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

--- Quote from: GK on June 13, 2018, 01:36:14 pm ---
--- 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 :)

--- End quote ---


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.

--- End quote ---


OK, better late than never; here is where I currently am at, construction wise, with the Model 1 TRS-80 clone. I basically have the whole thing designed on paper and it's now just a matter of building it. I intend to verify the design this time around, before laying out the PCBs, by building the complete computer on bread board. So far I have the video generation circuitry constructed entirely in CMOS logic to the extent that it will display from the uppercase page of the character set in high-res (64 horizontal characters) mode. My CMOS implementation is quite a bit different from the original design and I'm using an 8MHz pixel clock as mentioned previously in the quoted post.

The screen is just showing a random garble of characters at the moment due to the random contents of the video RAM at power up. Might start a new thread in a day or two  :)

Oh, and yeah, a couple of characters are a bit dicky (N for example), necessitating a character ROM revision - I did manually type this ROM out byte for byte in a binary editor after computing each byte on graph paper and am still not 100% sure if my character set is an entirely verbatim, pixel-for-pixel replication of the original.


wilfred:
I always enjoy reading your posts about progress in your latest project. But, wouldn't a dedicated thread for the TRS80 clone help people follow along better.
GK:

--- Quote from: wilfred on December 30, 2018, 10:44:17 am ---I always enjoy reading your posts about progress in your latest project. But, wouldn't a dedicated thread for the TRS80 clone help people follow along better.

--- End quote ---


Yes, already contemplating starting a new thread. I was hoping to maybe get some CPU action already happening before going out tomorrow night, but I just noticed that the two Z84C00s CPUs which I bought months ago now in anticipation of this project are in fact a pair of Z84C20s, which is a bloody PIO. Sometimes if I'm lucky a Digikey or Mouser order placed on a Sunday night will arrive the following Thursday or Friday arvo, but I can't see that happening now with NYE an all. Argh. Another week+ in limbo.
kizmit99:
 :-+
Looking forward to seeing what you do with this...

I've been planning to start a thread on my TRS80 project, but have been spending too much time trying to get the disk-controller emulator working.
Just for grins, here are a few shots of where mine's at...
First version of the PCB needs a few extra pullups:


It's able to read and write from "cassette":


And plays all of the games I've tried on it.  Here's Star Trek ala 1979...


Have been working on the disk emulator for far too long now...


But even though it's still very flaky (I'm getting more read failures than from a real floppy :)), it will boot (and somewhat run) LDOS:


Definitely looking forward to your approach.   :-+
GK:
I'm interested to see how your disk controller emulator works out  :)

I've decided to build my TRS-80 Model 1 clone and the expansion unit as separate units. The only non-standard thing I'll be doing, architecturally, is putting the full 48k of memory into the TRS-80 clone. As you likely know, the original TRS-80 keyboard unit was expandable to 16k maximum and you got to 48k with an additional 32k inside the expansion unit. My clone will be compatible with an original expansion unit too, with a small modification -  the RAM inside the expansion unit will need to be disabled by inhibiting the memory data bus read buffers.
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