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- Roy Colson
March 14, 2010, 12:53 am
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Re: USB Hub
Roy Colson wrote:
So are they powered or unpowered USB hubs? You didn't say. Unpowered hubs
need to be attached to the USB port on your system case and can supply a max
of 1/2 amp across all ports on that hub (along with sharing that max current
with the unused USB port back on the case with was one of the paired USB
ports there). Max current draw from the pair of USB ports (coming from the
same controller on the motherboard) is 1/2 amp max to all attached devices,
and that would include through an unpowered USB hub. If you have any
high-powered USB devices attached (USB-powered scanners, USB-powered
headsets with mics), they could consume so much current that you cannot
attach other USB devices to the other paired USB case port or to the USB
ports on that external but passive hub. There isn't much to tell the user
that they have exceeded the max current draw from the controller except that
the USB devices to the same controller become flaky or fail to operate.
It's not there is a red LED that shines through a translucent shroud around
the USB case port to alert you that the controller has shut down and
discontinued supporting the attached devices because the current draw got
Powered USB ports don't draw power from the USB case ports to which they
connect. Of course, you have the nuisance of plugging in a wall adapter to
supply the power to that powered hub. As I recall, there is a maximum of
127 devices that can be connected to a USB controller. That would be the
total across that pair of ports brought from that controller to the system
case. The controller has its own address. Each powered hub would appear as
another device and get its own address. This is so the computer can send a
reset signal on cold boot to put those devices in a known state. There are
probably 2 to 4 USB controllers on your motherboard (but all may not be
connected to a paired set of USB case ports). Without the power limitation
of passive USB hubs (i.e., by using powered USB hubs), you could have 127
devices (that include the powered USB hubs) on one pair of USB case ports,
another 127 on another paired set of USB ports, and so on up to the limit of
USB controllers you have available (plus you could install a USB
daughtercard to give you even more total devices).
How many USB devices were you planning to have concurrently attached to your
system case's USB ports? The more powered USB hubs that you chain together,
the greater points of failure in your USB setup. So get powered hubs with a
large number of ports (to minimize the number of links in your chain).
There is still a maximum bandwidth for the controller to all devices
attached to it. Although you could hookup 127 devices to that USB
controller, you will probably run out of bandwidth long before then.
Receiving audio (headset speakers), sending audio (headset mic), printing to
a USB printer, using a USB mouse, a USB keyboard, USB controlled speakers,
USB monitored UPS, USB camera, and so on are all competing for bandwidth.
That's why you usually want to have low traffic devices on one pair of USB
ports (mouse and keyboard), with a printer and scanner on a different set of
USB ports (since it is unlikely you will use both at the same time, and a
USB cable modem on a different pair of USB ports. You spread out the
bandwidth load across the USB controllers that you have and have been
connected to [paired] case ports.
Says nothing about how many USB controllers on on the motherboard or added
through daughtercard(s). You never mentioned a model; however, you can read
the manual yourself or just look at the case to count how many pairs of USB
ports you have.
Re: USB Hub
On Sat, 13 Mar 2010 19:53:44 -0500, "Roy Colson"
Plug each hub into a separate USB port on the compter.
Actually, you are even better off buying more hubs than the
# of ports you need, and dividing up the bandwidth used by
each so that any concurrent use bandwidth is split between
as many motherboard port-pairs as possible because each has
a shared bandwidth.
Diasy chaining is kind of a kludge solution, seldom does
anyone have that many low bandwidth USB periperhals that
they could use this topology without performance
degradation... BUT, it is a helpful feature if you need to
extent the length of the periperal cable from the system.
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