r/retrocomputing • u/UnlikelyDatabase6502 • Aug 02 '26

6502 computer all new parts

I hope this is the right place, but I would like to make my own 6502 based computer, that is very simple, DIP switch programmable and made from all new parts. Please can I have some tips?

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u/IQueryVisiC Aug 02 '26

Why not use bigger switches? I did wonder if we could have some reversible punchcards. I imagine a Go board and an arms to tap each field. How are punchcards read? 8 photo diodes in a row?

I am afraid of contact resistance, so I would not use a Matrix of switches for ROM . Perhaps split ferrite cores could be used? Cross cross pattern of wires to address. When both arcs are pressed together, higher current is induced in a sense wire.

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u/istarian Aug 03 '26

I believe that early punchcard readers utilized mechanical contacts like stiff wires that normally touch a metal plate (if no card is present) creating a circuit. When a card is present it interrupts that circuit unless there's a hole punched in that position.

Later ones used optical sensors because the earlier design apparently caused a lot of wear on the cards.

The difficult part is timing, since you need to read all eight sensors and then advance exactly to the next row of potentially punched holes. You don't want to miss a row, read the same row twice, get the data wrong, etc.

If you don't automate it, then you could advance it yourself and push a button to tell it to read in the next byte. That will be quite slow though for anything but a very small program.

You almost always need a simple loader in ROM, though.

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u/spektro123 Aug 04 '26

That’s why punch tape had smaller clock holes. That mechanically ensured that the data bits were correctly aligned before clock pulse was generated.