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[maxchainsplendid]

Hey all, first post heres. I've recently been messing around with SNES carts, and have attempted to make my own SOM2 cart. The sac cart I used was an Illusion Of Gaia, and I've been going of these instructions here: http://snesdev.romhack.de/som2.htm
The thing is, I'm a bit confused, as in the instructions it mentions to use a 74LS139 address decoder, but there is in fact one already mounted on my NTSC cart pcb. Given, the example shown is of the Illusion Of Time PAL pcb, it seemingly has different chips? I'm just wondering if I can wire directly to the one on board? Also, I should note, that I'm using 29F040B's, 8 of em. And yeah, I know it's much better to go with a larger chip, but the thing is, I just had a bunch lying around, and am just doing this for fun, to see if I can get it working. I've browsed the forum here quite a bit, and surely someone can throw out a few tips - Thanks everyone:]

[kyuusaku]

The 74139 contains two 2:4 decoders, meaning you give a binary number between 0-3 and it spits out 4 enable signals, which are only active when their respective binary input is present.

I believe both of the decoders in the 139 are already used by the cart: one for ROM decoding, the other for SRAM decoding, so you'll need another chip. Besides, if you're using 8 ROMs a 139 wouldn't be suitable for decoding 3:8 (8 ROMs) since it would require an extra NOT gate.

For decoding 3:8 use the 74138, that's it's purpose. Just wire the inputs (A0, A1, A2) to the SNES' A19, A20, A21 and the outputs to /CE of each ROM (the first 4M of the game goes on ROM 0, second 4M = ROM 1 etc) for the enable signals, wire E2 to GND, wire E3 to +5V and wire E1 to SNES' /ROMSEL signal and every ROM's /OE to SNES' /RD and you're set.

PS: I also think you shouldn't use 8 FlashROMs; 8 ROMs will be a mess and it's really an awful waste of Flash...

[maxchainsplendid]

Wow, what a fantastic reply! Thanks man - and I couldn't agree with you more on the usage of the flash roms as a whole. Like I said, I just kind of of picked this up on a whim, as I had all the necessary stuff already; thing is, the only roms I had lying around were those, and just thought I'd give it a shot before I spent any money. I'm sure I have an 74LS138 as well, so I'll see how she goes :]