The usb protocol can support up to 127 (or maybe it's 128) devices on
a single usb bus. Of course, in practice, this isn't the case if
each device requires power from the port. The usb port (talking 2.0
here, not 1.1 ports) can offer a max of 500 MA (that's miliamps) of
power draw. If the usb device needs to draw power from the usb port,
then you want no more than 2 or 3 devices on a single port, as the
current draw above this point is too much for the port to handle. If
however, each device is self-powered, then as mentioned above, you
can chain up to 127 devices on a single port, though, I'd suspect if
anyone managed it, the devices would be very slow, and extremely
error prone, since all the devices would be contending for resources.
The 1.1 stats are similar, but less than those for 2.0, though I
don't know what those are off the top of my head.
Anyhow, hope this helps.
You do have multiple usb ports on your mac, so use those first for
port-powered devices (such as thumb drives, and dongles), while the
keyboard ports (usually only 2 of them) should be reserved for those
devices that don't need either speed or can handle their own power
requirements.
This is of course, only a summary, but nonetheless, I think I hit the
main points. :)
, hth.
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