Motorola MC6802 repro (early 80s)

Any stories, collections or recollections of the earliest days of computers
or related projects
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Phil_G
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Motorola MC6802 repro (early 80s)

Post by Phil_G »




Another 'lockdown 2020' Retro-Computing project :-)
Back around 1980 I worked in BT's Measurement & Analysis Centre, which was a GEC2050 computer that made test-calls and collated results to improve the telephone service. In each exchange, as part of this setup was a TRT302 printer, which contained a 6802 microprocessor board, which we robbed from any faulty printers. I was a Z80 user and this was my first 6800, but I hand-wrote a monitor for it and me & pal Steve made development systems from these boards. 40+ years on, I still have that original board, but its forever breaking and I thought I'd make a reliable reproduction using the same hardware and the same monitor program. It has display memory, modify memory, goto program, a serial loader for 'S' records, breakpoint & register display, NMI handling etc... quite complex for a hand-coded project in 1980. The Repro is much smaller and neater but works exactly like the old one.
The original display on the TRT board was intended to use two TIL311s but they turned out to be too expensive in 1980, hence the homebrew eprom & calculator display arrangement. Today I have a few TIL311s so thats what I used - much neater! I need to find some more, they're so useful.
The noughts-&-crosses game is embarrassingly simple by todays software standards, but its typical of hand-coded, amateur efforts back in the day - of course, its unbeatable, it may be over 40 years old but it never tires of winning!
Getting a micro to play a tune, or to calculate prime numbers was an achievement back then, this was all fascinating new territory for the electronics hobbyist.
Cheers
Phil

Edit: just realised this isnt number 4, its 5 - I forgot the mini CP/M board !
Also, I overestimated the age of the original - it was early 80s, not '78 as I first guessed.
bluejets
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Re: Motorola MC6802 repro (1978)

Post by bluejets »

Remember a lot of that gear but couldn't get my head around it at the time.
Perhaps if the internet and tutorials were around for me then it may have been a different story.
Everything one needed to know was in books but which way to go. :shock:

RodneY Robot might have eventually even had a chance in my workshop.
Interesting stuff...... cheers Jorgo
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Mike_K
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Re: Motorola MC6802 repro (1978)

Post by Mike_K »

bluejets wrote: 19 Nov 2020, 06:26 Perhaps if the internet and tutorials were around for me then it may have been a different story.
For me it was the local public library and borrow the wonderful Rodney Zaks books, he did them for the 6502 and Z80 and also how to build complete systems. They were better in some ways than the modern internet tutorial as it was all in one book and as the book progressed it took you from novice to details only an expert would need. Now you need to read uninterpretable data sheets or trawl through umpteen forums to get the same detail. But unless you knew about those books or similar, it would have been an uphill struggle for most. If only he was still around and did an Arduino book, most on this forum would become Arduino experts!

MIke
Martin
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Re: Motorola MC6802 repro (1978)

Post by Martin »

Did you design the new red PCB Phil? I can't quite make out the URL from the video - something like http://sbprojects.com/projects/nano6802/index.php

You can emulate these early 8-bit microprocessors pretty accurately - and at full speed or greater - using a dirt-cheap modern chip such as an Arduino, ESP32, etc. You have to study the datasheet for the microprocessor and implement all its opcodes, addressing modes etc., Then probably the PIO chip and any other peripheral chips too. After a few weeks hard work you can eventually run the original binary code on your new emulated hardware. Writing the emulator is an interesting challenge and has its own charm - but it's different from running the real hardware. Best to do both!
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Phil_G
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Re: Motorola MC6802 repro (1978)

Post by Phil_G »

Hi Martin, no, as has happened before with many of the projects I found a PCB that was almost exactly what I wanted, just needed simple mods.
No idea who designed it but its on Seeed. Edit: I since found it was designed by San Bergmans, thanks San! The schematic is annotated as the 'Nano-SBC' but its literally the Motorola minimal configuration straight from their Applications Manual. The display and 4x4 keyboard are functionally identical to the old ones but like the PCB, much smaller, using the TIL311s. The display is on vero and the 4x4 keypad is a standard ebay twopenny item.

There are 6800 emulations out there but I wanted to recreate the setup we had back in the day, I have a few 68 family chips so there was no availability problem for me. I accidentally bought way more 6821 PIAs than I need, I keep doing that, forgetting I've ordered something and ordering it again... and 28C64s are current at RS.

I've no practical use for any of these retro computing projects - its purely about building one for the sake of nostalgia and all of them will probably end up filed away in forgotten boxes in the loft... :D
You cant really use them for battery powered stuff like R/C as they draw quite a bit of current compared to modern chips.

Cheers
Phil
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