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Posted By

Fuzzweed
on 2025-04-03
04:45:49

264 diag carts

Hi, just been looking / reading about diagnostic carts. It looks to me that A15+A14 are tied to vcc. I think this is how they work to force external rom.

But my question : is this 'bad' for the cpu or Ted. What if they are trying to drive these lines low at the same time they are being held high?

Posted By

SukkoPera
on 2025-04-03
05:09:36

Re: 264 diag carts

Diag carts (Well, actually there is only one: Diag264) are normal carts, they do nothing dirty to start up. The A14+A15 fiasco concerns external RAM expansions for the C16/116, which are a different thing and will never work correctly.

The only "problem" is that the 264 series has nothing like the Ultimax mode of the C64, which bypasses the KERNAL. This means that Diag carts might be unable to start up if the KERNAL is faulty, while on the C64 they can bypass that (in most cases).

This is why Diag264 also has a "KERNAL ROM" version, meaning that it's supposed to be run by being physically put in place of the KERNAL ROM, rather than as an external cart.

Posted By

Fuzzweed
on 2025-04-03
06:50:44

Re: 264 diag carts

OK, I found this pcb on ebay which says diag264, but I was doing continuity checks and 14,15 are vcc.

I can't find any info on it, maybe it's some random homebrew, it doesn't look like any other diag carts I googled.

Anyway that's not important I was just going to cut traces to use as a prototype board, but I was just interested in how the A15/15 hi thing would / could work.

https://www.ebay.co.uk/itm/356710996485?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=_fxzdgqmrxq&sssrc=4429486&ssuid=0wXDmSYxQKy&var=&widget_ver=artemis&media=COPY

Posted By

SukkoPera
on 2025-04-03
12:44:37

Re: 264 diag carts

You mean A14 and A15 of the EEPROM, right? That is in order to handle modern 64k (E)EPROMs. Individual ROMs are only supported up to 16 kB on the 264 family, so A0-13 would be sufficient, but the remaining address lines on the EEPROM must have some stable voltage, so they can either be connected to GND or +5V (or even switched between those values in order to have multiple "slots", like I did on OpenC16Cart. But then those lines are NOT connected to A14/15 of the CPU.

I don't see any mentions of Diag264 on that eBay auction, but anyway it's just a ROM like any other.

I suggest the you have a look at OpenC16Cart, it's open source, so you can see exactly how it's made, including schematics, etc. and hopefully understand the logic (it was one of the first PCBs I ever designed, so it might not be very very clear

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