The FireWire connector has six contacts, as shown in Figure 3-2. The connector pin assignments are shown in Table 3-2.
Figure 3-2 FireWire connector
When the computer is on, the power pin provides a maximum voltage of 24 V (no load) and up to 7 W power. The power is shared with the USB port, which can use up to 2.5 W. The power available for FireWire is reduced by the power consumed by the USB port.
Pin 2 of the 6-pin FireWire connector is ground for both power and the inner cable shield. If a 4-pin connector is used on the other end of the FireWire cable, its shell should be connected to the wire from pin 2.
The signal pairs are crossed in the cable itself so that pins 5 and 6 at one end of the cable connect with pins 3 and 4 at the other end. When transmitting, pins 3 and 4 carry data and pins 5 and 6 carry clock; when receiving, the reverse is true.