We’ve seen a few retro products using core rope memory, such as telephone autodiallers. Obviously, we’ve covered the Apollo program computers, but we don’t think we’ve seen a complete and functional DIY computer using core rope memory for program storage until now. [P-lab] presents their take on the technology using it to store the program for a Z80-based microprocessor demoboard, built entirely through-hole on a large chunk of veroboard.

For the uninitiated, core rope memory is a simple form of ROM where each core represents a bit in the data word. Each wire represents a single program location. Passing a wire through the core sets the corresponding bit to a logic 1, else 0. These wires are excited with an AC waveform, which is coupled to the cores that host a wire, passing along the signal to a pickup coil. This forms an array of rudimentary transformers. All that is needed is a rectifier/detector to create a stable logic signal to feed onto the data bus.

For this to work as a ROM with the Z80, the address bus is decoded to 16 individual lines using a CMOS 4515 4-16 decoder. These lines each drive a 2n2222 NPN transistor, pulling one end of the associated address wire to ground. The other ends of all address wires are tied to a common AC-coupled oscillator based around our good friend, the 555 timer. A simple rectifier is formed for each core sense circuit with a BAT85 Schottky diode and a 1 nF capacitor, which passes the sense signal along to a CMOS 4508 dual 4-bit latch. The output of which is passed back to the Z80 data bus via some multiplexing logic. Other than some indicator LEDs on the data bus, the only other IO or indication on this board are a couple of 7-segment displays. Obviously, the ROM is limited to a meagre 16 bytes of storage, making a meaningful demo rather limited. Luckily, it’s possible to code a Fibonacci sequence generator in that space, including driving the output latch to update the display. We wonder now what else could be made to fit in such a restricted code?

We covered DIY core rope memories before, like here and here; they’re not news to us. Compare and contrast a DIY magnetic core memory. Less efficient at this scale? We think so, too.

thanks to [Giuseppe] and [Survival Hacking] for the tip!