The FireWire connector has six contacts, as shown in Figure 3-2. The connector signals and pin assignments are shown in Table 3-2.
The power pin provides up to 15 W total power on both connectors. The voltage on the power pin is unregulated, ranging from 20 V under a typical load to 30 V under no load. The power pin also accepts external power at 8 to 33 V, in conformity with the P1394a draft standard, to keep the FireWire bus connected when the computer is turned off.
Pin 2 of the 6-pin FireWire connector is ground return 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.