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I have a Odroid-U3 minicomputer where its USB VBus and Ground pin are shorted, making the USB ports unusable.

But I was able to make a workaround. What I did was used an external power usb hub, taped the VBus pin to the USB plug to be connected to the USB Host (Odroid), and let the external power supply power my peripherals.

Now, I have one power supply powering Odroid and another to power the peripherals but they share the same ground.

It works but is it safe to let them share the same ground or since the data is transmitted via differential signals, ground is not needed?

EDIT: Here is the current schematic diagram of my odroid setup: Current Schematic Diagram

Also, there is no connection between the Vbus of the USB port on odroid to Vcc of the system power supply.

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    \$\begingroup\$ Hello, and welcome EE.SE! Please try to include a schematic (hand-drawing is acceptable, if it's detailed and readable) so that others can understand your question easily and provide useful answers. \$\endgroup\$ – KnightsValour Feb 22 '16 at 16:27
  • \$\begingroup\$ @KnightsValour, I have already included a hand-drawn schematic diagram. Let me know if it's enough. \$\endgroup\$ – Xegara Feb 23 '16 at 4:48
  • \$\begingroup\$ @ScottSeidman, sorry that I forgot to mention that there is no connection between the VBus of USB port and Vcc of the system's power supply. \$\endgroup\$ – Xegara Feb 23 '16 at 4:48
  • \$\begingroup\$ The picture you've drawn is exactly the correct way to do what you're trying to do. Obviously make sure the Vbus connection on the system (which is shorted to GND) is not connected to the 5V supply on the hub :-) \$\endgroup\$ – Jay Carlson Feb 23 '16 at 4:58
  • \$\begingroup\$ @Jay Carlson, wow and am glad to know that the circuit is correct. But I also wanted to know more the explanation on why it is safe for them to share ground signals and is it really necessary? Take for example the telephone lines, it does not have a ground, it only has the differential signals to send the data. Or am I wrong? :) \$\endgroup\$ – Xegara Feb 23 '16 at 5:07
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It's completely safe to ground both circuits together, and there's no need to add any high impedance resistence between sources. Actually, adding this resistor would only lead to increase in power consumption. It's important to understand that the goal of attaching both ground it's to maintain equal electric reference among both circuits, otherwise you could create hazard difference of potential between the circuits which could lead to damages.

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  • \$\begingroup\$ I didn't understand the part that it might introduce a hazard difference of potential when they don't share one ground. For instance in my schematic above, what if the ground from usb host is not connected to the usb plug of the usb hub, only the D+ and D- are connected between the two circuits. Will be differential signal be affected without the ground? \$\endgroup\$ – Xegara Feb 23 '16 at 17:26
  • \$\begingroup\$ Ok, I'm not familiar with USB voltage standards, but let's assume your circuit works with D+(5V) and D-(-5V) to transmit a logic 1. Also assume the state when your left circuit "A" is transmitting 1 to your right circuit "B", and GNDA is not connected to GNDB. "A" will create a potential of 5V in D+ and -5V in D-, and this is important to realize, referenced in GNDA! Now, circuit "B" receive these potentials as a difference of 10V, but its reference is GNDB, what means that for circuit "B" D+ can be 200V and D- 190V or D+ can be -100V and D- -110V, depending on GNDB's potential w.r.t. GNDA \$\endgroup\$ – PDuarte Feb 23 '16 at 20:24
  • \$\begingroup\$ In real circuits the inputs of "B" have specifications that allows a maximum of x Volts on its inputs, with reference to its ground, which in this case is GNDB. That situation leads to circuit "B" damages. \$\endgroup\$ – PDuarte Feb 23 '16 at 20:26
  • \$\begingroup\$ I edited this answer to add some clarity that should help explain this comment thread. \$\endgroup\$ – Jay Carlson Feb 24 '16 at 3:46
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Yes it is safe, as long as you keep the current pathway between the supplies high impedance, if you design the circuit wrong you have a potential for voltage to flow from the higher voltage to the lower one. I've drawn a simplified circuit diagram, you could simplify your loads on each supply as R1 and R2 and whatever circuits between them as R3. R3 has to be acceptably high, meaning it could be 1k if that is acceptable for you. If the voltages are the same then you could have a low impedance between them, but voltage supplies are never quite the same...

A good engineer will be able to circuits and be able to simplify them into simpler circuits to answer questions. A microprocessor or IC can be thought as a changing resistor, if you know what your current draw is and the voltage you can find the load value of the resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I see what you're trying to do here, but having an R3 in this circuit is confusing and irrelevant to what the OP is trying to do. While there are USB signals shared between the circuits, they are referenced to GND, not to Vbus. \$\endgroup\$ – Jay Carlson Feb 23 '16 at 5:00

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