It's long overdue - Cosmac Elf II build

[HigashiJun]

:bravo:

[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.

[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.

[HigashiJun]

:yay2: :girldance: :cheer: :girldance: :cheer: :yay2:

[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

[rubber_jonnie]

: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 :)

[stephen_usher]

Yay! It works!

[rubber_jonnie]

Yay! It works!

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 :)

[rubber_jonnie]

OK, so that's the last of the parts ordered for the CDP1861 replacement ordered - in the correct TSSOP package this time!!!

[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.