Build a mechanical computer

That’s mechanical television. I’m building a mechanically scanned TV for my Concept Development class. It will have only 32 horizontal pixels by 48 vertical pixels, so this ain’t HDTV. It is, however, the way amateurs received TV images in the 1920s and 1930s.

I dunno if this will ever be part of a mechanical computer, but it would sure be cool.

I never thought I would say this, but bit-serial computers are just plain neat. Until I started poking around on the web, I had supposed it would take at least 500-1000 active devices (transistors, vacuum tubes, relays, etc.) to make any sort of useful computer. Well, the Bendix G-15 had 180 vacuum tube packs and 300 germanium diodes, the Librascope LGP-30 had 113 vacuum tubes and 1450 diodes, and the D-17 Minuteman guidance computer had 1521 transistors, 6282 diodes, 1116 capacitors, and 504 resistors.

The LGP-30 is particularly interesting to me because it apparently had just 15 flip-flops total; all other storage was on magnetic drum, including the CPU registers.

Now, even if every vacuum tube used was a dual triode, that’s still just 226 active devices, and couple dozen of those were used for the drum R/W head amplifiers.

This stuff is so cool that I can hardly wait to try out a few marble logic designs. And maybe I’ll even try a few transistor designs, too.

Beat, as in beating a drum, but more specifically magnetic drums, the sort of thing used in the 50’s and 60’s for bit-serial digital computers. I’m thinking of machines akin to the IBM 650 or the Bendix G-15, or even the Autonetics D-17 (the guidance computer for the Minuteman ICBM).

Serial hardware would reduce the number of pieces to make, though it makes it much more of a juggling act to design, literally so if the machine uses marbles for computation.

I want to build a completely mechanical computer someday, but I’m unlikely to find the time and money to do it up right any time soon. I figure I’ll settle instead for building a mechanical binary calculator, something that just adds and subtracts, to start.

My excuse is teaching my boys how computer logic works, but really I would want to do it even if I didn’t have kids; it’s that artsy steampunk streak in me again.

I reckon I might use marble logic; there are several binary marble adder designs on the web, so that part should be relatively straightforward. The only wrinkle here would be implementing 2’s complement arithmetic, something I haven’t yet seen in marble logic. it should be manageable, though.

The hardest part will be decimal input and output. I want it to work just like a calculator with keypad input and a multiple-digit decimal display output.

So, there is the I/O hardware itself: keypad and display and the decimal-to-binary and binary-to-decimal conversions. Instead I could do all the math in BCD (something else I haven’t seen in marble logic), because it’s much easier to do decimal-to-BDC and vice versa. The question is how hard 2’s complement BCD arithmetic would be to implement…