It's long overdue - Cosmac Elf II build

[rubber_jonnie]

OK so the improved LED board is built:

IMG20260415214559.jpg

Now just waiting on the connectors for the mainboard and we should be good to go :)

[rubber_jonnie]

Ho hum...

The GALs arrived for the DCP1861 video chip replacement, but do I have a TSSOP24 adapter for my programmer? Nope, turns out I have pretty much every sort of adapter apart from that one!!! Also the closest one I have has the wrong pin pitch so can't be pressed into service. Adapter on order...

In addition my replacement CDP1802 CPU arrived and the Q LED still won't light up with the correct code.

So, either the code is wrong, or something is preventing the code from running, or the code isn't arriving in memory as expected or there is some sort of electronic issue.

Now if I lift the Q leg of the CPU or remove the CPU the Q LED comes on, and it's a very simple circuit:

Screenshot 2026-04-18 at 11.45.50.png

So the Q line of the CPU is just a latch as I understand, and is held low, but I see no activity on it when I run the code. If I pull up the Q pin on the CPU then the Q LED lights.

This is suggestive that the Q circuit works as expected. So what next?

Well the CPU has power and a clock, and it seems that I am able to enter data and step through memory to see random stuff from power on. so it seems I either have the wrong code or the CPU isn't running it.

I think I will try connecting the RAM battery backup to and then I can power off/on to see if the memory contents are as expected and if I have truly entered the code as expected.

And my connectors for the improved display are still in the post... What's annoying about that is that they were supposed to be from a UK seller but the tracking looks like they have been drop shipped...

Merdé

[rubber_jonnie]

Hmm interesting...

I tried to use the battery backup and I think it works, however I accidentally found that the Q LED would sometimes light.

So I'm thinking that the memory location for the Q latch had been accidentally set when the machine starts, so I'm convinced that the CPU is good and there's perhaps an addressing issue.

I need to get the 6 digit display sorted so I can see exactly which memory location is appearing on boot.

Hopefully the sockets I need will turn up soon.

[rubber_jonnie]

OK, so I think I've realised whats going on.

With the battery connected the RAM is acting like a ROM so if whatever randomness was in there caused the Q LED to illuminate it will continue to do so when it loads at boot, which means that the CPU is proved good because whatever is in there now causes the Q LED to light when I power on.

Not being able to see what memory location I'm at is a bit of a bind so hopefully I can get the 6 digit display sorted soon. Beyond that it'll be nice to get the CDP1861 replacement built soon as it should give me a composite out and as I understand that also shows a 6 digit Memory:Contents monitor style display which was common for monitors of the period.
.

[rubber_jonnie]

And finally the missing parts arrived so the new display is in.

IMG20260423121422.jpg

Also, I am now able to get the Q LED to light up and I realised I was using the 3 position switches incorrectly. In the mid position they are off, so they have to be either all the way back or all the way forwards or they don't work as expected, and I think this is the mistake I made earlier.

Having the extra 4 digits to show the RAM made a big difference in understanding just what was going on.

I think I have a working Cosmac Elf II!

:girldance: :girldance: :girldance:

So next up is the CDP1861 build to be completed to see if I can get a video display out of it :)

[HigashiJun]

Love the retro feel.

8-)

[rubber_jonnie]

Love the retro feel.

8-)

It's as retro as you'll get :)

[stephen_usher]

Yay! :-)

[rubber_jonnie]

So, as described in the GitHub repo, with this display board the 7805 runs quite hot, even with the 7805 bolted down, so I am going to replace it with a Traco I think and we'll see how that gets on.

[rubber_jonnie]

OK, so the 2 GALs labelled Line and Frame are now programmed, so I'm just waiting on the CD4040 and CD4014 to arrive then I can assemble the CDP1861 and we'll see if I can get an image on a display :)