NROM - PRG replacement for an Eprom

NROM - PRG replacement for an Eprom
by on (#16868)
Hey dudes!
I just got this great idea to play Megaman on my real NES.
I have a Gyromite cart over here that has an NROM board and I was wondering...is any modification or rewiring needed to fit a 32pins eprom on the board?
Or is it just pin 1 to 28 in its place and that's it?

In other words:
Please help me making my Megaman 1 cart

by on (#16869)
Rewiring an NROM board to UNROM is too complex to be a good first project.

by on (#16870)
I'm sorry to say that I'm pretty sure no versions of Gyromite were ever the right board type to play Megaman 1. NROM is just a bare board with a PRG and CHR rom. Megaman used the UNROM board, which uses a couple of extra standard logic components to allow for bank switching.

I myself used Silent Service for making my own UNROM cart, and was just playing Megaman on it the other day. I didn't exactly give a super-detailed explanation since most other folks I know aren't electronically inclined enough to understand it, but you might find some use in the parts that I did explain, along with the pictures.

I guess with my breadboard version, technically I converted it from NROM to UNROM at one point, but you might find it tricky to try and shove all those things into an NROM cart, as well as finding a piece of SRAM for the CHR ram (since UNROM generally never used a CHR rom; Megaman didn't, in any case).

Depending on the board though, you can probably fit a 32-pin socket on it, you just might end up moving capacitors out of the way. On mine, for example, I had to move the one next to the PRG rom to the underside of the board. But the nice thing about UNROM is that you'll only need to worry about replacing the one chip, since as already mentioned, they rely on ram in place of CHR rom.

Matching up the pinouts is the tricky part, so if you aren't absolutely sure what should go where, feel free to ask or show what you come up with for folks to verify for ya.

by on (#16871)
Ok so converting it to UNROM board is a ba idea.
What about putting an EPROM with a game on there.
I know the PRG is 28 pins and my EPROM is 32 pins.

So now I need to know what pin goes where.....
So I can put MachRider on there.
Btw.... if I remove the PRG from a US gameboard and replace it with an EPROM with a PAL rom on there, will the timing be PAL or wil the timing be in NTSC?

I guess the timing is gonna be in PAL cause the PPU is PAL.

by on (#16875)
PAL boards are identical to NTSC boards, only the software and cartridge label differs.
Rewiring a NROM board to UNROM is absolutely impossible, and only a few board can be swapped manually without much trouble (For example UNROM->UOROM, SNROM->SUROM, TKROM->TNROM). For almost anything else, you'll need a board identical to the one your software use.
NROM cannot run programms larger than 32kB, so if your EPROM is 32 pins it is at least 128kb or more. You'd rather found a cart with an UNROM board. Anyway, you'll run in the same problem again because UNROM has only 28 pin slots for 128kb mask ROMs, and 128kb EPROMs have 32 pins. You need to manually bend up the pins 1, 2, 31 and 32 in order to solder your chip. And you'll also need to rewire those pins proprely.

by on (#16878)
Useful references:

NES mask rom pinouts.

EPROM pinouts. Or better yet, check the datasheet for your specific chip.

NES to EPROM conversions. I didn't use this one myself since I was using an eeprom, but it seems to have all the good stuff for normal eproms.

Also something to keep in mind is that Mach Rider apparently uses horizontal mirroring, where as Gyromite apparently uses vertical, so you'd need to tend to that. Some boards actually have two little split solder pads, only one of which having a dollop of solder to specify the mirroring. These usually have an "H" and a "V" marked near'em. If this is the case, it should be an easy fix, by removing the solder on one and putting some on the other. Otherwise you might have to cut a trace and manually reconnect it. Basically, mirroring is changed by connecting either CHR A10 (for vertical) or CHR A11 (for horizontal) to CIRAM (vram) A10.

