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Re: "Active" USB extension cables?

At 7:07 PM -0800 12/5/07, R.L. Grigg wrote:
I've heard that the use of passive USB extension cables is discouraged

Extension cables are specifically called out in the spec as being disallowed. The reason for that is while an extension cable may work in specific circumstances, they will not work in all circumstances and may cause devices to fail potentially in subtle and annoying ways. The spec wants to provide a good user experience so extension cables, with their potential for confusion and bad user experience are banned.

but what about the new "active" extension cables that are being sold? Are we asking for grief if we use one? Do they really work chained up to 80 feet? What happens after 80 feet? Our planned use would be for a UPS system that must be located in a shielded room away from our systems (too much RFI and noise). So there wouldnt be much data traffic going over it compared to using it for a router etc.

http://catalog.belkin.com/IWCatProductPage.process?Product_Id=160362

Those look like a 1 port hub which cheats on the power issue. If you chain multiple hubs together every other one should have its own power supply. If these are hubs, they're ignoring that and hoping the power numbers add up because there's only one port.

At 11:04 AM -0800 12/6/07, Clark Martin wrote:
I don't know about active extension cables but you might want to look at these:

<http://www.networktechinc.com/usbc5.html>

Those look like other "extenders" which are not USB compliant, whatever the blurb says. The ones I've seen work by inserting extra NAKs in to transfers to cover up the extra propagation delays.

Both of these solutions are not compliant with the spec, so you may get them to work, or they may fail. Apple has only said it'll work with complaint USB devices.

The furthest USB can go and still be compliant is 80ft is you're using hubs which at least every other one is powered. If you really want to do USB at greater distances, you need to have a host controller at the remote end. That's the only way to do this and still be compliant with the spec. There are 2 ways of arranging that, either have some sort of USB server at the remote end, or have a computer at the remote end.

The difference between a USB server and a computer is a little blurry. You could put a complete computer at the remote end (sacrifice a Mac mini, or something cheap you found on eBay), and talk to it via the network. There are application specific servers out there, which is fine if what they're serving is the sort of thing you're interested in (which it probably isn't). An example of that is the AirPort base stations will remotely serve USB printers and disks.

Or you could use something like a Keyspan USB server <http://www.keyspan.com/products/homepage.2.productList.USB.spml>. Which is a host controller at the end of the network. It has drivers to make it look like a local host controller. So should work with most devices. Some drivers may get confused by unexpectedly high latencies.
--
Barry Twycross
email@hidden
---
USB, it's not a Dyslexic BUS. (Thanks to TC.)
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 >"Active" USB extension cables? (From: "R.L. Grigg" <email@hidden>)



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