Electronics > Projects, Designs, and Technical Stuff

Cloning a Commodore PET-2001

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james_s:
The TL-866 programmers that are sold all over the place ought to do what you need. I have one and use it frequently, it works as advertised.

GK:
Yeah, that's the model I linked to in my opening post. I have one in transit; departed China yesterday evening. The W65C02 and other bits are due tomorrow. To get these bits right before the long weekend with everything else set to go but no programmer to flash the ROMs. Arggggh! So frustrating! I guess I'll have spend this weekend finishing off my Video Pong instead.

GK:
I think I'll be able to have this PET 2001 clone fully built on perf. board with sockets for the programmable chips before the programmer arrives.

I'm sorting out the master clock 1MHz/"Turbo" switching circuitry now. This manual switch-over isn't necessarily as trivial as it might first appear. I've never played with a 6502, but there is the possibility that it is susceptible to runt clock pulses. Suppose you just asynchronously switch the master clock frequency source while the uP is running. Chances are greater than not that you'll create a runt pulse or two whilst switching over; that is a pulse too short for the uP to properly count. That pulse might be ignored completely by the uP, or, if it's duration is within a critical range, it might only successfully clock part of the uP's internal circuitry, causing a crash/lockup/whatever.

I've designed a preliminary circuit to do the clock frequency switching synchronously, only passing whole cycles of either clock whilst engaging/disengaging, but just pondering the issue I'm curious as to how the master clock circuitry of old 286 and the like motherboards with a "Turbo" switch were designed.

Googling so far isn't delivering answers. Does anyone have any idea or a source of really old "IBM compatible" motherboard schematics showing the clock generation circuitry? Much of these of PCs were constructed out of TTL/CMOS logic, so I presume that may have included the clock generation circuitry.

GK:
I seem to to mostly talking t myself here  :P I'm eventually going to layout a single large PCB for this complete PET-2001 clone, but I'm building it up on proto board first to develop the design and iron out any potential bugs first.

Here is the (currently ~3/4 complete) 74HC, 40 column by 25 row character-based video generator. At the moment all that still needs to be wired in is the static screen memory and address/data bus remote-access multiplexing, so right now all I can put on the screen is a single selected character in every character position. I found an old UV-EPROM with a character set already programmed into it, from a video generator project dating back to ~2000, so this is what I currently have plugged into the character ROM socket until my programmer turns up and I can flash the OTP-equivalent PROM with the PET ("PETSCII") character set. Most of the character set in the EPROM is corrupt now as it, although it has spent most of its life in semi-darkness, hasn't had a sticker over the window for who knows how long.

In the photo the eight ROM address lines which select one of the 256 available characters can be seen jumpered to the breadboard, which is how I am currently addressing the displayed character. These lines are about to be soldered to the screen memory SRAM (the empty socket beneath the EPROM) data bus.

The second photo attached below shows the displayed character in reverse field, which is how the PET blinks the cursor.



GK:

--- Quote from: wilfred on June 10, 2017, 12:46:27 pm ---No. I'm reading along. Can you show more about the video circuit.
--- End quote ---

Sure, but that will have to wait until I'm done soldering it together and testing, after which I will transfer pages of scribbled diagrams to Protel.


--- Quote ---Is it your own or did you find it somewhere?
--- End quote ---

Entirely my own of course.




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