How did the 28 work?


Hi all.

One question's always fascinated me:

I can fathom how a unibody calc (45, 67, 33E, 38G, etc.) can communicate with the CPU. Although the 28C/S are clamshell in form factor, how did the left keyboard communicate with the CPU?.


The keyboard is basically a four-layer flex circuit. Twelve signal lines (six scan and six return) cross through the hinge with three signals per layer. It's quite ingenious.

See the August 1987 HP Journal (PDF), page 19 and 20.

Edited: 8 Mar 2012, 1:53 a.m.


Eric, thanks for that link to the HP Journal. It is absolutely fascinating.


Earliest use (in a calculator) that I know of is inside a Sharp EL-8009, manufactured around 1975: EL-8009 on flickr


Flex circuit, you say? What a fascinating idea. Never thought that circuits could be pliable and elastic (read bendable). I always though circuits needed to be rigid and flat.

Well, that's one new thing I learned today.



One of the most commonly seen uses of flex circuit technology is in printers, where there needs to be electrical connectivity from the printhead to the fixed electronics, it needs to be very low mass, and very reliable over millions of flexes.


Unlike every flip cell phone i ever


Cell phone! And I didn't even think of that! GEESH, I use a Motorola RAZR everyday and the flex circuit didn't even occur to me!


There are different kinds of flex circuit for different applications. They have varying materials, thicknesses, and production processes. The kinds most commonly used for the main electronics in cell phones and cameras aren't designed to withstand millions of flex cycles like that in printers.


Different, yes. But I get the gist of the tech involved.


I think the biggest difference between printers and cell phones is the layout of the flex. In printers the flex to the print head is relatively loose with a large bending radius, so there is almost no stress in the flex. On cell phones the flex is tight with a small bending radius, increasing the stress in the circuit resulting in eventual failure. The reduction in stresses is probably one of the reasons the 28's flex is in a U shape (and not directly across).


The thickness also tends to be different. Sometimes the material is different; when they don't need to endure as many flex cycles, polyester is sometimes used rather than polyimide.


Another common application of flex circuits is hard disk heads. Those are constantly moving and see even more usage than a typical printer head cable.


Common, yes, but not commonly seen.


Thanks Eric.

I especially like the fact that the electrical connection between the two halves doesn't bend, it twists. See the article for details, but basically the connection between the clam shell halves is a U-shaped circuit running half the length of the calculator. The part that bridges the hinge remains flat inside the hinge and the legs of the U twist to allow this.

I'd love to see similar in-depth articles on the design of newer calculators. Perhaps in HP-Solve.

Edited: 9 Mar 2012, 8:01 a.m.

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