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The PCI bus supports an IEEE 1394 FireWire controller IC with a maximum data rate of 400 Mbits (50 MBytes) per second. The controller IC implements the FireWire link layer. A physical layer IC, called a PHY, implements the electrical signaling protocol of the FireWire interface. The PHY supports two FireWire ports by way of the external connectors on the rear panel.
The computer is capable of accepting external power through the FireWire connector to operate the PHY when the computer is turned off. While the PHY is operating, it acts as a repeater from one port to another so that the FireWire bus remains connected. For more information, see FireWire Ports .