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Assembling Multicart ROM Image

Posted: Fri Jun 17, 2022 9:13 am
by RetroHawk
Hi all,
I'm a little slower than most, and I'm only just building my first MegaCART from the design by Cheshire Noir and Clockmeister.
It turns out that the only PROMs I have are AM29F040, half the size of the 27C801 specified in the design.
I figure that I can just modify the address lines that go to the selection switches to handle only up to A18 (not A19) and tie A19 low.
My question is, when compiling the ROM into the Flash EPROM, do I just make sure that the BIN files are loaded in at 32K boundries?
Do I have to do anything fancy with the 8K/12K/18K ROM files?

Re: Assembling Multicart ROM Image

Posted: Fri Jun 17, 2022 9:37 am
by Scouter3d
Hi RetroHawk,

i guess you do not need to modify the Adresslines, as the "highest" DIP-switch will pull A19 LOW...

But please have a look in the 29F040 Datasheet how PIN1 (VPP instead of A19) should be treated while reading, maybe it needs to be set "high" (5V)

Each ROM needs to be 32k and its 2x16k need to be "bankswapped" (find swapped ROMs attached, hope this is OK?)

Please be aware that there is a small flaw in this MegaCart design, that prevents it from working on MKII Creativisions (because of a different pinout of the Cartridgeport!) If you have an MKII i can provide you with the "patches and cuts you have to make on the PCB)

Cheers, TOM:0)

Re: Assembling Multicart ROM Image

Posted: Fri Jun 17, 2022 10:07 am
by cheshirenoir
I should really make a fix in the master file for that issue :-/


Re: Assembling Multicart ROM Image

Posted: Fri Jun 17, 2022 11:44 am
by RetroHawk
Thanks for the prompt response Tom.

The reason that I need to modify the address lines is because on the 040, A18 is on the same pin as A19 is on the 801. I'd like to be able to ignore the first switch (I would have only soldered in a 4-way switch if I'd realised earlier). The WE pin of the 040 is on the same pin as A18 for 801. I shouldn't need to use that pin in the cart, but will need to ensure that it is at the correct logic level.

I had read about the swapping of the ROM banks, but I couldn't remember how they were swapped. Thanks. You saved me the trouble of doing it. :D

I'm pretty sure that my Dick Smith Wizzard is Version 1, according to what I saw on Madrigal's site. If I find it needs swapping, I'll follow your instructions here. viewtopic.php?f=10&t=330&start=20

Thanks again for your assistance. I just finished printing a case from your design, but I'm finding it a little tight. I also need to add a cutout for the EEPROM. I realised that I won't need many of these carts, as even with a 29F040, I will still be able to fit 16 games on it. I really only want one version of each game, and maybe some of the extras. I'll make up a second cart for new homebrew and demos. I would dearly love to see Scramble finished. It is one of my all time favourite arcade games.

I've just been checking out yours and Chesh's keyboard project. It looks great. I think you should consider replacing the Atari style joysticks with SNES controllers. I bought a couple with sockets to experiment with for my VZ200/300. They have a serial interface and use only one additional I/O pin for each additional controller.

I'll try and keep up with what you are doing.


Re: Assembling Multicart ROM Image

Posted: Fri Jun 17, 2022 2:00 pm
by RetroHawk
Quick followup. The ROM files you provided worked great, thanks.

The only issue I had was that I initially left WE floating, and after it didn't work, I checked the datasheet, and sure enough, it needed to be HIGH for READ. So I added a 10k resistor to Vcc and it worked a treat. I only have 16 games in the 512K, but that is one version of each of the commercial games. I'll put BASIC and some other utilities on a different cart.

Very happy, thanks,

Re: Assembling Multicart ROM Image

Posted: Fri Jun 17, 2022 4:53 pm
by Scouter3d

Cheers, TOM:0)