USB communication differential signaling

[peterpops]

Hi Everyone!

From what I've heard USB communication uses differential signaling. That is the difference between D+ and D- decides whether its a '1' or a '0'.
To me (with my basic knowledge of electronics) that means if I have an USB device I can power it from a PSU and then I only need the D+ and D- from the computer to communicate with it
(see attached picture)


This won't work, however if i connect the GNDs from the PSU with the Computer it will work

why is this?

 

[Lo_Volt]

Hi Everyone!

From what I've heard USB communication uses differential signaling. That is the difference between D+ and D- decides whether its a '1' or a '0'.
To me (with my basic knowledge of electronics) that means if I have an USB device I can power it from a PSU and then I only need the D+ and D- from the computer to communicate with it
(see attached picture)
View attachment 94911

This won't work, however if i connect the GNDs from the PSU with the Computer it will work

why is this?

Differential signaling still needs a ground reference. D+ is high when it's a '1' and low when it's a '0' while D- is low when it's a '1' and high when it's a '0' but both are still measured with respect to ground.

The receiver IC on either end has limits on the signal voltages on each of the input pins D+ and D-. If you do not connect the ground between the device and the computer, you cannot be assured that both are at the same reference. Therefore, D+ and D- may be outside the required input voltage range on one end or the other. Note that the protective ground on the power supply is not low impedance enough to assure a close reference, the ground reference needs to run along the same path as the twisted pair containing the D+ and D- signals.

This is true for USB as well as RS-422 and RS-485 communications systems. Ethernet is a different case, though. Ethernet is essentially a current loop with a transformer or balun at each end. It relies on current coming into one pin being the same as the current leaving the other pin.

 

[peterpops]

Lo_Volt, thanks for the answer!

2 follow up question:

1. So D+ and D- are measured with respect to ground? I thought differential meant that D- was subtracted from D+ (or vice versa) and that the difference decided whether it is high or low.
For example
D+ = A
D- = A+0.2

Ddiff = A - A+0.2 = 0.2 = High

and

D+ =
D- = A
Ddiff = A - A = 0 = Low


2. Can GNDs be connected in what ever way you like? I got a feeling that either the PSU or the computer would "suffer" from it? I mean the PSU's GND can be at 10V higher than the Computer and connecting them must change something? Or have I lost myself somewhere on the road?

 

[sivamit]

Hi Peter,
USB requires a shielded cable containing 4 wires.

Two of these, D+ and D-, form a twisted pair responsible for carrying a differential data signal, as well as some single-ended signal states. (For low speed the data lines may not be twisted.)

The signals on these two wires are referenced to the (third) GND wire.

The fourth wire is called VBUS, and carries a nominal 5V supply, which may be used by a device for power.

In practice, the signal source ground and the computer ground will/may not be exactly the same voltage as
they are connected through the ground returns of the equipment . The difference between
the ground voltages forms a common mode voltage i.e. a voltage common to both inputs which a differential input is designed to reject up to a certain limit.

Hope the above explanation helps.!

Thanks.