Toggle Switch Bankswitching

I'm trying to put a few different Sonic ROM hacks onto one cartridge. I'll have some 2MB EPROMs (m27c160) next week to play around with, and I was wondering if I could just switch between banks with a simple toggle switch. If each game is 512k, then I can just put one switch on each of the last two address lines (A18 & A19) and use those to toggle the four banks, correct?

Yes you can do just that

Thanks! I'm getting antsy waiting for the EPROMS. Also, are the lines active high or active low?

Could you provide a basic schematic for this once you have it made? I would like to do something like this for myself.

The lines use both states, not just one or the other.

I've seen a repro for all the Phantasy Stat side stories done with switches like this, it's a cool concept. The final iteration though I believe he a cap hold/increment the states so pressing the reset button would cycle through each active ROM.

If I'm understanding correctly, to select bank/game 3, I pull A18 low and A19 high, or is there more to it?

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Originally Posted by Helder

Could you provide a basic schematic for this once you have it made? I would like to do something like this for myself.

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Sure.

00 - game 1
01 - game 2
10 - game 3
11 - game 4

One more question: What voltage do I connect the pins to to pull them high? is connecting them straight to the 5V pin out of the Genesis okay?

You connect to power supply of the ROM chip which is 5V. Anything above will cause damage :P

Update: my bankswitching cart works as it should. Thanks TmEE! I'll put up a simple schematic soon.

Cool, so how many roms do you have on the cart?

Only 2 right now. Here's the schematic you requested. To combine the roms, simply open a command prompt and use "copy /b rom1.bin + rom2.bin outputrom.bin"
http://i1346.photobucket.com/albums/...ps0cc20a43.png

So you combine the roms onto one single flash chip? I was under the assumption that you had 1 rom per chip and how does the bank switch know where the 2nd rom starts if they are joined?

Quote:

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Originally Posted by Helder

So you combine the roms onto one single flash chip? I was under the assumption that you had 1 rom per chip and how does the bank switch know where the 2nd rom starts if they are joined?

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Let's say we have two 1MB roms on a 2MB EPROM. These roms are each exactly 1MB, so when we look at the data on the EPROM, there is a line exactly down the middle dividing the 2 roms. For game 1, we connect pin A19 to ground. This means that the game will use the first 1MB, as it normally would. In order to access the second game, we connect A19 to 5V. Now, all cartridge memory addresses have a 1 tacked onto the front, which means that the system requests data from the first 1MB, and the game thinks it's sending data from the first 1MB, but in reality, it's sending data from the second 1MB of the cartridge. Here's a simplified example using an 8 bit address instead of a 16-bit one, for simplicity: The system starts reading at what it thinks is 00000000, but the most significant but has been pulled high, so it receives data back from 10000000. This continues, so that a request for data at 00000010 returns data from 10000010, a request for data at 00000011 returns data from 10000011, and so on. You could use 2 separate chips as you had suggested, and toggle which one is enabled, but that's more complicated to wire and potentially more expensive than this solution.
Also, I know the 68000 is big endian, but I used little endian numbers in this example because big endian ones are confusing.