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Repairing a 1960s-era IBM keypunch: controlled by mechanical tabs and bars (righto.com)
23 points by sohkamyung 11 months ago | hide | past | web | favorite | 6 comments

It's interesting to see that the number keys are almost exactly in the same spot as they are in the integrated numpads of modern laptops' keyboards, although the layout is "telephone style" with 123 at the top.

Since each character can have punches in any of 12 rows, you can think of cards as using a (mostly-sparse) 12-bit encoding.

The sparseness of early punched-card formats has always seemed a little odd to me --- instead of storing up to 960 bits per card, this encoding only holds 480 bits (80 columns * 6 bits/63 characters each.) Perhaps people just didn't understand binary very well at the time?

The way that the mechanism works also reminds me of early teletypes --- they encoded and decoded characters using a similar system, essentially doing serial/parallel conversion entirely mechanically. Here's one example:


People understood binary perfectly! This was only about 50 years ago!

It's a function of the electromechanical technology: It's a lot easier and more reliable to have a wheel with ten index positions on it than to have four wheels with two index positions.

Many early electronic computers used binary coded decimal as a bridge between the electromechanical and the purely electronic worlds.

I suspect the sparse encoding was used so that punched cards were stiff enough to survive being fed through a reader.

(related: https://en.wikipedia.org/wiki/Lace_card)

The mechanisms of early computing devices --- the gears, levers, springs, cams --- somehow strikes me as more ingenious than modern silicon and software. That all those moving parts worked well and fairly reliably is amazing to me. And to think of the development effort... No testing in VM that you can just start clean whenever you need to. Of course that's also why the basic bar and bail mechanism was used for three decades.

Those keycaps are gorgeous.

What does it feel like to type on?

Slow. You can only press one key at a time. Like an old manual typewriter.

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