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Re: It's long overdue - Cosmac Elf II build
Posted: 11 Apr 2026 13:52
by HigashiJun
:bravo:
Re: It's long overdue - Cosmac Elf II build
Posted: 13 Apr 2026 11:07
by rubber_jonnie
I've been doing some research into the Elf II, because it is, if I'm perfectly honest, a bit odd.
I found this article
Cosmac Elf and whilst it is for the original Elf not the Elf II it neatly answers the question "Why is there no ROM?".
Well it seems the CDP1802 has an unusual mode that allows the CPU to run without ROMs or any other non-voloatile memory (Such as battery backed RAM) and using the 'LOAD' switch directly enter programs into memory.
No monitor or any other software is required, just direct entry. I think that is pretty cool TBH and it explains why there are only 2 7 segment displays on the base board. It drops you at a specific spot in memory and you can add enter data directly then run it :)
Very neat indeed.
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 11:28
by rubber_jonnie
And the Elf II lives!!!
The final part arrived today, a 74HC173, the last part for the keyboard.
Lo and behold I can step through RAM contents and see the data there, and the value of 77 was entered by me :)
IMG20260414110638.jpg
Next steps are to get the CDP1861 replacement built and the 6 digit display finished and we should be in a good place, but it's good to know it appears to run :)
Annoyingly one of the parts for the CDP1861 arrived today and I ordered the wrong footprint, but since I still have parts on the way from AE I think I have time to fix that error.
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 11:45
by HigashiJun
:yay2: :girldance: :cheer: :girldance: :cheer: :yay2:
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 11:49
by rubber_jonnie
D'oh I'm a dimwit...
:dizzy:
I just realised I ordered both IC's, a 4014 & 4040 in SOIC and it should be TSSOP. I should check these things more carefully.
Merdé
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 11:51
by rubber_jonnie
HigashiJun wrote: 14 Apr 2026 11:45
:yay2: :girldance: :cheer: :girldance: :cheer: :yay2:
Thank you :)
Now I need to find out the code I need to turn on the 'Q' LED to see if that runs :)
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 11:52
by stephen_usher
Yay! It works!
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 11:56
by rubber_jonnie
I know, very surprised it seems to have worked first time.
The Memory Protect switch also works as well as the Load switch, and if I put random code in and hit the Run switch, it hangs, so I'm thinking the Run switch works too :)
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 14:09
by rubber_jonnie
OK, so that's the last of the parts ordered for the CDP1861 replacement ordered - in the correct TSSOP package this time!!!
Re: It's long overdue - Cosmac Elf II build
Posted: 14 Apr 2026 16:11
by rubber_jonnie
Hmm.
I think I may have a DCP1802 with a bad Q latch...
I tried running a quick prog to turn the Q LED on, it's just a latch in the CPU, but sadly it doesn't work, but with the leg lifted on the CPU the LED illuminates.
So very likely a bad latch in the CPU. Boo hiss.