nesdev.org

Hello new friends,

I'm modifying Mega Man 2 and I have a few things I need help on. It's mainly the hardware that's giving me the uncertainty. I see that Mega Man 2 is an SGROM-02 cart, and I was planning to use Rad Racer as a donor. Rad Racer is also an SGROM-02 cart, however has only a 28-pin PRG-ROM, as opposed to Mega Man 2's 32-pin PRG-ROM. The board has holes for 32 pins, so I can use a 32-pin DIP; but do I need to? I have a pair of Atmel AT28C256 flash chips- is there a way to make these work? Or must I find a 32-pin? If I can't use mine, and I wanted to avoid UV EPROMs and stick with flash chips, would you recommend a part number?

I've read that the SGROM wiring should be identical to the SNROM on multiple sites, so I'm going to go with this being truth unless told otherwise. That means I'll be following the SNROM mod steps (bend pins 1,2,24,30,31... etc) correct?

I guess the first question I need to get out of the way would be what anyone recommends for a 32 pin flash chip if I cannot use my existing AT28C256s...

Sources:
NesCartDB - Mega Man 2
NesCartDB - Rad Racer
Atmel AT28C256, page 2

X-posted in /r/NesDev
Thanks SpaceCowboy58

WReXx wrote:I have a pair of Atmel AT28C256 flash chips- is there a way to make these work?

No. Memory chips are named in kilobits, as opposed to kilobytes.. This means that the AT28C256 can only hold 32 kilobytes, 1/8 of what Mega Man 2 needs. Also, 64KB is the largest a 28-pin programmable chip can be. Non-programmable ones, like the ones found in original Nintendo carts, can go up to 128KB with 28 pins.

I wanted to avoid UV EPROMs and stick with flash chips, would you recommend a part number?

AT29C FlashROMs have worked great for me. You'd need an AT29C020 (128KB 256KB) or an AT29C040 (512KB) if you decide to go with those.

Well don't I feel silly. I guess I'll hop to it for some new chips

Also, for clarification- modding an SGROM cart, I only need to change the PRG-ROM and not the CHR-RAM, correct?

Thank you



edit: the AT29C020 would be a 2Mbit or 256KB chip, correct? Just checking, as you typed 128

WReXx wrote:modding an SGROM cart, I only need to change the PRG-ROM and not the CHR-RAM, correct?

Yes. The RAM chip doesn't contain any data, it's filled during run time by the program. Some times you'll need to replace a CHR-RAM chip with a CHR-ROM one (easy) or a CHR-ROM chip with a CHR-RAM one (not easy if the CHR write pin is missing from the cartridge connector - it often is). This is not the case now though, since both the donor and the new game use CHR-RAM.

edit: the AT29C020 would be a 2Mbit or 256KB chip, correct? Just checking, as you typed 128

Yup, my mistake! =)

So the first thing I did was take Rad Racer apart. I noticed some differences...

- The security screws were not the standard Nintendo 3.8mm variety; they were almost flatheads.
- The label is the North American style label with no "NES-RC-CAN" text. It seems to look USA-released.
- The two-digit code on the back is "12" which aligns with the USA distribution... However!
- The CHR-RAM is by NEC; 64S20 (and 8730MX) are written on the chip. This does not correspond to any of the Rad Racer carts listed on NesCartDB. The Europe PAL-B has an NEC CHR-RAM but that's the only similarity.
- The PRG-ROM is possibly NEC; NES-RC-0 PRG W77 (and 8734KX) are written. The W77 stands out as the Canadian version of this cartridge.
- There is no C3 resistor, but there is a C4 resistor. The picture on NesCartDB shows a C3 but no C4 resistor. Mega Man 2 shows a C3 but no C4- and also no C2.

That's quite a bit of difference between the information provided. Other than that, the board looks great.

1. Can I still use this board as a donor?
2. When converting this 28 pin PRG-ROM to a 32 pin, is there anything I need to do differently than if it had a 32 pin chip to begin with?


Pictures:
Front of cartridge
Cartridge disassembled
Front of board
Rear of board

WReXx wrote:- The security screws were not the standard Nintendo 3.8mm variety; they were almost flatheads.

Not all NES games used security screws. (In fact, my copy of Rad Racer uses the same screws as you described.)

WReXx wrote:- The CHR-RAM is by NEC; 64S20 (and 8730MX) are written on the chip. This does not correspond to any of the Rad Racer carts listed on NesCartDB. The Europe PAL-B has an NEC CHR-RAM but that's the only similarity.

It's just a RAM chip, it doesn't really matter what kind it is.

WReXx wrote:- The PRG-ROM is possibly NEC; NES-RC-0 PRG W77 (and 8734KX) are written. The W77 stands out as the Canadian version of this cartridge.

