[mythtv-users] OT: USB inductance (or other) problems with external peripherals

Stephen P. Villano stephen.p.villano at gmail.com
Tue Oct 15 14:25:20 UTC 2013


On 10/15/13 5:26 AM, warpme wrote:
>
> Marius,
>
> I think we can consider following hypothesis:
>
> 1.USB transfer errors due RJ45 impedance mismatch compared to USB specs.
> 2.Non sufficient power supply due RJ45 cable resistance.
> 3.USB transfer errors due RJ45 cable cross-talks (with other cables).
> 4.USB data signals levels biasing due ground loops.
>
> Ad1: USB cables are called 'USB Cables' not only because they have USB
> connectors but also because they have correct wave impedance.
> Impedance depends on frequency and there is high impedance difference
> because of ISDH freq ( order of 200kHz) and USB1.1 (multiple MHz).
> So RJ45 will have wrong impedance for USB because of: 1\different
> spectrum of frequencies and also because 2\it is electrically designed
> for different impedance (ISDN requires 120ohm) while USBequires 90ohm.
>
> Ad2: USB requires given resistance on +Vusb and GND not higher than.
> RJ45 cable never was designed to carry high currencies required to
> power-up low-voltage devices like USB.
>
> Ad3: USB cable has practically always D+ and D- as twisted pair. Not
> only for achieving given impedance - but also for reducing sensitivity
> for external electro-magnetic fields which can cause induction of
> distortion signals in data path.
>
> Ad4: this is always problem when You have multiple power sources...
>
> Order of hypothesis I presented is IMHO from most probable to least
> probable.
> For verification of them I propose however to go with following order
> of tests:
>
> Start with p2: use voltmeter on end USB device and check voltage.
> If voltage is OK (not lower than 4.75V) - make sure there is no
> ripples by spikes of load.
> Ideally will be to use oscilloscope. In You don't have one - simple
> add 470-2200uF electrolytic capacitor to filter ripples.
>
> When 2 not helps: test p4. Simply power-up ALL HW from single AC
> source. You can use temporarily AC power extender cable i.e..
>
> When this not help time is to eliminate p1+p3: try use dedicated, good
> quality of USB cable.
> I would look in comp.store for special USB reach extending cable. You
> may choose at least USB2 supporting cable as they have to meet higher
> impedance requirements and consider to choose famous vendor (HAMA, etc)
>
> PS: I personally will start immediately with good USB cable as I'm
> pretty sure Your problem is with wave impedance mismatch....
>
> Good luck!             
>
>
>
> _______________________________________________
>
It isn't always impedance causing a problem, especially with USB. USB
uses a lower frequency than gigabit ethernet.
But, one can suffer from ringing on long runs, secondary to inductance
in the line.
That is why many long length USB cables come with built in Schmitt
triggers as buffers.
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