Electronics > Projects, Designs, and Technical Stuff
Cloning a Commodore PET-2001
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ale500:
How about a CoCo clone ? :), the three important chips are sitting in one of my parts drawers since... a couple of years :)

Fantastic work !
GK:
Okaayyyyyyy......

I've basically have the motherboard schematics finished now and drawn up in Protel. I was just about ready to start laying out the PCB Sunday night after "updating"/importing all of the component footprints to the PCB template from the schematics.

However I've made quite a few significant changes to the design over the initial crappy prototype and I just decided that it would be good to see the final circuitry working before committing to the PCB, just because. The changes were primarily regarding the manner in which the data bus from the CMOS W65C02 was buffered to the TTL-level-IO memory and N-version peripherals and the read/write decoding.

So this evening, to save time cooking and eating and all of that, I picked up a whopper with cheese and large fries from Hungry Jacks (burk!) on the way home from work, scoffed it down and then went straight into the shack. It was going to be way too hard to majorly rewire the crappy initial soldered prototype, so I unplugged all of the chips and went for breadboard instead.

Four hours later and I have the computer built to the absolute minimum hardware (nary a wire beyond) required for a system boot. The boot-up message didn't display for me on the initial power-up as I missed the wire from the address decoder to the !CS pin of the system RAM! I hope I'm not coming down with early-onset Alzheimer's disease or something.
 
Tomorrow if I feel like it I'll wire in PIA#1 - that will give me the blinking cursor and allow me to connect up my keyboard interface. Right now though I'm going to bed.

GK:
I have finally finished the motherboard PCB layout.

I've added a few features not available in the original PET too. The ROM holds both V2 (sometimes referred to as V1) and V3 (sometimes referred to as V2 or "upgrade") BASIC. The version of BASIC to run is switch-selectable. There is also a RESET button, which saves having to power-cycle when the computer crashes, which it will if you change the position of the BASIC version select switch when the computer is running.

The original PET-2001 used 1/60s "jiffies" as the time interval for updating its software real time clock. The vertical frame rate from the video circuitry was the frequency reference. Since I designed my video generator to operate at a frame rate of 50 Hz, the real time clock will not keep accurate time., so as a work-around I included on the board an accurate 60 Hz clock source. A jiffy clock selector switch permits the CPU interrupt/sync to be taken either from the 60 Hz source or the video vertical frame rate.
However in the 60 Hz mode, the CPU will no longer be synchronized with the video frame rate and the PET's synchronized "snow free" display update mode won't operate without the snow. This is only a minor inconvenience as the 60 Hz switch need only be flicked when you want the PET to accurately measure time, and such applications generally aren't graphics intensive anyhow.
As an aside, AFAICT, the PET's original jiffy clock wasn't all that accurate anyway, as the 60 Hz field/frame must have been a little bit off frequency as it was derived by integer division in the video generation circuit of the 8 MHz master clock. 60 doesn't divide evenly into 8M. I used a stock 3.932160 MHz crystal in my oscillator, which is divided by 2^16 = 60 Hz exactly (a 74HC4060 followed by both halves of a 74HC74).

For sound effects/music there is an LM386 audio amplifier on board with volume control to drive an internal speaker, a source select switch and a -10dBV line-out buffer amp. The 4-pole rotary source select switch positions are OFF, VIA CB2, Cass. read & Cass write. VIA CB2 is for sound effects as demonstrated in this thread previously and the other positions allow you to listen in to either the input or output data streams of the cassette drives.

For the keyboard interface, I elected not to incorporate a PS2/USB interface on the motherboard, keeping the circuitry as per the original PET in this regard.
My PS2/USB keyboard interface will be a separate PCB design which will simply plug into the IDC26 keyboard matrix interface connector on the motherboard as an alternative option to either an original CBM keyboard or one hacked/rewired to suit.

That's all that's interesting that I can think of right now as it's past time for dinner. I'll be giving the whole thing one more thorough checking over again tomorrow. Hoping to have the board to load with bits in a couple of weeks.
       
GK:
Motherboard schematic teasers................



















GK:
Amazing service from Elecrow. I posted the Gerber files and ordered Sunday evening and the boards arrived Friday (yesterday) afternoon. I wasn't expecting to receive the boards prior to this weekend and haven't organized all of the parts yet, but I just loaded up what I had this afternoon.

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