AGP Compatibility
- Speed is always backwards compatible
- Signalling voltages can be incompatible
- Connectors are keyed so that you can only use electrically compatible
equipment
AGP Speed
Speed is always backwards compatible. For example, a 4x device
must be able to run at 2x and 1x. No compatibility issues here.
Signalling Voltages
Both the motherboard and adapter card must signal at
the same voltage to function. Different speeds require different signalling
voltages for operation. For example, AGP 8x signals at 0.8V, while AGP 1x can
signal at 1.5V or 3.3V. Physical keys in the connectors dictate which signalling
voltages the devices is capable of operating at.
| Speed |
Signalling Voltage |
| 1x/2x |
1.5V or 3.3V |
| 4x |
0.8V or 1.5V |
| 8x |
0.8V |
AGP Connector Keying
AGP connectors are physically keyed to specify the
signalling voltages the device is capable of operating at. AGP specifies two
keys, a 3.3V key and a 1.5V key. Unfortunately they added a third voltage, 0.8V.
If a device operates at 0.8V it uses the 1.5V key and must be tolerant of 1.5V
signalling, but not necessarily capable of operating at 1.5V. A key is
manifested as a raised area in a female connector and a gap or absence of pins
in a male connector. The key physically prevents an electrically incompatible
card from being inserted into a slot.
If a device has the 1.5V key and claims operation at 8x speed you know that
it is capable of 0.8V operation.
AGP Connector Keying
AGP Pro
AGP Pro is a extention to the standard AGP connector on both
sides to provide additional power to an AGP card.
It comes in two flavors,
AGP Pro110 provides for 50-110W of power and requires two adjacent PCI slots for
cooling. AGP Pro50 provides for 20-50W of power and requires an adjacent PCI
slot for cooling.
An AGP Pro card will not work in an AGP slot.
An AGP card will work in an
AGP Pro slot, however the card must have a registration tab.
AGP
Pro Slot
AGP Registration Tab
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