Harry Porter's relay computer is a loud, clunky, heavy thing of beauty.
While the rest of the world may be focusing on miniaturization, Harry Porter is headed in the opposite direction. A professor in computer sciences, Porter has a strong interest in showing just exactly how these systems work. To that end, he's built a very impressive relay computer, which occupies a place (make that a big place) of honor on his living room wall.
Harry proudly displaying his relay computer.
Porter's relay computer consists of four physical units: arithmetic logic, register, program control, and a sequencer unit. Each is housed in a nice wooden frame with a glass front for display. Everything is organized logically and LEDs are in place so you can actually see the data flow through the system while the 415 total relays emit a familiar and satisfying cacophony. You can see and hear it in the video below.
The specs, taken from Harry's documentation, are as follows:
Data Bus (8 bits)
Address Bus (16 bits)
All relays are identical (Four-Pole-Double-Throw, 12 Volts)
Max Power Consumption: Estimated 12 Amps @ 13.5 Volts (160 Watts)
Porter completed the computer in 2007. When I asked him if it still worked, he replied "Yes, it is still functional. But it doesn't get much use. I tend to read email on my iPad instead," which is completely understandable. It now waits on standby, ready to be powered on if an excuse presents itself.
Registers and switches.
Click the image above to view the entire slideshow of 17 images.
Programming is quite an arduous task that involves first selecting an address, then using switches to select the byte you wish to enter and then push that into the memory. Repeat this process over and over until you're ready to begin your computation. There are a few sample programs documented on Porter's site that demonstrate how the machine does simple addition, subtraction, and multiplication.
Harry has done a fantastic job of documenting the build. He shares not only a wonderful set of pictures of the process, but also PowerPoint presentations, schematics, and a 60-minute detailed breakdown of the relay computer's design.