whartung wrote:
I'll just summarize my 2 bits.
Whatever components end up being used, they're effectively worthless without a curriculum to coincide with it. This is a combination of applicable theory, design agenda, assembly instructions and experiments.
I don't think it should be a PCB. I think it is more interesting and more accessible as WW or breadboard. Populating a PC board is not interesting, that is an exercise in soldering. Physically wiring the bits together adds more to the process, since the user must think about each wire, why it's there, where it's going. It's the difference between cutting and pasting code in to an editor and typing it in your self. The physical process of assembly is educational in its own right, and simply soldering chips in to a board is too high level and skips large aspects of the design.
Assembly should be done in testable, provable stages so that there is confidence that things are working as assembly progresses. Things like making sure your power is working properly, that your clock is working properly, that any decode logic is working properly, that the CPU runs (someone mentioned hardwiring NOPs and watching LEDs blink -- that's an excellent experiment), etc. etc. So that when you finally get that last components in to their sockets, not only are you confident it's not going vanish in a whiff of ozone and a curl of blue smoke, but that it's going to function. And if it doesn't, you have a good idea how far back you need to go since it "worked last". I fully expect assembly stages to possibly undo work in the previous stage that was their solely for testing (like the NOP wiring...).
Finally, some interface experiments to work with the "real world" via either blinky lights, relays/SCRs/motors, simple LCD displays. thermometers, etc.
A 6502, a static RAM chip, a VIA of some kind, an EPROM, a MAX-232 and some discrete decode logic is simple, it's functional, and it's easy to build. It has all of the major components laid out in a 3D, functioning block diagram.
An EPROM should be provided with the necessary software, I don't believe this is a software problem, so the software should be effectively solved until the machine is running. Once it's running, the students can program it for the interface experiments. I think it's fine that the students may well need to reprogram the EPROM throughout the assembly and testing process. It would be more interesting if a single EPROM could contain all of the software for all of the stages, with perhaps some jumpers telling it which particular stage to run to avoid the necessity of needing an EPROM programmer.
At the end, there should be some advanced experiments documented with more components: adding a keypad and display, adding more RAM, adding another VIA, moving the components to a permanent PCB and housing, etc.
To me, refinements to this design would head in the direction of making it cheaper and more fool proof for a student, especially a solo student, to participate. Simply, if you're going to change the design, provide the answers to "is it cheaper" and "is it simpler" and why.
It would be exciting to see if the combination of components and tools to assemble and test this minus a host PC, a USB -> Serial cable and the EPROM programmer could be done for < $200. It would be so awesome if this can be done without a scope.
So feel free to update.
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