Q1: Is it acceptable per the IEEE 1394-1995 standard to only supply power to the PHY via the 1394 cable?

It is acceptable, but not recommended since a large number of nodes may require a large amount of power to be supplied on the cable. A better solution would be to have an alternative power source that takes over supplying 5/3.3V to the PHY whenever the cable power is not available. See our EVM schematics on the TI 1394 External Web Page or the IEEE 1394-1995 standard section 4.2.2.7. Please also see sections 5 and 6 of the Cable Power Distribution specification. If you must power the PHY by the cable only (separately from the link), then be sure to do a software reset of the link layer each time the PHY is powered up.

Q2: Is a DC/DC converter necessary for isolation?

No. A DC/DC converter is required if you plan to be a cable power provider to the bus but do not isolate the entire node with an isolating power supply. The DC/DC converter will isolate the PHY ground from the cable power source ground.

Q3: Does TI have any recommended isolated DC-DC converters needed to supply power over the 1394 cable?

No. TI has not used any DC-DC converters in test situations. We do not have an application with isolated PHY power from a DC-DC converter. In the lab, we have always used a bench supply. If you plan to be a power provider, in addition to powering your own PHY (reference Figure 5 in the TI Galvanic Isolation App Note), an off-the-shelf converter should work.

Q4: What are some isolation suggestions?

If you are implementing isolation you MUST either isolate the power being supplied to the PHY from the link power by using a DC to DC converter or you must supply the PHY from another power supply isolated from the link power. This is one of the uses of cable power if it is supplied from a source isolated from all link power domains. The typical means of isolation is two separate ground planes, one for the physical layer circuitry and 1394 ports, and one for link layer circuitry. Many people also have a separate ground for chassis ground. In these cases only the ground planes need to be connected by a parallel combination of resistors and capacitors. Experience has shown that larger capacitor values are better for this. Please refer to the TI Galvanic Isolation App Note for more information.