Total cable length is usually limmited to 5 meters?
I see your point, but I suspect the logic used effects the lag.
Thanks.
USB travels in "hops", for all practical purposes.
You can get a passive cable, which is 5 meters long. But if
you want to extend it, you plug in a hub, then another 5 meters of
cable and so on. (The USB spec is in nanometers, and if someone
were to change the "time of flight", perhaps a small alteration
in reach could be made. Really long reaches, are done with bridging.
The USB spec assumes a cable propagation delay of 5.2ns/meter.)
Since cable+hub is a common configuration, they also make "active" 5 meter
cables, which have a "one port hub" on the end. That blob, terminates
and effectively buffers the signal.
Since bus powering is affected by all that length of cable, best
practice is to put a powered hub on the end of the chain. So perhaps
your "last" extension cable is passive, then a wall-wart powered hub,
then your USB peripheral plugs in there. That config covers cases
where the peripheral needs 500mA, and the wall wart powered hub
can provide that current. Active buffer cables (running off bus power)
would be limited to 100mA loads, as far as I know, so only things like
a keyboard or mouse, would go on the end of a bunch of active cables.
All of the info is here, if you need it.
http://www.usb.org/developers/usbfaq/
One other quote for you:
According to the USB Specification, the maximum limit of hubs cascaded in
series cannot exceed five.
(Figure 7-41 of USB20 spec, has the maths nearby for this.)
http://www.usb.org/developers/docs/usb_20_071411b.zip (extract USB20.pdf from the zip)
So now the picture looks like this. This is an example of a 30 meter
reach, with full power available at the end (500mA). The wall powered
hub down near the end, ensures you've got good quality power.
MB
SB 5M
USB --active_cable--(hub) 5M
\--active_cable--(hub) 5M
\--active_cable--(hub)
\
(hub)--active_cable--/
5M / 5M
Wall_powered_hub --passive_cable--
/
\--passive_cable--500mA_USB_peripheral
5M
Note: Certain Intel chipsets of very recent vintage, have a "hub"
inside. Intel decided, after having the same USB architecture
for eight or nine years, to just arbitrarily change it. The
USB1.1 and USB2.0 logic blocks traditionally provided in the
Southbridge (SB) have been changed to one unified "hub" block.
As far as I know, that cuts down the external reach to 25 meters.
If you buy a USB2 add-in card for $10-$15 or so and slap that
into the motherboard, you can go back to 30 meters again :-(
Much longer reaches are possible with various bridging approaches. And
you can spend the whole night researching those. I see Newegg lists
some cables, of dubious length, with no technical jargon to back up
the product.
This is an example of a 5 meter cable, using a USB hub chip in the "blob"
on the end. You could buy four of these, as part of the above diagram
(the rest of the reach being done with passive cables). Note that not
all these active cables are built equally. Years ago, there were
some which could not be cascaded and a chain of two would fail. And at
the same time, other brands worked. Now, I see at least one brand
has managed to make cables where they're all bad out of the box. Bravo!
Always read the reviews, before you buy, and make sure that *somebody*
has tested that the cable works when cascaded. If there aren't reviews
of your liking on Newegg, check out Amazon reviews instead.
http://www.newegg.com/Product/Product.aspx?Item=N82E16812339009
So 5 meters is the limit per hop, unless something non-standard is used
for an implementation. And then it's up to the manufacturer to guarantee
it will work.
HTH,
Paul
The first item, that