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Hello there,

For learning purposes, I'm working on a custom 2*4k cartridge using standard 2732 eeproms.

I have issues finding the exact memory map for the cartridge. In a manual I found a cartridge between $C000 and $FEFF, here it is $C000-$FF00 ("Cartridge ROM or DOS ROM") and also $8000 - $FF00 ("Basic and Cartridge ROMS")

Regarding the CoCo memory map, it's $C000 - $DFFF ("8k Cartridge ROM").

So I'm a bit confused... Looks like the starting address is $C000. Adding 8k gives a stop address of $DFFF but I'm not sure. Regarding the existing PCBs it looks like there is both 8k and 16k carts.

So is there a "standard" memory map for 8k games ?

Thanks !

The 'R2' line on the cartridge port is active (low, IIRC) for all addresses $C000-$FEFF (map type 0). For 2 * 4K EPROMs I guess you'll also need to decode that with A12 for your chip selects (and A13 if you don't want your 8K of data $C000-$DEFF to be "shadowed" to $E000-$FEFF - 256 bytes missing there as they're SAM register access addresses).

Thanks. $C000-$FEFF make sense, I'll retro-analyze a cartridge pcb to see if the IC are working in this area.

But maybe I'm missing something but I don't have any 'R2' signal on my cartridges pinout. Is the Dragon pinout similar to the CoCo one ? I do have a /CTS ("Cartridge select signal") witch is used in my sample cartridge.

Oh, yes, pin 32 - CTS. Sorry, it's labelled 'R2' on the schematics.

..ciaran

Thanks !

You confirm that the CoCo cartridge pinout is the same than the Dragon one ?

I can confirm 99.8% that they're the same (maybe there's a tiny difference I'm not aware of? I have some nagging memory that some Cocos have 12v and some don't on the cart port). But I know that a Coco cart game works when put in a Dragon, and the CocoSDC floppy emulator works in both.

tjewell wrote: ↑Mon Jul 20, 2020 9:32 am I have some nagging memory that some Cocos have 12v and some don't on the cart port.

I believe the CoCo 1 has 12V, but the later models do not.

Thanks guys ! I'll study this.

CoCo 1 has 5V,12V *AND* -12V.
Dragon has 12V and 5V
CoCo 2,3 have only 5V.

What is important though is the line that carries -12V on the CoCo1, carries +12V on the Dragon. Though unless you are using them they can be left unconnected. Certainly for a ROM cart you shouldn't need them.

Cheers,

Phill.

That's make sense. And yes for a simple cart we don't need it.

Thanks !