[Ti] Firewire Port Frying External Hard Disks

[Jerry Krinock]

After burning up several external Firewire hard disk enclosures in the last
couple months, I've finally found a possible pattern: I believe it happens
whenever I connect one of them to our old 867 MHz Ti Powerbook G4.  Upon
connecting an external drive to this old Powerbook, I hear a snapping sound
inside of the external drive case, smell some faint smoke, and when I take
the drive case apart I often see that the case of the Firewire interface IC
has a crack or a hole it it, and this seems to be the source of the smell.

It happens using either a 4-pin cable (signals only) or 6-pin cable (signals
+ 30VDC unregulated power).  It happens whether or not I provide external
power to the external firewire drive.

According to the official IEEE 1394 pinout, pins 1 and 2 are supposed to be
one differential signal pair, and pins 3 and 4 are another differential
signal pair.  The outer shell is ground.

When I measure the DC voltage presented by the suspect Powerbook, pins 1 and
2 are both at +3.3 VDC while pins 3 and 4 show no voltage source.  On a good
Powerbook, pins 1 and 2 are at +2.3 VDC instead of +3.3 VDC.

So, possibly the firewire driver inside the powerbook is shorted to +3.3V
and is frying whatever I connect to it.  But most chips can handle at least
0.5V more than their VCC, which looks like it's probably +3.3V for Firewire.
Also, the powerbook does not seem to mind if short one of these +3.3V
sources to ground.  If I use a resistor of 10-100 ohms, it initially shows a
source resistance of 100 ohms or so, but then after a few seconds the
voltage reduces to 0 as though a capacitor is discharing.  So, it's not like
a direct short to a powerful supply, and I'm surprised that I'd be frying
external devices so consistently, unless there is an initial current
transient which I cannot see using my Radio Shack DVM.

Anyone with any knowledge or experience in these matters is encouraged to
add to this story!

Jerry Krinock