Stacking EPROMs with 74HCT139

I am trying to stack two M27C801 EPROMs together to make a Harvest Moon SNES cartridge (childhood favorite) and I have come across this wiring scheme:



However, I am not sure how to tie in pin 31 of the swap binned EPROMs. Do they all get tied together and connected to position 24 on the PCB?

Thanks!

 

well i might be able to help, but im not exactly sure your question is, so.
The 74HC139 looks like its directing the Output enable signal on each EPROM.
this allows the data on the specific 1 of the 4 chips to be readable on the system bus.
the address lines A18 or pin 31 on the eproms should be connected together,
to the pin on the cart connector that is A18 or what typically would be used.
each A## is a address line so connect each of the same pins together to eachother,
so each pin (1-12,23,25-32) on the actual eprom to the others and back to the snes connector.
the data lines should go like pin (13-15,17-21) connected like the address lines were.
Vss = Gound, Vcc = Supply Voltage, E = Chip enable and
VGpp = Output Enable(Readable); <- All those pins must be seperate from eachother,
hence the 74HC139 inplay to allow those purposely separate 4 pins,
to access the new additional EPROM for your DIY SNES cart.
you can pm me further if you would like but i dont know everything lol ;P

http://datasheet.octopart.com/M27C801-100F1-STMicroelectronics-datasheet-12523557.pdf
http://www.nxp.com/documents/data_sheet/74HC_HCT139.pdf

 

That picture's from my blog

But yes, you connect all the EEPROM Pin 31s together onto pin 24 of the donor cart. Good luck!

 

Sorry to hijack the post, but I was wondering if you would need a second 139 chip to use Sram?
I'm trying to wire a 16mbit game into a pcb with two maskrom slots and sram. The donor already have a 74LS139 chip, but I can't seem to figure out how to rewire it to work with two 27C801's and Sram.

 

No idea, sorry.


The answer to my question however was to wite according to the diagram and tie pin 31 on each EPROM together to position 24 of the mask rom slot.