How over 5 m USB extension cables work - Page 1
Forum section: electronics / projects

author: jmaja
author_rank: Frequent Contributor
timestamp: December 19, 2016, 05:12:42 pm
content: How do these cables work? They seem to have something active at the device end. Nothing every 5 m? Is the other end just passive? I would need to cut the cable in order to get it through a cable gland and then put a new USB connector.E.g. thishttp://www.tripplite.com/shared/product-pages/en/U02610M.pdf.USB 1.1 is enough and the device takes less than 100 mA.

author: Fungus
author_rank: Super Contributor
timestamp: December 19, 2016, 06:05:36 pm
content: USB is a differential signal. I'm guessing the power lines are more of a worry than the data lines. There could be quite a voltage drop if you try to draw 500mA over several meters of USB cable.Quote from: jmaja on December 19, 2016, 05:12:42 pmThe datasheet says there's a 150mA limit with that cable. Maybe there's a 5V DC booster in the female connector to help deal with the voltage losses.

author: Brutte
author_rank: Frequent Contributor
timestamp: December 19, 2016, 06:10:43 pm
content: I'd say that USB specification forbids any (passive) wiring with a total length of more than around 5m in one piece. You won't certify such thingy as a USB cable.However, in theory this could be a USB hub, Hi-Speed (certified) with one input and one output (standard USB hubs have 4 inputs and 1 output). Interestingly this one does not have USB certification. Possibly this is just a repeater (only low level MAC+PHY) but as USB is half-duplex and with voltage/current drive, that would be quite a complex Si.If I were to build such thingy, I'd consider tying ULPI back to back with some high speed bus. 480Mbps is not a trivial thing.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: December 19, 2016, 06:18:06 pm
content: I seem to recall these active cables use a hub to re-drive the signals. The USB 2.0 spec limits the passive cable length and also the number of passive cable segments between hubs (5, I think) to limit the round-trip delay of the signaling.

author: SeanB
author_rank: Super Contributor
timestamp: December 19, 2016, 06:32:56 pm
content: AFAIK they simply use a 4 port hub at the one end, and simply do not use 3 of the differential outputs on the chip, just bond the input, power and one output of the chip to the board. dispense with the power switching, add some local decoupling and use a chip with onboard clock generation and you can fit it all into a standard USB cable end.

author: Fungus
author_rank: Super Contributor
timestamp: December 19, 2016, 06:35:04 pm
content: Quote from: Brutte on December 19, 2016, 06:10:43 pmMaybe that, too, although it would only be useful if you're chaining these things together (which they say you can do, so... hub it is!)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2016, 09:22:59 pm
content: FYI, USB isn't full differential.For example, applying a common mode filter will close the eye, not open it. Mind: the high speed data portion of a packet is fine, but it's the unbalanced start and stop signals (JK or whatever they call them) that screw everything up. Between that, and the source terminated, bidirectional design, you can't effectively filter it, or run any unshielded length, because you run out of input common mode range, or input (regular single-ended CMOS input level) threshold.This is why USB cables and connectors are fully shielded: the signals are utterly hopeless without that!For similar reasons (the source termination, and the protocol itself), USB is point-to-point only, and not useful over long runs. Hence the 5m limit. You could probably get away with more, if you used, say, a high quality CAT-6 STP cable, maybe 10m, maybe longer, while maintaining High Speed compatibility.So, as noted above, most or all extension cables have a repeater inside.Tim

author: jmaja
author_rank: Frequent Contributor
timestamp: December 19, 2016, 11:07:16 pm
content: So you are saying that those 10 m or 20 m USB extension cables have an USB hub at 5 m intervals although it looks like just a cable with normal USB connector on one end and something bigger at the other end?

author: Brutte
author_rank: Frequent Contributor
timestamp: December 19, 2016, 11:40:51 pm
content: Quote from: jmaja on December 19, 2016, 11:07:16 pmUSB standard imposes some requirements on the wiring and hub parameters. Soup to 4* (any USB compliant cable + any USB complinat hub) + any USB compliant cable + any USB compliant devicemeets the worst case USB scenario (is guaranteed to work). That means you can extend the bus up to 25m "legally".From above you cannot infer something non-compliant in that chain will work or will not work with above setup if it is 25.1mAs for the hubs with 10m wire attached - I do not think USB standard allows that explicitly (but I am not an expert). I also do not believe there is a hub in the middle of that 10m wire (4*2=8 and USB won't work with 8 hubs).As for this particular solution - hard to say how it operates internally without tearing it apart. I can understand that in theory one could use better shielding, thicker copper or cutting edge drivers/receivers than "5m USB standard" imposes to meet the USB performance requirements, even with 10m spacing, but I seriously doubt that particular cable works that way. I think it is a raw hub + raw USB cable cut in 10m lengths.