The title is quite a mouth full, but I have been working on an adapter board for use in the NES. While EPROMs are still available for burning ROMs, (for development purposes of course) they are running out.
27C, 29F, 49F series chips, etc. They are no longer produced and as stocks dwindle, prices increase. Some companies do still make ROM chips though! The newly produced chip are rarely made in a DIL package though, so an adapter is needed, else the user very carefully hand solders each pin. Even with each pin hand soldered, how are you supposed to program the ROM to it? With an adapter, thats how.
So say you have an adapter for one such new ROM chip. Is the final pinout the same as a CHR mask ROM, PRG mask ROM or the old 27Cxxx/29Fxxx/49Fxxx pinout? How about all three? The thing that my adapter board has, which others may not, is solder-pad jumpers to change the pinout of the two rows of pins. Each pad is named so the user can see which pads to use when they want the pinout of a CHR ROM, PRG ROM or normal flash ROM.
The only draw back is that the Chip that I used, the GLS29EE010, is only a 1 MegaBit EEPROM. That is only 128 KiloBytes. Many NES ROMs are small enough to use these, but many complex RPG's are much larger. Now for a beginner NES programmer, 128KB is plenty of room.
Anyhow, the biggest reason that I wanted to make this adapter is so I can configure the board to be a 29F010, program it with my Willem Programmer and then reconfigure the pads for either a CHR ROM or a PRG ROM. This makes it so I do not have to modify the traces on an NES cart or connect wires all over the board.
In the pictures, you can see how far I have come. I still need to place the solder pads in convenient locations and the last thing I will do is make the board smaller. As you can see, the board protrudes past the through-hole pins. This small amount of extra board may not cause any problems, but it is best to be safe.
The above pictures are of my finished board. The top side contains the filter capacitor, a pull up resistor for the Write enable pin and the 128 KByte ROM itself.
The bottom side has all of the jumpers which are labelled accordingly. If you want the pinout of a CHR ROM, solder the jumpers which are labelled CHR, but if you want to program the chip with your EPROM burner with factory settings, the solder the 010 jumpers and set your programmer in software as a 29F010. Its as easy as that!
I have also reduced the size to 41.91mm x 19.05mm. The filter capacitor is optional since the standard cart will have one immediately next to the power pin of the original mask ROM. The pull up resistor is required unfortunately. Many IC's now-a-days have internal pull-up or down resistors so that pins can be left open or hanging, but the datasheet does not say anything about pull up resistors and this is not a pin we can leave to self-oscillate!