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 indicato ..
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