I recently found an IBM Model M keyboard at my local Goodwill store. There is rarely any computer hardware in there worth having, so this was quite a surprise.
As far as I can tell, this Model M has never been used even though it was made in 1993. It came in what appears to be the original box, with a styrofoam insert. The coiled, un-stretched cable still has plastic wrap on it. There is no visible wear on the keys. There are no finger grease stains. It is a virgin, IBM-branded, Lexmark-manufactured beige beauty. Even the box doesn't look 20 years old.
The model M was built before USB was a thing, so it has a PS/2 port. My main rig has a "legacy free" motherboard, so it doesn't have PS/2 ports. It only has USB ports.
The keyboard I have been using lately is from long-gone Micron computer. I bought a computer from Micron in the 1990s and the only remaining evidence of that is that keyboard. (Micron was a popular computer vendor at that time, though there isn't much evidence of that anymore.) Normally, I use a PS/2 to USB convertor to connect my Micron keyboard to my computer. That has worked great for several years.
When I unplugged the old Micron and plugged in my new Model M, they keyboard was unresponsive. No lights, no nothing. Worse, the mouse that is plugged into the same PS/2->USB converter also stopped working.
In the spirit of the IT Crowd, I turned it off and turned it on again. No change in behavior. I plugged my old keyboard back into the computer. Everything works. This wasn't the best way to start the week.
I thought that my siren-like keyboard was dead, but I carried on and kept experimenting. It turns out that if I plug the Model M into the PS/2 port on my Thinkpad dock, the keyboard works.
The PS/2->USB converter I was using is an Adesso Ez-PU21 "Smart Adaptor". It's got a snazzy logo, a serial number and some other information written on it. I've been using it for so long I don't even remember where I got it from.
While researching the problem, I found a detailed explanation of how the PS/2 interface works. It has links to a history lesson on keyboards and detailed information on the communications protocols used for keyboads and mice, keyboard scan codes and more. There is a more approachable article describing the PS/2 interface on Wikipedia.
The new Model M has four pins in it's connector, my Micron PS/2 keyboard has six pins and so does my mouse. I have another, older and grungier Model M that also has six pins. The two pins that are missing are shown as "not implemented" on all of the PS/2 connector diagrams that I can find. The two "extra" pins were sometimes used to implement non-standard mouse-and-keyboard combination cables. Those missing pins shouldn't make a difference, yet they do.
I dug out the other, no-name, beige-grey converter that I own. I had thrown into the bottom of my parts pile years ago, last using it with my Pentium III/933 desktop. There is no writing on it other than "Made in China" and the date it passed Quality Control in June, 2006. I've got no ideal who made it. It works. No problem.
Once again, persistence wins over genius. I've got a great, "new" keyboard from 1993.