Change of product direction, good and bad news!

[TalosThoren]

I always wanted it to be a DIY kit and will absolutely prefer to consume it that way. To me this is simply good news.

I’m definitely more interested in the original X16 than the described X8. I mostly work with Zilog line CPUs and I’d prefer a single, full-featured X16 to serve as my single reference system for this CPU.

[rje]

5 hours ago, VincentF said:

Also for troubleshooting / maintenance, if a chip get fried it's easier to replace and certainly cheaper. 

Except for all the little bits that could be combined into a CPLD -- perhaps.

[D. Kile]

I didn't have strong opinions about the first two questions. I would probably buy an X8, though. When I first heard about the project I thought that $400-$500 would be a both realistic and reasonable price range. I would also consider contributing to crowdfunding.  A kit would do me no good. I've always sucked at soldering on a good day, and now I have what is probably permanent neuropathy in my fingertips.

[BruceMcF]

3 hours ago, rje said:

Except for all the little bits that could be combined into a CPLD -- perhaps.

Yes, the use of a CPLD in the X16c would be a compromise, because in the cost cutting version of a board, you have to compromise.

But if it functions identically to the X16p, I can understand it at the level of the circuit it is simulating, even if I may never understand it at the level of its VHDL specification and how the circuit optimizer turns that into a pattern of NANDs and NORs and latched bits.

8 hours ago, Brad said:

What group of students are we talking about, specifically? For a young kid, having individual components that can be visually segregated definitely helps. Once you hit a certain level of understanding abstraction, the physical representation is less important, but foundationally I think it's very useful.

What "students"? Sheesh, I'm a college Econ professor, how could I spend time on that in a class about the Great Depression and looking at the first two decade's worth of GDP value numbers?

My grandkids, of course!

In many cases, the "students" would be exactly the kind of interested beginners whose questions about what is the best 8bit computer to buy to explore 8bit computing was part of the original inspiration for this whole project.

[Carl Gundel]

18 hours ago, Brad said:

What group of students are we talking about, specifically? For a young kid, having individual components that can be visually segregated definitely helps. Once you hit a certain level of understanding abstraction, the physical representation is less important, but foundationally I think it's very useful.

Agreed.  It really depends on individual learning style, but generally speaking young people need things to be more concrete before they learn to see things in the abstract.

[rje]

12 hours ago, BruceMcF said:

But if it functions identically to the X16p, I can understand it at the level of the circuit it is simulating, even if I may never understand it at the level of its VHDL specification and how the circuit optimizer turns that into a pattern of NANDs and NORs and latched bits.

You know... his question got me to thinking about the relative value of 7400s versus CPLDs, and how different digital logic is today from back-when.   I guess there's still value in doing it with 7400s.

[BruceMcF]

1 hour ago, rje said:

You know... his question got me to thinking about the relative value of 7400s versus CPLDs, and how different digital logic is today from back-when.   I guess there's still value in doing it with 7400s.

Heck, I see that the SD card SPI CS and the serial ROM SPI CS is side by side in a control register in the X8, and there might be one spare pin (if it isn't a pin stranded by lack of logic resources), and I'm like, "hey, that's a job for a 74x138, just send three bits out on pins undecoded, and you get 7 alternative SPI selects."

...

... OOH! WAIT! It might be possible to finesse away the need for a spare pin!

Use the next decoder! A 74x139 dual active low 2>4 decoder ... that's the ticket!

See, if you include a GPIO extender, you can use some of that GPIO for system uses. So you have one decoder, /EN tied to ground so it's always on, tied to the two pins that were original SD and serial flashROM CS's. So %01 SD card, %10 serial flash ROM %11 GPIO extender ... %00, SPI expansion bus.

Now, the other decoder has its two address pins tied to two of the GPIO (probably the top two, for convenience of addressing), and the active low for the first decoder's %00 tied to ITS /EN line. So you address the GPIO pin functions, set the top two pins to output, then address those pins as b0 and b1 and store the desired SPI slot number, 1, 2 or 3. The three four active low outputs go out on four of the pins on the 5x2 block header, plus MOSI, MISO, SCLK, +3.3v and GND. If there IS a spare pin, it can be used as an Alert pin, with a pull up resister, tied to the GPIO extender alert line and also brought in from a pin of the block pin header.

It's BETTER if there is a spare pin to be used for addressing, but it's still workable without ... to "turn off" the extension "slots", just set the GPIO selecting the extension slot to "slot 0", which is NC ... and it's probably a higher priority to use a spare pin for Alert if it's available.

[Wavicle]

2 hours ago, rje said:

You know... his question got me to thinking about the relative value of 7400s versus CPLDs, and how different digital logic is today from back-when.   I guess there's still value in doing it with 7400s.

In re-watching David's original dream computer video, he mentions that the VIC-20 was simple enough that a hobbyist could understand what every chip on the board does. I assume he didn't use the C-64 because it had a PAL to handle the address decoding complexity. I think that is driving the 74xx series vs. GAL/SPLD decision. While internally an SPLD is a very simple circuit, the implementation is still hidden from you.

[Scott Robison]

Just now, Wavicle said:

In re-watching David's original dream computer video, he mentions that the VIC-20 was simple enough that a hobbyist could understand what every chip on the board does. I assume he didn't use the C-64 because it had a PAL to handle the address decoding complexity. I think that is driving the 74xx series vs. GAL/SPLD decision. While internally an SPLD is a very simple circuit, the implementation is still hidden from you.

This is why I want to create a dream computer made with vacuum tubes. Transistors are just too dang magical. ?

[VincentF]

Vacuum tubes are too complicated, let's use solenoids and other electromechanical-stuff-that-i-dont-know-the-name, with 120V it's safe enough for children ?