USB keyboard pinout - Vss

[tmcw]

I have a keyboard with a squashed USB plug, tried bending it back to shape, but not much better was the result.

The busted port was on a lead that terminated on the circuit board inside the keyboard to 5 solder points, Vcc, GND, D+, D- and Vss.

So I was looking at a few other USB leads I had in the parts stash, and cables in other keyboards, but I can only see cables with 4 wires, none of the 4 going back to the USB plug shield. The few I looked at just seem to be 4 wires inside the cable, no shielding along the USB cable length.

Is this standard practice, no wire from the plug shield to the internal keyboard circuit board?

And will I get away without connecting to the Vss? Any likelihood of damage to keyboard or PC?

[japlytic]

Re: USB keyboard pinout - Vss

If you reverse the polarity of the D+/D- wires, generally no harm will occur; it simply won't work.
But if you reverse the Vcc and Vss (GND) wires, damage could occur, especially if the USB is not current limited (some are like that).

[stj]

Re: USB keyboard pinout - Vss

vss and vcc are your power, ground is for the shield, but it's probably best to not connect it so you dont create a groundloop - it's already connected at the other end.

unscreened cables are another matter and should be binned.
they can be unreliable because they can pick up noise, more importantly they radiate noise that effects other things.

i'm sure the u.s. government loves uncreened cables - tempest v2 and all that, but fuck them - use screened cable.

[tmcw]

Re: USB keyboard pinout - Vss

Thanks for the input.

I had another quick look at the keyboard that has the squashed USB plug...

The Vss connection connects to a big metal plate that sits in the lower portion of the keyboard, under the membrane (in turn, connected to the shield on the USB plug). The Vss doesn't connect to any other traces on the circuit board inside the keyboard.

The "GND" and "Vss" aren't connected together (so GND isn't connected to the plug shield).

Is the Vss like another ground?

It's a Dell KB1421 BTW.

[stj]

Re: USB keyboard pinout - Vss

yes, vss = ground in your case.

nothing surprises me with dell, prison-labour manufactured junk these days.

[cheapie]

Re: USB keyboard pinout - Vss

But if you reverse the Vcc and Vss (GND) wires, damage could occur, especially if the USB is not current limited (some are like that).

I've burnt up a flash drive that way. I had the USB header connection on the motherboard connected in exactly the "right" way to reverse Vcc and Vss, and D+ and D-, and leave the shield unconnected. The flash drive got very hot and stopped working.

[budm]

Re: USB keyboard pinout - Vss

Vss (5V return) is tied to PC circuit ground which is also tied to the safety ground of the incoming AC inlet, same for the shield.