Analog Discovery - Not Fully USB 2.0 Compliant

[AKH]

I would like to incorporate the Analog Discovery into a piece of equipment, but I cannot get it to work over a longer USB cable.  In fact, sometimes I can't get it to work reliably over the short USB cable that is supplied with the device.  I have tried all of the tricks suggested on the forum to no avail.  I have also tried a powered USB hub.  It appears that it will mostly only work with the cable supplied by Digilent.  This makes the device functionally useless to me.  I need to be able to separate the computer and the Analog Discovery by more than 18 inches.  When I try to use a cable that is 45 inches, the Analog Discovery becomes unreliable and looses communications or fails to load at all.  It appears that the Analog Discovery is not fully hardware compliant with USB 2.0.  To be compliant, the Analog Discovery would be able to operate over the maximum specified cable length, which is up to 16 ft.  Anything less than 16 ft is noncompliant.  The message that is provided is not informative enough because these solutions seem to not reliably solve the problem of Analog Discovery USB 2.0 noncompliance. 

I am very disappointed with this nonconformance characteristic, because this little device is very powerful and very useful when it works.  So, the problem is not that the cable needs to be shorter, it's that the Analog Discovery is not fully compliant with USB 2.0 hardware specifications. 

Since the hardware is noncompliant, is there a possible software solution that could render the Analog Discovery useful over longer cables?  Something like a degraded performance bandwidth due to lower communications speed, or something like that?

Thanks. 

[attila]

Hello,

The 16 inch cable length limitation of USB 2.0 is for data transmission signal integrity. In case you need more than this you use up to 5 USB hubs.

The Analog Discovery needs about 400mA (2W) to function, less than the 500mA USB 2.0 limitation at 5V.
Most computers can provide more than 2A without any problem (like external HDDs need this to spin up), having the surge protection above this limit.

However long and/or bad quality cables have high resistance and the voltage will drop too much on the device side, below the ~4V minimal requirement for the AD.
http://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/

There can be huge differences between cables. The best cable I have is a thin unshielded 6 feet one, some others are thick and looking good but have way more resistance. It seems the plug contact resistance is a very important factor since these can oxidize easily.
With a good cable I see 0.1V drop (4.9V), with other bad ones up to 0.8V (4.2V) when using with AD taking 2W.

Try using different USB cables, check the device voltage in the WaveForms application status bar.
You can also try the Analog Discovery 2 which can be powered from 5V auxiliary supply.

[AKH]

I have tried powered hubs, and the communications still fails.  The message displayed by the failure to connect is too generic.  I cannot connect therefore I am not able to read the voltage that would have been measured by the Analog Discovery to determine whether voltage drop is the problem or not. 

After some fiddling around with various USB cable configurations, I found one that worked for me.  It looks like the USB cable that came with the Analog Discovery was limiting the USB performance.  I put a different cable in it's place and got it working.  The different cable was a 3 foot long micro USB to USB A cable from Dynex.  The moral to this story is that shorter is not always better.  In fact the voltage drop on the Dynex cable is less than the voltage drop on the Analog Discovery supplied cable, even though it is longer.  The patch cable I subsequently successfully added in series has 28 AWG on the pair, and 24 AWG on the power conductors, and it is a 3 foot cable as well.  Essentially, I have 6 feet of cable between the Analog Discovery and the computer, with an extra coupling in the middle.  This works for now.  It is clear to me that the 24 AWG conductors in the patch cable are making a difference in the circuit

I don't know what the Dynex cable is made of (probably a larger conductor size for the power), therefore I am not able to identify the short comings of the Analog Discovery supplied cable.  It clearly performs poorer in this circumstance.  A better constructed cable supplied with the Analog Discovery would go a long way to solving some of the interconnection problems people are having with it.

Thanks for your help.