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The current flow direction in USB V2

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xmen_xwk

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I'm building a digital switch between USB ports. And I want to turn off all 4 connections (VCC, D+, D-, GND) when a port is switched off for Smart Usb hub.

There are 2 Laptops(Host) and one client. Now what I want is to digitally connect/disconnect client to any laptop. To do that I'm using both AND gate and OR gate.
But I've few question, like how current flows for D+ and D-? I presume it flows from host to client for D+ and client to host for D-, like VCC and GND?

Does the direction change like AC? Or stays in one direction like DC?
Is it even possible to build switch like this?

PS: Talking about conventional current.

Thank you.
 

D+ and D- are logic signals and currents are not specified. At any time the communication is half-duplex. Logic voltage levels are specified in the USB standards.
 

I do know it's logic level current, my question is about the current direction. Please read the first post again. Thank you. ;)
 

my question is about the current direction
Click to expand...
USB data lines are bidirectional and have no "current direction" (or data direction, which might have been the actual question).

Is it even possible to build switch like this?
Click to expand...
You can buy a ready made USB change-over switch for little money (starting at < 10 $).

Don't know if they implement analog switches or hub-like repeater function for the data lines. Interrupting the GND line isn't feasible for an electronic switch. Making your own work for high speed USB might be tricky.
 

Well that's the problem if current is bidirectional. And I can't use change over switch, I need to make my own.

Is there any possible way to do the digital switching for data lines ?
 

Well something along the lines of a level shifter with auto direction sensing might work like this, but don't expect the result to be high speed USB with it in there.

I think most of these USB "switches" on the market are basically lobotomized USB hubs.
 

xmen_xwk said:
I do know it's logic level current, my question is about the current direction. Please read the first post again. Thank you. ;)
Click to expand...

There are no logic level current, they are logic level voltages. The current loops were used (rarely) in some serial ports because current loops were more noise-resistant. The direction is half-duplex, it means at any given time, one is a receiver and the other is a transmitter. I do not know the bit encoding. Both D+ and D- independent channels (any current flows to ground).

- - - Updated - - -

The bit encoing is NRZI (https://en.wikipedia.org/wiki/USB). D+ and D- are differential pairs.
 

There's a number of dedicated USB analog switches, e.g. TI TS3USB221A.
 

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