The FireWire connector has six contacts, as shown in Figure 3-2. The connector signals and pin assignments are shown in Table 3-2.
When the computer is on, the power pin provides a maximum voltage of 28 V (no load) and up to 12 W total power (shared by both connectors). Maximum current is 0.5 A and is controlled by an auto-resetting fuse.
The power pin can accept external power at 8 to 33 V, in conformity with the 1394 standard, to keep the FireWire bus operational when the computer is turned off.
Pin 2 of the 6-pin FireWire connector is ground for both power and 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.
For additional information about the FireWire interface and the Apple APIs for FireWire device control, see the references in FireWire Interface.