unused pins on Replica I processor

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Offline
Joined: Sep 18 2006
Posts: 2

I'm learning about circuitry from the Apple I Replica Creation book. On pp 191-192, it says:

"IRQ - This line is not used by the Apple I."
"RDY - This line is not used by the Apple I."
"NMI - This line is not used by the Apple I."

Later on page 192, it says:

"RDY, /IRQ, and /NMI are tied to High with current-limiting resistors."

My question is: What is this point of this? Why not just leave them disconnected? And even if we do want to keep these lines High, what purpose do the resistors serve?

This is mostly out of curiosity, but also, I'm interested in building the Replica I on a breadboard, since I think that will be more educational than just soldering the chips onto the printed circuit board. And my breadboard is pretty small, so anything I can leave disconnected is helpful.

Thanks,
Jeremy

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Offline
Joined: Dec 20 2003
Posts: 249
Since those pins are inputs,

Since those pins are inputs, leaving them disconnected can create an interrupt when you don't want one. RDY line if brought low, halts the CPU so if it floats below 2.5V it will stop the 6502. Why resistors? Always use a resistor when pulling up 5V or pulling down (grounding) a input pin. Well, almost Smile

Building the replica from breadboard is fun, I did it initially when I designed the replica. It is a lot of work but with patience it should work fine.

In the design, I have reduced as many parts as possible so this is a pretty trimmed down system, not much you can cut out.

Vince

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