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I found an IBM Model F keyboard (the AT version, not the PS/2 or Terminal versions) at a yard sale for $1. When I opened it up, the cord has a black box with a wire and a ring connected to it. What is it (and the wire with the ring) and what do they do? Do I need them? The section of the cord that passes through the case looks possibly damaged (I cannot get the keyboard to work), so I was considering cutting out this section of the cord and twisting the wires together. It is, however, right next to the black box. See the picture below.

IBM Model F keyboard cord

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The "And this" part looks much like a ground connector. –  Thomas W. May 4 '14 at 21:18
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I like keyboard chords. Especially the inverted ones. –  John May 4 '14 at 23:41
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And this is supposed to screw into the case to ground it. –  JFA May 5 '14 at 16:24
    
Before cutting any of the cable open, use an ohm-meter to check for continuity. Each pin on the connector will almost certainly connect to exactly one pin on the other end of the cable. I don't know if the ground strap is connected to any of the pins, but it will likely be connected to the shell of the round DIN connector that plugs in to the PC. You can probably find the official wiring diagram of this cable as well, it was made at a time when IBM published full schematics of everything. –  RBerteig May 5 '14 at 21:01
    
@RBerteig, that's good advice for diagnosing this. After reading a few of the comments on here about the build quality of this keyboard, I didn't have the heart the cut the cable open, so I will try that. –  Scribblemacher May 9 '14 at 11:01

2 Answers 2

up vote 36 down vote accepted

Manufacturing techniques have changed a lot in the past 30? years. That cable assembly is complicated, and build quality is high.

The ring-connector is for being screwed in to the keyboard chassis (which is metal, underneath). This will ensure that the keyboard ground is at the same potential as the actual computer. It will also discharge any static that builds up on the user, keys, etc. Having a ground-strap is not surprising, but what does surprise me is that the grounding is separated from the main cable bundle where it is.

I suspect that during manufacture, the 6# 'tail' of the ground cable is separated out from the other cables, and terminated with the ring connector. Then the other conductors (data signals) are terminated in the connector-block. The cable is then re-molded and insulated with the strain-relief block in a separate molding process.

The Strain Relief is visible on the outside of the keyboard, and the molded block sits inside the keyboard, creating the mechanical mount for the strain relief. The block probably contours well with the inside of the keyboard, and prevents 'wiggling'. It also protects the junction where the earth conductor re-joins the rest of the bundle.

Reasons for building cables like this are not normally for cost. The 6" tail is probably needed for regulation purposes... You can bet that at the time the keyboard was built, there was a requirement that "all devices need to be grounded within 6 inches of where any power cable enters the device" or something. Using a complicated system like this is probably cheaper than having the cable routed differently to get the data to the circuit-board.

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Back In The Day (tm) IBM built stuff to last. I'll bet that keyboard weighs several pounds and is noticeably heavier than keyboards built today. I used to love the IBM 3270's at work. Even the cheap ones weren't cheap - and they weren't built cheap, either. –  Bob Jarvis May 5 '14 at 20:57

If the box contains a ring of metal, it is a choke. It reduces electromagnetic interference and helps to keep the signals being passed down the wire clean. It is common to find them on good quality VGA cables for example.

The part with the star-shaped whole appears to be an earthing strap. Designed to be bolted to a larger piece of metal.

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