26/Jun 2002

21/Jul 2002 (mod)

Dead FireWire

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248_ PowerBook G4 DVI FireWire port

248_ PowerBook G4 DVI FireWire port

Ratoc system Corp. announced detailed analysis about the FireWire Port trouble issue. Because of a fault in Apple's circuit design, there is a risk to damage Firewire PHY chip TSB41AB1 when you connect/detach Buspower-enabled Firewire devices.

I found kapton tape on the very circuit which supply buspower on PowerBook G4 DVI motherboard. I know it's not intentional, but it seems as if they veils the whole problem...


249_ FireWire cable power source

In the case of PowerBook G4, it supplies about 17-12V without load. This circuit is switched by MOFSET and connected with Firewire connectors through anti-backword diode, self-restoration type fuse (polyswitch), filters, and so on.

Pic beside shows relevant parts. Ratoc analysed that there's a line to 3.3V regulator come from between diode and polyswitch. They explained this is for enabling repeater function as driving Firewire PHY by external buspower while PowerBook is turned off.

Since buspower is "power source with non-fixed voltage", as is written in Developer Note, voltage rises and falls in wide range when you connect or detach cables. Ratoc pointed that a noise derived by cable insertion / extraction may spoil Firewire PHY.

249_ FireWire cable power source


250_ 3.3V Step-down DC-DC converter and ESD chips

250_ 3.3V Step-down DC-DC converter and ESD chips

As PowerBook has multiple-layer motherboard, it is not obvious how all parts are connected with each other at a glance. There doesn't seem any direct connection between buspower source, which I marked with yellow line, and 3.3V DC-DC converter nearby, but probably they are connected via MOFSET switching circuit or something. They set as close as in 1cm.

To explain the voltage change in Firewire signal line discovered by Ratoc, it is natural to think that spike excessed DC-DC converter's permissive range, and leaks to TSB41AB1 (Firewire PHY). It may caused by capacity shortage of two condensers beside DC IN, slower response of DC-DC converter, or filter's weakness. There are any possibilities at this stage. Ratoc points that Apple added an ESD protection circuit on PowerBook G4 DVI. It must have good circuit performance, Ratoc says.

251_ PowerBook G4 Gigabit Ethernet (original)

This is the original motherboard of PowerBook G4 Gigabit Ethernet.

Parts layout is a bit different, and diode and Power Mofset is placed on the back. Firewire PHY is set on upper side between 24.576MHz crystal and diode.

You can see chip-ferrite-beads on Firewire buspower line and other anti-noise parts are neatly located. There's no ESD protection circuit which PowerBook G4 DVI had.


251_ PowerBook G4 Gigabit Ethernet (original)

252_ modified FireWire Port

Mr. Masuya send me a photo of revised logicboard he cannot found until 6/25. He says at least till 6/23 this revision model doesn't exist, and he saw 3 revised logicboard that day.

These pics enable us to guess Apple's understanding and measures about this problem. Blue jumper wire and new chip near the FW port are added. What do they mean? Let us show an enlarged pic next.\


252_ modified FireWire Port

253_ close up modified FireWire Port

Enlarged pic around the port. See 4 parts stacked there. I thought this was a MOFSET at the first time, but a reader suggested this may be the stacked 2 diodes. This seems a kind of protection circuit to block noises from outside going through Firewire PHY, by leaking excessed signal level to GND and Vdd. It has same meaning with the ESD parts added on PowerBook DVI, as Ratoc says.

Mr. Watanoki comments that since ESD chip for highspeed serial interface such as Firewire is prand-new product and not shipped until recently, as you can see on NEC's release note, and he took it granted for Apple not to add ESD protection circuit on earlier motherboard.


253_ close up modified FireWire Port

Failure example 42/108 (japanese)

The example that broke down repeatedly 11/108 (japanese)

Normal examples 55/108 (japanese)


I was able to obtain motherboard photography of a PowerBook G4 with Gigabit Ethernet showing the countermeasures that were taken after the repair of a Dead FireWire port.

When we examine Picture 253 which is a close up of the modified FireWire Port, an ESD circuit seems to have been added.

Ratoc system Corporation, points this out, but it seems to be as Picture 250 shows, a 3.3V Step-down DC-DC converter and an ESD chip. This ESD chip are standard on a PowerBook G4 DVI.

Is this a design flaw of Apple with all the  older model machines that do not have ESD protection circuitry. The PowerBook G4 DVI now has ESD protection circuitry.

I received the following information from Mr. Watanoki.

Please refer to "Release of small-volume ESD protection element for high-speed interface". As for us, the ESD chip was not available until recently.

Performance of the diode in the ESD chip that is used for the high-speed serial interface of the FireWire system is excellent. For example, according to ESD Protection IEEE 1394 Data Lines(PDF), aberration of the wave type is minimized if capacitance between the terminals is kept to about 3pF. A small revision in the content specifications of IEEE 1394b is expected.

And the Ratoc company also points out, and this is a special point, that there is a design flaw with the power supply circuit and bus power of FireWire PHY. They are not separate, however, they should be.

Next please have a look at the fifth page of USB & IEEE 1394 Circuit Protection Solutions(PDF) which explains the mounting of a self repairing type fuse.

In the standard circuit, the regulator circuit (V Reg) of FireWire PHY and the bus power are not divided.

When we examine the failures, it seems they are easily damaged when  external bus power devices are connected or disconnected.

It may not be a design flaw, but the absence of ESD protection elements and the deficiency of the power supply circuit are largely to blame for the trouble.

And there are many cases of machines breaking down repeatedly. I think that the connection of external FireWire devices is the cause.

If we can keep the Firewire circuitry separate from the bus power, we may lower the rate of trouble or failure. Care in particular is necessary for pre PowerBook G4 Gigabit Ethernet models which are not equipped with the ESD protection circuit. At this point it is still not totally known if this is a design flaw, but this cannot be ruled out completely. The cost of the repair is expensive, especially if the computer is outside warranty.

Special thanks, Mr. D. Andrews.

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