 3.0 Card 1.5V slot Supports only 0.8V signaling. Available speeds 4x, 8x.
Universal 1.5V AGP 3.0 Card 1.5V slot Supports 1.5V and 0.8V signaling. Available speeds 1x, 2x, 4x at 1.5V and 4x, 8x at 0.8V.
Universal AGP 3.0 Card Double slotted Supports AGP 3.3v, 1.5V, and 0.8V signaling. Available speeds 1x, 2x at 3.3V and 1x, 2x, 4x at 1.5V and 4x, 8x at 0.8V.
*Different slots connectors have different position of key

The AGP connectors on the motherboard are keyed to prevent insertion of AGP cards which would be damaged if plugged in. An AGP 3.3V motherboard connector can only accept AGP cards which have the 3.3V slot. If you try to insert a card without a 3.3V slot into an AGP 3.3V motherboard connector, the card will bump into the connector key and cannot be inserted. Likewise an AGP 1.5V motherboard connector can only accept AGP cards with the 1.5V slot. An AGP universal motherboard connector has no keys and therefore can accept any kind of AGP card. An AGP card with both voltage slots can be plugged into any kind of AGP motherboard connector. If you can plug an AGP card into an AGP motherboard connector, then neither the card nor the motherboard will be damaged (assuming they obey the AGP specifications).

AGP pinout

 
3.3 Volt Boards
Universal Boards
1.5 Volt Boards
Pin # Side A Side B Side A Side B Side A Side B
1 +12V OVRCNT# +12V OVRCNT# +12V OVRCNT#
2 TYPEDET# +5.0V TYPEDET# +5.0V TYPEDET# +5.0V
3 Reserved 5.0V Reserved 5.0V Reserved 5.0V
4 USB- USB+ USB- USB+ USB- USB+
5 Ground Ground Ground Ground Ground Ground
6 INTA# INTB# INTA# INTB# INTA# INTB#
7 RST# CLK RST# CLK RST# CLK
8 GNT# REQ# GNT# REQ# GNT# REQ#
9 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3
10 ST1 ST0 ST1 ST0 ST1 ST0
11 Reserved ST2 Reserved ST2 Reserved ST2
12 PIPE# RBF# PIPE# RBF# PIPE# RBF#
13 Ground Ground Ground Ground Ground Ground
14 Reserved Reserved WBF# Reserved WBF# Reserved
15 SBA1 SBA0 SBA1 SBA0 SBA1 SBA0
16 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3
17 SBA3 SBA2 SBA3 SBA2 SBA3 SBA2
18 Reserved SB_STB SB_STB# SB_STB SB_STB# SB_STB
19 Ground Ground Ground Ground Ground Ground
20 SBA5 SBA4 SBA5 SBA4 SBA5 SBA4
21 SBA7 SBA6 SBA7 SBA6 SBA7 SBA6
22 Key Key Reserved Reserved Reserved Reserved
23 Key Key GROUND GROUND GROUND GROUND
24 Key Key Reserved 3.3Vaux Reserved 3.3Vaux
25 Key Key Vcc 3.3 Vcc 3.3 Vcc 3.3 Vcc 3.3
26 AD30 AD31 AD30 AD31 AD30 AD31
27 AD28 AD29 AD28 AD29 AD28 AD29
28 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3
29 AD26 AD27 AD26 AD27 AD26 AD27
30 AD24 AD25 AD24 AD25 AD24 AD25
31 Ground Ground Ground Ground Ground Ground
32 Reserved AD STB1 AD STB1# AD STB1 AD STB1# AD STB1
33 C/BE3# AD23 C/BE3# AD23 C/BE3# AD23
34 Vddq 3.3 Vddq 3.3 Vddq Vddq Vddq 1.5 Vddq 1.5
35 AD22 AD21 AD22 AD21 AD22 AD21
36 AD20 AD19 AD20 AD19 AD20 AD19
37 Ground Ground Ground Ground Ground Ground
38 AD18 AD17 AD18 AD17 AD18 AD17
39 AD16 C/BE2# AD16 C/BE2# AD16 C/BE2#
40 Vddq 3.3 Vddq 3.3 Vddq Vddq Vddq 1.5 Vddq 1.5
41 FRAME# IRDY# FRAME# IRDY# FRAME# IRDY#
42 Reserved 3.3Vaux Reserved 3.3Vaux KEY KEY
43 Ground Ground Ground Ground KEY KEY
44 Reserved Reserved Reserved Reserved KEY KEY
45 VCC 3.3 VCC 3.3 VCC 3.3 VCC 3.3 KEY KEY
46 TRDY# DEVSEL# TRDY# DEVSEL# TRDY# DEVSEL#
47 STOP# Vddq 3.3 STOP# Vddq STOP# Vddq 1.5
48 PME# PERR# PME# PERR# PME# PERR#
49 Ground Ground Ground Ground Ground Ground
50 PAR SERR# PAR SERR# PAR SERR#
51 AD15 C/BE1# AD15 C/BE1# AD15 C/BE1#
52 Vddq 3.3 Vddq 3.3 Vddq Vddq Vddq 1.5 Vddq 1.5
53 AD13 AD14 AD13 AD14 AD13 AD14
54 AD11 AD12 AD11 AD12 AD11 AD12
55 Ground Ground Ground Ground Ground Ground
56 AD9 AD10 AD9 AD10 AD9 AD10
57 C/BE0# AD8 C/BE0# AD8 C/BE0# AD8
58 Vddq 3.3 Vddq 3.3 Vddq Vddq Vddq 1.5 Vddq 1.5
59 Reserved AD STB0 Reserved AD STB0# Reserved AD STB0#
60 AD6 AD7 AD6 AD7 AD6 AD7
61 Ground Ground Ground Ground Ground Ground
A62 AD4 AD5 AD4 AD5 AD4 AD5
63 AD2 AD3 AD2 AD3 AD2 AD3
64 Vddq 3.3 Vddq 3.3 Vddq Vddq Vddq 1.5 Vddq 1.5
65 AD0 AD1 AD0 AD1 AD0 AD1
66 Reserved Reserved Vrefcg Vrefcg Vrefcg Vrefcg

The AGP bus is 32 bits wide, just the same as PCI is, but instead of running at half the system (memory) bus speed the way PCI does, it runs at full bus speed. This means that on a standard Pentium II motherboard AGP runs at 66 MHz instead of the PCI buss 33 MHz. This of course immediately doubles the bandwidth of the port; instead of the limit of 127.2 MB/s as with PCI, AGP in its lowest speed mode has a bandwidth of 254.3 MB/s. The AGP specification is in fact based on the PCI 2.1 specification, which includes a high-bandwidth 66 MHz speed.

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