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The FireWire connector has six contacts, as shown in Figure 3-2. The connector pin assignments are shown in Table 3-2.
When the computer is on, the power pin provides a maximum voltage of 24 V (no load) and up to 6 W total power (shared by both connectors). Maximum current is 0.5 A and is controlled by an auto-resetting fuse.
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.
Voltage--up to 15 V--is still present at the power pin when the computer is in Sleep mode or when it is off but the power cord is still plugged in, and devices that use less than 1 W can continue to operate.
WARNING
Devices that draw several watts of power from pin 1 should not attempt to operate while the computer is in Sleep mode or is turned off. Doing so is likely to exceed the 0.5 A maximum current and blow the fuse.
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, developers should refer to the resources available on the Apple FireWire web site at: