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 RCA 1802 reverse-engineering progress 
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Joined: Wed Jan 09, 2013 6:54 pm
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The RCA 1802 was an early CPU, and a very early CMOS CPU, used for example in the COSMAC ELF evaluation board/kit computer.

A chip was imaged in 2011 at high resolution by Greg of the Visual6502 team, with images here and some supporting analysis here.

More recently, AmyK has produced a schematic of the control logic, now found at the last link above, and has found a couple of parallel investigations: Chuck Bigham has blogged and posted on the cosmacelf list about his efforts to simulate the control logic, and Dave Schultz has posted an analysis of several circuits:

Image
Image

(See also ChuckT's post about the 8085)


Last edited by BigEd on Sun Aug 04, 2013 8:15 am, edited 1 time in total.



Mon Feb 25, 2013 12:01 pm
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That is very exciting. Cosmac ELF was the first computer I built. The register set architecture was pretty amazing for the time - it was practically a RISC machine if I remember correctly


Sun Aug 04, 2013 2:49 am
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Ed, the 2nd- and 3rd-from-last links (the images) are no longer available. Are you able to replace them with wherever they were moved to?

The Elf was the first computer of a friend of mine too, in 1978 or '79. He bought it in kit form, but, judging from my look through my COSMAC data book, he apparently had the evaluation system which was quite a lot more complex than what's shown in Ed's linked pages. The 1802's performance was really, really poor; but it was one of the first CMOS and rad-hard processors, so it went into a lot of spacecraft IIRC. This friend used it in his senior project as an EE major with the excuse that with so many registers, he didn't need any RAM.

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Sun Aug 04, 2013 4:45 am
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images relinked.


Sun Aug 04, 2013 8:15 am
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I have thought about using this CPU, although being that it is rare I dont think I could get many perhiperal chips for it.

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Tue Jan 14, 2014 6:37 pm
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You should be able to use most peripheral chips that use RD/WR control signals. Those chips requiring E and R/W* will be more of a challenge. You will need an address latch, because the address is output 8 bits at a time.

The 1802 had some interesting I/O and DMA capabilities, but you're not required to use them. :)

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Mon Jan 20, 2014 12:22 pm
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So would the 8255 be ideal.

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Tue Jan 21, 2014 2:46 pm
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James_Parsons wrote:
So would the 8255 be ideal.
There is a wide variety of peripheral I/O chips that use /RD and /WR control signals (as opposed to E and R/W). As noted, with 1802 you will need an address latch -- but that's fairly simple, and you will need it anyway if you want to use generic RAM and ROM chips.

BTW, remember that basic bitwise I/O can be as simple as a 74xx573 or 74xx574 attached to the data bus. IOW, the fancy LSI peripheral chips aren't your only option. Also be aware that the 1802 has a successor, the 1806, which is slightly more capable.

cheers
Jeff

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Tue Jan 21, 2014 3:21 pm
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Slightly off topic, but perhaps interesting: "The COSMAC ELF Membership Card" http://www.sunrise-ev.com/membershipcard.htm

Lee Hart and friends rebuild a slightly modified COSMAC ELF in a tin box. Something I couldn't get around :lol:


waiting the kit to arrive...


Thu May 03, 2018 9:35 pm
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Quote:
waiting the kit to arrive...
Anything come of this?

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Mon Oct 08, 2018 12:32 am
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Oh well, yes, ages ago :)

Thank you for your question.

The kit assembles easily and works seamlessly. It was much fun to toggle in some small programs. As you can check the state of the toggle switches during runtime you have a degree of interaction available for your programs. Together with the 8+1 LEDs there are some funny little "applications" possible. The possibility to slow down the clock frequency helps to "see" how a program works without the need for delay loops if you feed the LEDs with whatever byte you wish to examine.

I cannot but recommend this kit to everyone who like such a barebone system and/or like the 1802 (which was the 3rd microprocessor I had back then ~1980).


Regards,
Arne


Tue Oct 09, 2018 6:11 pm
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Thanks for the reply. I may order one for myself.

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Michael A.


Wed Oct 10, 2018 4:02 am
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Enjoy yourself! You won't be sorry!


Wed Oct 10, 2018 6:24 am
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