Also, don't implicitly trust that the H and V on the board (if yours uses the solder pad method) will actually specify horizontal or vertical mirroring. On my Silent Service board, H is actually vertical mirroring, for some silly reason. I remember doing a continuity test at the time, because I thought the game might actually be horizontally mirrored despite what nesmapper.txt and such said about it, but sure enough, it's vertical after all.

Perhaps the H and V on the boards means "scrolling" instead of "mirroring"?

by on (#16881)
FyberOptic wrote:
Also, don't implicitly trust that the H and V on the board (if yours uses the solder pad method) will actually specify horizontal or vertical mirroring. On my Silent Service board, H is actually vertical mirroring, for some silly reason. I remember doing a continuity test at the time, because I thought the game might actually be horizontally mirrored despite what nesmapper.txt and such said about it, but sure enough, it's vertical after all.

Perhaps the H and V on the boards means "scrolling" instead of "mirroring"?

[[Mirroring]] says you're right. The NES homebrew community appears to have adopted the reverse of Nintendo internal terminology (which is, in at least one sense, a good thing because it shows we had no access to trade secrets).

by on (#16884)
Well, "vertical mirroring" means that the screen repeat vertically, so that the are arranged horizontaly one text to eachother. This makes a lot of sense to call it "horizontal arrangement" or something like that, and vice-versa.

by on (#16887)
ok forget the whole UNROM thing....that was a typo.
And I'm gonna forget about megaman on this board.
Ok so now my question is as followed.

What if I take this board (NROM - Gyromite; american cart) and put an EPROM on there with a PAL Machrider rom.
Will it fully take the PAL timing and in this case play it right?
So indeed I'm gonna search for the pinout of this Eprom and rewire it to the board.

by on (#16889)
NTSC and PAL use different lockout chips. Unless you've cut pin 4 of your console's CIC, you'll get blinking.

by on (#16894)
Use a 27C256 (or 27C512, since the extra address line won't be used anyways) with your NROM board, and you won't have to rewire anything at all.

by on (#16909)
tepples wrote:
NTSC and PAL use different lockout chips. Unless you've cut pin 4 of your console's CIC, you'll get blinking.

Right, but I assume the board is identical. Only the CIC chip itself differs.

by on (#16918)
ok so if I have a PAL rom on a board with an NTSC lockout chip and I have cut pin 4 of the lockout chip in my console....will it play in pal or NTSC?

by on (#16920)
If it's an PAL NES it'll run as PAL. Since the NES has the PAL versions of the CPU and PPU (and clock).

The lockout chip only checks that the region for the console matches the cart, when it's disabled it'll let anything run.

by on (#16989)
Ok so now I finally got a Boardtable list on my computer which tells me what board is used for what game.
Now I found a game which'isn't on the list.

I found "Yie-Ar Kung Fu"
I compared the iNES header with the gyromite one.

The Gyromite header says :
PRG-ROM Banks = 2
CHR-ROM Banks = 1
Mapper No. = 0

The Yie-Ar Kung fu header is like this:
PRG-ROM Banks = 1
CHR-ROM Banks = 1
Mapper No. = 0

Now I was wondering....will Gyromite go as a donorcart for Yie-Ar?
Reason I'm checking is because the iNES header isn't fully the same.

by on (#16990)
Yes, but if you use a 27256 EPROM, you must "double up" the ROM (copy /b yiear16.prg + yiear16.prg yiear32.bin) otherwise you can just put the game on a 27128.

by on (#17006)
kyuusaku wrote:
Yes, but if you use a 27256 EPROM, you must "double up" the ROM (copy /b yiear16.prg + yiear16.prg yiear32.bin) otherwise you can just put the game on a 27128.

Unless you run +5V to the upper address pin. This emulates a 27128.

by on (#17015)
Or GND; either way you must fill the EPROM.

by on (#17016)
He'd probably be better off tying to the address lines like normal and just doubling up the rom image on the chip, then he can use the same board for other bigger roms in the future.