again from the MH listserv:
http://www.tidbits.com/article/11993"Thunderbolt’s Blasts -- Thunderbolt is a fascinating mix of
old and new:
Despite what the tech spec pages say, Apple’s version of
Thunderbolt has up to 20 Gbps available in each direction (full
duplex), not 10 Gbps. While the Thunderbolt specification talks
about 10 Gbps to and from a host, Apple’s version incorporates
two channels over the same cable: one is apparently dedicated to
DisplayPort for video, and the other for PCI Express data. This
allows raw throughput up to 20 Gbps and reportedly a substantial
fraction of that in true throughput in each direction.
This amount of bandwidth would let you run two high-resolution
displays (which require bandwidth in the gigabits-per-second
range) and a super-fast RAID drive (demonstrated by Promise
Technology) or multiple drives that can work at full speed. On
the new MacBook Pros, Thunderbolt manages both the internal
screen and an optional external display, which is why you
can’t drive two external displays. On a future Mac Pro or Mac
mini that won’t be an issue, nor will it be a limitation on a
future iMac, as long as it provides multiple Thunderbolt ports.
Because Thunderbolt provides two channels on the same cable, a
display or hard drive can be in the middle of the daisy chain
without interrupting the flow of the other channel.
Target Disk Mode is supported under Thunderbolt. Until now, this
mode only worked over FireWire connections. When a Mac is booted
in Target Disk Mode, it acts as a hard drive for another
connected Mac.
You won’t be able to boot a Mac (yet) from a
Thunderbolt-connected drive, unlike with USB and FireWire. Andy
Ihnakto has this factoid, and I tend to trust him. I will be
surprised if this isn’t added later. We need a way to boot
from external drives, and if Thunderbolt eventually takes over
from FireWire, then it has to boot Macs, too.
While Thunderbolt is backward-compatible with DisplayPort, and
the connector uses the same 20 pins as DisplayPort, you can’t
use a DisplayPort cable to run a Thunderbolt connection. You can
use a DisplayPort cable, with an existing Mini
DisplayPort-to-DisplayPort adapter, to connect a DisplayPort
monitor to a Thunderbolt port, and you can also use a
Thunderbolt cable with any of the existing DisplayPort adapters.
The Thunderbolt controller automatically adjusts the signal
output to be correct for DisplayPort-native ports on the other end.
The Thunderbolt port carries 10 watts of power, a significant
amount for powering drives and other peripherals (though nowhere
near enough to drive a large external display). Apple’s
hardware with a single FireWire 400 or 800 port (or one of each)
can deliver 7 watts to the bus. USB 2.0 can push out a maximum
of 2.5 watts, while USB 3.0 can hit 4.5 watts. Apple’s
high-power USB 2.0 can generate 5.5 watts, which is enough to
charge an iPad while it’s plugged in and in use. Thunderbolt
devices can also boost power downstream: an AC-powered display
could push 10 watts out the port on the “far” side from the
computer in the daisy chain. (Apple’s external iPad USB-to-AC
charger is rated at 10 watts, but it’s just a USB plug
connected to power, not a data connection.)
Thunderbolt will allow splitters and other baroque
configurations of adapters, Apple told me. For instance, you
could have a DisplayPort adapter with two Thunderbolt ports for
daisy chaining. Apple has no plans to discuss here, but
there’s clearly room for a robust market of cables, hubs,
adapters, and other elements to make it easier to use legacy
video standards.
It should be possible to build Thunderbolt-to-eSATA and
Thunderbolt-to-FireWire adapters that enable connectivity with
older gear that you already own."