Friday, June 10, 2005

Why Firewire ports fail (and USB too)

Macintouch: FireWire Part 5

At some point during this past Minnesota winter I'd plug a USB cable into the front of my PC and the machine would reboot. Turned out it was a static charge effect (the machine seems ok, but it's incredible how much voltage was traveling into that machine). I started grounding myself on the metal case and the problem stopped. It sounds, though, as though I should have grounded the cable as well.

This is a significant problem for Firewire and USB port designers:
Michael Johas Teener

I read Ron Doerynck's experience (and numerous others over the years) about FireWire port reliability ... and I wish there was a simple answer, but it's actually quite complex:

1) The static discharge protection on most FireWire ports is actually quite good and meets all industry standards (basically, it meets the 'body model' ... the expected amount of energy delivered by a charged-up human body when it touches the computer). Indeed, the FireWire 400 and FireWire 800 sockets and plugs have special features to intercept the spark and send it to chassis ground rather than let it get into the FireWire silicon. Unfortunately, the 'body model' is a bit optimistic if you live in Alberta (or Montana, or Wyoming, or ... especially in the winter), so you can deliver some truly huge jolts.

2) The other big problem is that frequently it's the FireWire *cable* that builds up the big charge (just walk across the rug in an Alberta winter holding that cable) ... the cable is *NOT* body model ... it's a really big, long capacitor and can build up an even bigger charge ... and the spark can be delivered right to the FireWire interface when you plug it in. The protection against this is to touch *BOTH* the computer *AND* the plug with your finger, or to touch the plug to the chassis before plugging into the socket.

3) There have been some devices (computers/boards/peripherals) with 'suboptimal' FireWire protection circuits, particularly when the cable is a bit out of specification and fails to make a good solid ground connection. The systems that shipped in the last few years all have pretty robust interfaces (although I haven't looked at the Mac Mini, so I can't comment on it). The interfaces on the Xserve are particularly robust.
If you have a ground nearby, ground your USB or Firewire cable prior to plugging it in during high-static weather. This is harder for laptops, I wonder if that's the reason laptops seem most vulnerable to port failure.

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