What is it?

This is a bare board only. You must purchase parts and assemble the computer yourself.

You know you can make logic gates out of relays, right? How about a whole computer! The Single Board Relay Computer (or Relay Trainer) is a computer whose CPU is made out of electromechanical relays (like Konrad Zuse's Z3 from 1941), but with memory and front panel interface built from modern semiconductors and integrated circuits (like the 1970s and 80s single board "trainer" computers, such as the KIM-1).

83 relays are enough for a simple but surprisingly powerful CPU. The semiconductor memory allows it to be small and practical, replacing the equivalent of 14,000 relays.

There is much more information at the project website.

Why did you make it?

Over the years others have built a number of awesome hand-wired relay-based computers. Why? Many reasons! To learn computer architecture, to design and build a fun and challenging project, to create a nice teaching and demonstration tool, and all in homage to Konrad Zuse's Z3, which was the first working programmable computer.

The Single Board Relay Computer is my take on this idea. I was originally going to make a relay version of one of the smallest computers I knew, the classic PDP-8. But after running through the design I had a number of realizations: at 227 relays it would have been fairly large and expensive, yet too slow to run existing programs in any reasonable amount of time- meaning that although it was a nice goal, cloning an existing ISA (instruction set architecture) is not all that useful. This led me to clarify my goals: what I really wanted was a small computer that would be fairly inexpensive, still easy to program (not merely Turing complete), and not too slow (so parallel, not bit serial and it had to be some kind of single cycle RISC machine). To be really practical it should use printed circuit board technology and fit on a single board.

The Single Board Relay Computer is the design I eventually came up with. It took three PCB revisions to a get a working manufacturable computer, but now it is ready.

What makes it special?

Here are some of the basic characteristics of the Single Board Relay Compter:

• Relay count: 83
• Datapath width: 8 bits
• Instruction width: 32 bits
• Instruction type: two-address (each instruction specifies two memory locations and the result is written back to one of them)
• Memory size: 256 32-bit words
• Registers: 13 bits total including 8-bit Program Counter, 1-bit carry flag, and 4-bit output register
• Performance: one cycle per instruction, up to ~12 Hz
• I/O: 4 switch closure inputs and 4 relay contact outputs brought to screw terminals. Programs can also communicate with keypad and serial port.

Supported concepts:

• Subroutines (supported with self-modified code: the return address is inserted into a jump instruction)
• Indexing (supported with self-modified code: the index is inserted into a memory reference instruction)
• Multi-precision arithmetic
• Multiply and Divide implemented as subroutines
• Complete set of conditional jumps
• Single-instruction loops with "incjne" (increment and jump if not equal to zero) instruction

Other basic features:

• Keypad and LED display allow you to display and modify memory, halt / run / single step
• Knob to vary the clock speed
• Discrete LEDs show the current instruction, Program Counter, Carry flag, condition result, instruction arguments and I/O port bits
• Powered by 12V through 5.5mm barrel connector.
• 512 bytes of Non-volatile memory is provided to save and restore the RAM
• A 9600 baud serial port on 6-pin Arduino compatible header provides access to built-in assembler, disassembler, instruction trace, and debugger. You can load and save programs through this serial port.

Your purchase includes:

• Bare Board
• Programmed microcontrollers

The board is all through-hole for easy assembly. Even though the relays need only 8 pins, the board allows you to use standard 16-pin DIP IC sockets for them. Although they will add to your cost, I highly recommend that you use sockets to allow the relays to be easily replaced.

13 of the relays must be matched with holding resistors and it is recommended that you use relays as specified in the BOM (otherwise you may have to find different holding resistor values). The remaining 70 may use any standard 12V DPDT DIP relay. You may be able to save money by buying surplus ones.

You will also need a 12V, 1.5 A minimum power supply using 5.5 mm x 2.1 mm barrel connector (positive on center terminal). I recommend 2A for extra power for peripherals.

Also you will need a USB to TTL serial adapter cable with socket for the standard 6-pin header. I recommend this one from FTDI: TTL-232R-5V

The BOM, schematics and assembly instructions are available here.