I am building a homebrew computer using core memory. What sort of a ferrite core will I need?
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Look for a core that saturates in a low H field (lower power needed) and the core should have a wide hysteresis curve so that it stays magnetized when current is removed. No air gaps of course. Try ferroxcube, they do a good range.
You should consider to purchase an old core memory plane from an ancient computer. You could connect your own circuits to drive the plane and save the huge amount of work trying to wire up 1000's of magnetic cores. You could even just use one corner of the plane if your computer project needs a small amount of memory.
Check eBay - you can find a fair number of folks selling core memory planes. Here is one example:
You might find it easier using four wires through each core.
X drive Y drive
Sense and digit inhibit by using two diodes on the end of each drive wire you can reduce the number of bidirectional drivers to something like the square root of the number or x wires you then need the same for the y wires.
This will work for any number of bits ie planes in the stack. You will need a sense amplifier for each digit and a digit inhibit wire for each bit. TI use to make chips for doing the bidirectional driving.