I'm fairly sure that's the grounding wire.
nope
That wire connects to the cable shield, which shields it from EMI. It also connects to the plate of the keyboard (sometimes just the ground plane) to make the keyboards plate also shield it from EMI.
IBM model Ms have them as well.
You can see the ground wire as well. It's the black one on that four-pin connector.
I always thought they are the same. You can see it in other electronic equipment that the cable shield is connected to the ground(-).
They are "kind-of" the same. Both are supposed to be at 0V potential or grounded. But they have to be "sort-of" separate to function properly. Just like the Neutral and Ground pin on your wall plug: both should be at 0V but they are different.
Proper grounding and shielding of electrical equipment is actually a very complicated subject.
here is a good article about how it pertains to audio equipment. They are mainly trying to prevent ground loops, but the "best practice" remains the same.
Basically the shield can act as a faraday cage to prevent EMI (like from a wifi router) from inducing voltage in your wires. This isn't usually a huge deal for digital signals, but I think it has more to do with FCC part15 compatability. You also should have your plate grounded because otherwise you don't want it to build up static shock and shock the user. Also it'll help shield the switches / PCB from signals (since it has a bunch of huge holes in it, it can only protect against certain frequencies) The whole front side of a cherry PCB board is used as a shield plane in this way. (which is a total waste but whatever.)
That's sometimes how you can tell "good" usb cable from "cheap" USB cable. The good stuff will be shielded in aluminium foil for excellent EMI blocking and will usually also have a tinned copper braided shield for redundancy. Cheaper cable is usually thinner and missing one or both of these. If you cut open the USB from a cherry cable you will see.
Anyway, the point of all this is that you need exactly one ground path. The easiest way to do this is to have all the grounds tied together at one point. The wire helps them tie them together and also make sure it's at one point. If your grounds are tied together at one point, you can't have a ground loop and you know for sure which way any induced voltage will go to ground (through the ground wires instead of through your sensitive electronics.
I always thought they are the same. You can see it in other electronic equipment that the cable shield is connected to the ground(-).
Cable shield and signal ground are usually connected at one end of the cable, but they might be separated past that, depending on how the system is set up. They have to be connected somewhere for it to work, and if they were the same, then engineers would eliminate one of them to save money.
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As always with EE topics: Remember I haven't had any official training or classes in this. I encourage everyone to learn for themselves instead of taking my word for it, etc. If anyone knowledgeable will want to correct me, please do so.
