Electronics > Projects, Designs, and Technical Stuff
Cloning a Commodore PET-2001
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GK:

--- Quote from: ebastler on June 10, 2018, 05:33:21 pm ---
--- Quote from: GK on April 26, 2018, 02:45:15 am ---The (Z80) TRS80 model 1 is next on my list. I don't think that there are too many other "mini" computers from the era which can be cloned this way (not requiring NOS chips no longer manufactured).

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How about the Apple I and II? Somewhat similar to the PET, of course. But digging into Wozniak's graphcics design may be worthwhile?

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I've got the schematics and details for the Apple1, but didn't really find it all that interesting. I hadn't looked at the Apple II until now, but it does look like a very good contender for a clone....... unfortunately though, there are only so many hours in a day. If only I was payed to do this in my day job...........
GK:

--- Quote from: kizmit99 on June 10, 2018, 04:41:01 pm ---It sounds like you did exactly what I had to do for that font ROM - at least it was only 5x7...  It sounds like you've already done it, but if not I can provide a hex file with the "floating a" if you need it.

My plan for the floppy controller is to emulate the WDC1771 with an Arduino Mega talking to a flash card.

OK, back to your current project...  I'm gonna have to steal your keyboard template idea.  It will beat going back to the vhdl file to figure out where I mapped the Break key :)

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Maybe I should just make provision for an external '1771 emulator to be plugged in? Hm, I will see. Your project sounds interesting. Did you get as far as of yet as testing any or all of your VDHL on an FPGA development kit or the like?

There were some weird design decisions made with the original TRS-80; for example the odd-ball 10.6445 MHz pixel-shifting clock frequency. Perhaps that was a commonly available crystal 40 years ago, but you certainly can't buy a 10.6445 MHz crystal off the shelf today. That high frequency meant that all 384 pixels of a complete row were serially shifted out in just 36uS of the complete 63.13uS line period. Unless displayed on a TV/monitor having a tweakable (expandable) width control for the horizontal picture size, that will result in a fairly squished up display horizontally. Also 10.6445 MHz is pushing the video bandwidth limitation of your typical TV-based display monitor a bit too far and it was a complaint back in the day that the high-resolution video characters were ill-defined and blurry on the screen. Maybe RS just had a preexisting stockpile of 10.6445 MHz crystals to use up?

To mitigate these issues and to avoid having to source a 10.6445 MHz crystal, in my clone design I've lowered the pixel-shifting clock frequency to 8MHz. The master clock source will be a generic 16 MHz oscillator. The CPU clock in the original TRS-80 was 1.774 MHz (10.6445 MHz divided by 6). I'll divide my 16 MHz by 9 for 1.778 MHz, which is more than close enough.

james_s:
It really surprises me that the character ROM image is not already out there somewhere. There are many projects to emulate vintage computers, and other projects to archive ROM dumps from these and related hardware. Couldn't anyone with one of the machines dump the ROM easily enough?
kizmit99:

--- Quote from: GK on June 13, 2018, 01:36:14 pm ---Did you get as far as of yet as testing any or all of your VDHL on an FPGA development kit or the like?

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I had the base system (without the expansion interface) running on a Cyclone IV demo board like this one:


The final design should fit into a Cyclone II, so the idea is to eventually use this board with a custom daughter board:


Wrote a PS2 keyboard interface which memory maps the current set of pressed keys so it appears to be an original keyboard.  Instead of generating the original video signal it outputs a monochrome 800x600 VGA video signal.  Had to duplicate the original cassette audio input with a few analog components, then used an old iphone to feed the cassette audio input.  Definitely a real blast-from-the-past to be running StarTrek.  Haven't been able to make the cassette audio out work to the point I can actually csave anything though.
james_s:
Those little Cyclone II boards are great. I have nearly a dozen of them running as various retro computers and 70s arcade games. They're ridiculously cheap, cheaper than buying the bare FPGA. It's a rather old part now but still perfectly adequate for this sort of thing, and making the most of limited hardware is kind of in the right spirit for this stuff anyway.
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