newbie building a scratch-built computer

[Tony Duell]

> I've just got to a major milestone on a 6502 based prototype (ie it actually
> runs my code :) and I've got to say I felt I'd actually achieved something

Well done!

> special after debugging with nowt but a meter and  led+resistor probe(s).
> Being so restricted in the tools I had really made me *think* about what to
> test and the end result is that I know exactly how the thing works.

This is true, and it's one reason for knowing how to use simple test gear 
effectively. I have debugged a lot of things that way, watching the 
brightness ot the LED to see if siagnal is stuck high, 50-50 (possibly a 
clock waveform) or something else :-)

On the other hand, I once spend a sunday afternoon tracing a non-fault in 
the memory arbitration circuitry of a Whitechapel MG1, using a simple 
logic probe. I nearly went sane (!). The next day I sent off almost all 
the money I had and bought a LogicDart....


The reason I san non-fault is that the thing reported a multi-bit DRAM 
failure, and it appeared the CPU wasn't finding any RAM at all. That's 
why I thoguht it was a control/arbitration problem, not a memory chip 
failure. In fact, there was no fault at all. The boot ROM needed more RAM 
than was on the mainboard -- IIRC it needed a pair of memory expansion 
boards too. Nowehere was this mentioned...

> Sorry Tony, but I was really glad I was working on a breadboard!

Why? I would have thought having known-good connections and low-impedance 
power and ground connections (neither of which tends to be the case on a 
breadboard) would be very helpful in only having to find the _real_ faults.