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I have a keyboard with a fixed cable that I'm looking to reconfigure into a detachable one. It's a hefty cable that contains two separate USB 2.0 cables and some shielding. The shielding connects to the large metal plate that the keys are mounted to, which is then connected to ground on the PCB underneath.

One of the cables is for the keyboard itself (the one with the extra shielding connection on it. The other goes to a type A port on the back of the keyboard to use with peripherals. Since it's just a simple extension it can be used for anything USB 2.0.

So this is really a two part question. First, will leaving that shielding disconnected cause any issues with interference with the two USB cables running right next to each other? And if it will, since I won't have a PCB in the modded cable, can I attach the shielding to the metal body of the male USB I will be using to connect it to the keyboard?

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    \$\begingroup\$ If this is just a keyboard, it's going to be running at USB 1.1 speeds, and the shielding won't really matter. But if it's just a keyboard, I doubt it would have two USB cables (that's actually really strange; what exactly is this thing? I'm not sure that's even strictly allowed by the USB standard.). \$\endgroup\$
    – Hearth
    Dec 15, 2021 at 16:39
  • \$\begingroup\$ The second USB cable in the main cable is connected to an A port on the back of the keyboard for plugging in anything you want. I mainly use it for a mouse, or a thumb drive. It's really just a glorified USB extender. \$\endgroup\$
    – Logarr
    Dec 15, 2021 at 16:42
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    \$\begingroup\$ You'd think it would be easier to just build a USB hub into the keyboard; I've seen plenty that do that. \$\endgroup\$
    – Hearth
    Dec 15, 2021 at 16:50
  • \$\begingroup\$ It probably would be. But it works to my advantage as I can simply bridge the two JST XH connectors inside to turn the port into the keyboard's connector. It means making an A to A cable though. \$\endgroup\$
    – Logarr
    Dec 15, 2021 at 16:54
  • \$\begingroup\$ "will leaving that shielding disconnected cause any issues" - why would you want to do that? \$\endgroup\$ Dec 15, 2021 at 19:24

2 Answers 2

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For low speed signals like a keyboard shielding may not be strictly necessary. But consider static protection. If a static spark jumps from your finger to the metal plate in the keyboard, where will it go? Worst case it will jump to the keyboard's electronics and fry them, or jump to the USB data lines and fry the USB port on your computer. Provide a safe path for that static discharge. Generally that is case ground which eventually leads to earth ground. The shield on data cables can be used as an extension of case ground.

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  • \$\begingroup\$ The shield in the cable is only grounded at the keyboard side to that metal plate. Path to ground for static discharge on the plate would be through the ground on the PCB > ground on the keyboard's USB cable > motherboard's ground > case ground. \$\endgroup\$
    – Logarr
    Dec 15, 2021 at 18:03
  • \$\begingroup\$ Then the shield in the cable is only for signal integrity. Best practice in that case is to only connect one end of the shield (to avoid ground loops). You may get away without it. YMMV. \$\endgroup\$ Dec 15, 2021 at 18:08
  • \$\begingroup\$ Assuming the terminal for the shield won't be inside something with a PCB, what should I connect it to? Can it be connected to the "shield"/body of the USB plug, or to the ground wire of the USB connection? \$\endgroup\$
    – Logarr
    Dec 15, 2021 at 18:11
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    \$\begingroup\$ @Mr_Engineer - No, it's not considered best practice to only connect one end of a shield (to ground), at least in the high speed digital & RF world. Our EMC engineers would never pass a design that only has one end of a shield connected. What's another name for a shield connected at one end? Antenna! \$\endgroup\$
    – SteveSh
    Dec 15, 2021 at 18:53
  • \$\begingroup\$ @SteveSh For a USB cable like this where should the other end be connected? Does it go to the body of the USB plug, or to the ground pin? \$\endgroup\$
    – Logarr
    Dec 15, 2021 at 19:27
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A shielded cable, if the shield is left floating, won't protect the signals in the cable from most noise. A floating shield will protect a signal from very high frequencies, but in many environments, much of the electrical noise is mains frequency or some multiple of mains frequency -- too low frequency for a floating shield to provide protection.

Thus, a shielded cable should be grounded, or at least a low impedance path to ground should be provided. The grounding should occur on at least one end of the shielded cable.

There are cases where grounding on both sides is advised, and there are cases where grounding on only one side is advised. And there are sometimes acrimonious debates over whether to ground on one side only, or both sides.

However, in either case, at least one side of your USB cable shield should be grounded, or the shield will fail to do it's job properly.


Of all (several dozen) the USB devices I have taken apart, this is the first I've had where there is a dedicated grounded shield wrap. Most don't even have metal braiding around the wires. But this is the first dual USB cable I've had, so the question is more about whether leaving the shield disconnected will cause a problem with this cable more so than any regular unshielded single USB cable. As in, does a dual cable like this need the shielding to prevent crosstalk issues?

Shielding around a group of data conductors has virtually no effect on cross-talk within that group of conductors. What does have an effect is the twisting of pairs, and changing the twisting pitch between pairs. (In some cases there is an outer shield, and then inner shields around individual twisted pairs, but that is more of a specialty (read expensive) cable than something you would generally find in most consumer products.)

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  • \$\begingroup\$ Of all (several dozen) the USB devices I have taken apart, this is the first I've had where there is a dedicated grounded shield wrap. Most don't even have metal braiding around the wires. But this is the first dual USB cable I've had, so the question is more about whether leaving the shield disconnected will cause a problem with this cable more so than any regular unshielded single USB cable. As in, does a dual cable like this need the shielding to prevent crosstalk issues? \$\endgroup\$
    – Logarr
    Dec 15, 2021 at 16:15

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