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? |
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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. |
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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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Posted By
Fuzzweed
on 2025-04-04
02:28:11
| Re: 264 diag carts
yeah, I'm an idiot of course when I checked the A14/15 aren't actually connected to the edge contacts. Duh.
Yes I have seen the opencart but I have been trying to make my own one just for fun. What do you think about this idea:
128K banked cart from 2 x 64k eeprom. Address select on C1/2/L/H and then use CS0 CS1 on each output enable (via OR/AND gate, if cart selected then enable chipselect) . Of course you would get 64K banked to low memory and different 64K banked to high memory, but it could be useful. Or just have two identical EEPROM to make it banked 64K |
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Posted By
SukkoPera
on 2025-04-04
04:58:01
| Re: 264 diag carts
I'm not exactly sure how you intend to do 128k, for that you would need to do some form of bank switching.
I have done something similar for the next version of OpenC16Cart, but it's limited to 64k, indeed. It's not yet released but I can send the schematics. |
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Posted By
Fuzzweed
on 2025-04-04
06:17:55
| Re: 264 diag carts
I was thinking if you use the rom select signals to address a 16k bank of eeprom. C1 lo/hi addresses 4x 16k on erprom1. C2 lo/hi same on eeprom2. and then cs0 and cs1 to choose which eeprom is visible on the data bus.
Plus some logic that eeprom is not output enabled if cartridge rom select is not active. |
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Posted By
SukkoPera
on 2025-04-04
07:10:28
| Re: 264 diag carts
I think you misunderstood how /CS0 and /CS1 work. Check the +4 schematics! |
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Posted By
Fuzzweed
on 2025-04-04
14:02:52
| Re: 264 diag carts
I was thinking they were active on activity to 8000+ and c000+, but you are the expert, I will look further before I start soldering
But yeah if you're happy to send opencart schems that would be great. Fuzzweed at Gmail
Later.....
So this is the post I was basing my idea off https://plus4world.powweb.com/forum/50556/Hardware (near the bottom, not sure how to link to specific posts. and the url tag isnt working anyway...)
If I set A14 and A15 of the EEPROM using C1_LO and C1_HI and then read from $8000-BFFF this will bring CS0 low (attached to O/E on the EEPROM) and put 16K of EEPROM 1 on the databus no? And then I just need some logic so that any read to $8000+ doesn't also enable the EEPROM unless C1_LO or C1_HI is also set.
And same same for EEPROM 2 at $C000+ / CS1 |
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Posted By
SukkoPera
on 2025-04-05
03:54:04
| Re: 264 diag carts
Here are the schematics of my 64 kB cartridge:
The /C2 signal is generated as follows:
If you want, I can send you free PCBs, etc etc. |
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Posted By
Fuzzweed
on 2025-04-05
14:09:23
| Re: 264 diag carts
Free stuff is always good  I have eeproms waiting to be used. Send me a message....
Do you have any comment on my 8 x 16k idea? The lack of enthusiasm makes me think I'm missing something obvious... |
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Posted By
SukkoPera
on 2025-04-06
07:26:07
| Re: 264 diag carts
Your idea is not clear enough to me. If you draw schematics I can have a look. |
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