On Tue, 14 Jul 2020 21:04:11 +0100, Paul wrote:

Mark Lloyd wrote:
On 7/13/20 9:25 AM, Commander Kinsey wrote:
How do I make a USB port in Windows 10 just provide power and not a
data connection? For example my satnav has a low battery and I just
want to use it indoors. But when it sees the PC, it just becomes a
removable drive and I can't use it.

A charge-only cable?

IIRC the data lines are cut on the controller (computer) side and
shorted on the device side.


For charging, you don't always cut D+ and D-.

There is a strap resistor scheme for signaling.

Before some of those smart cables came along.

If you bought a charging cable, and found something
other than D+ shorted to D- or so, you'd get out the
ohm meter and check for the strapping patterns.

These weren't necessarily standardized either. Whereas
the later USB specs defined more complicated ways of
doing the signaling.

https://www.instructables.com/id/Mod...n-iPod-iPhone/

"Nowadays the iPhone expects a certain voltage on those
two pins to decide how much current to absorb from the charger.

Putting a 2.0 V voltage on both the pins the iPhone will
absorb about 500 mA

While with 2.8 V on D- and 2.0 V on D+ it will absorb about 1000 mA."

Computer ------ strap resistors ------------ voltage pattern the peripheral
D+ to rails, D- to rails can see, and adjust its appetite
accordingly.

And you select the resistor scheme used, so that if D+ and D-
start driving out, nothing gets burned. There are two diagrams
on the page, showing some 10K resistors used for the
strapping. They use voltage dividers.

Is it a differential voltage? If so disconnecting them produces a difference of 0, which is the 500mA option.

The resistors are for the peripheral to read. If the expected
pattern is not there, it might even refuse to charge.

Ah, I have had that probably. I have a few wall wart USB things, some 1A and some 2A. The 1A ones won't power one of my devices (I forget which), but the 2A ones will. But the device uses way under 1A. Maybe the 1A (cheaper) warts don't have any signalling.

I did wonder why people aren't blowing up USB 2.0 (500mA) ports charging things that take 1 or 2A. I thought perhaps if you wanted over 500mA you had to actually communicate digitally over the data lines. Or perhaps the voltage just dropped if the socket was overused to limit the current. So it's as simple as resistors?