And my (USA) copy of Super Mario Bros. says "HVC-SM-0 PRG". The region and manufacturer of the ROM chip is not important, only its contents.

WReXx wrote:- There is no C3 resistor, but there is a C4 resistor. The picture on NesCartDB shows a C3 but no C4 resistor. Mega Man 2 shows a C3 but no C4- and also no C2.

Those are capacitors, not resistors, and they're usually not too important. Someone more familiar with electrical engineering can tell you if you need to add C3 or not.

WReXx wrote:1. Can I still use this board as a donor?

I don't see why not.

WReXx wrote:2. When converting this 28 pin PRG-ROM to a 32 pin, is there anything I need to do differently than if it had a 32 pin chip to begin with?

AFAIK, if the board has the holes for the extra 4 pins, you can treat it as if it used a 32 pin chip to begin with. It wouldn't hurt to check the connections with a multimeter, though, just to be sure.

Joe wrote:

WReXx wrote:- The security screws were not the standard Nintendo 3.8mm variety; they were almost flatheads.

Not all NES games used security screws. (In fact, my copy of Rad Racer uses the same screws as you described.)

WReXx wrote:- The CHR-RAM is by NEC; 64S20 (and 8730MX) are written on the chip. This does not correspond to any of the Rad Racer carts listed on NesCartDB. The Europe PAL-B has an NEC CHR-RAM but that's the only similarity.

It's just a RAM chip, it doesn't really matter what kind it is.

WReXx wrote:- The PRG-ROM is possibly NEC; NES-RC-0 PRG W77 (and 8734KX) are written. The W77 stands out as the Canadian version of this cartridge.

And my (USA) copy of Super Mario Bros. says "HVC-SM-0 PRG". The region and manufacturer of the ROM chip is not important, only its contents.

WReXx wrote:- There is no C3 resistor, but there is a C4 resistor. The picture on NesCartDB shows a C3 but no C4 resistor. Mega Man 2 shows a C3 but no C4- and also no C2.

Those are capacitors, not resistors, and they're usually not too important. Someone more familiar with electrical engineering can tell you if you need to add C3 or not.

WReXx wrote:1. Can I still use this board as a donor?

I don't see why not.

They very well could be capacitors- that would explain the C# designation which I thought was incorrect at first. I imagined that they were there to bias the CMOS gates, but who knows. The funny thing is that I'm an electrical engineer...

Seems to me like we're still in business though.

tokumaru wrote:

WReXx wrote:2. When converting this 28 pin PRG-ROM to a 32 pin, is there anything I need to do differently than if it had a 32 pin chip to begin with?

AFAIK, if the board has the holes for the extra 4 pins, you can treat it as if it used a 32 pin chip to begin with. It wouldn't hurt to check the connections with a multimeter, though, just to be sure.

Yes, there are the extra four holes- I compared the traces and they appear to be the same.

Update- pictures!!!

http://imgur.com/a/oET0D

So you destroyed a Mega Man 2 MaskROM when removing it? Or was this Rad Racer?

I asked because you said something about clipping and removing something from the holes in another step. If you desolder then the MaskROM is still useful. If you damage the legs it makes it harder to impossible to reuse. Its bothersome when someone would use a great game like Mega Man 2 for this. It's not like there aren't plenty of absolutely terrible games to use up first. Or just use a brand new board like InfiniteNESLives makes.

MottZilla wrote:So you destroyed a Mega Man 2 MaskROM when removing it? Or was this Rad Racer?

I asked because you said something about clipping and removing something from the holes in another step. If you desolder then the MaskROM is still useful. If you damage the legs it makes it harder to impossible to reuse. Its bothersome when someone would use a great game like Mega Man 2 for this. It's not like there aren't plenty of absolutely terrible games to use up first. Or just use a brand new board like InfiniteNESLives makes.

I would never damage a Mega Man 2 for this. I snipped the legs off on Rad Racer! It's less time consuming this way. I suppose I could solder suck it all out and mess around for an hour to get it to come out smoothly, but mehhhh.

Desoldering can definitely be a pain. Good that you didn't choose to use Mega Man 2. I don't really think Rad Racer is as good of a game. You should consider using new boards though. It's much easier and cleaner than reusing old boards.

Aaaaaand black screen. I don't know what I'm doing [wrong] but...

Tried my ROM, split with TNINES 2.6 into a single PRG file.

Burned ROM to EPROM just fine... No errors. Used binary format, tried fill with '00' and 'FF'. Black screen.

Then tried original Rad Racer, split, burned PRG. No worky.


I rechecked my hardware...
There are NO shorts or stray cable strands making contact with something they aren't supposed to be.
There aren't any socket pins that are shorted (I checked with a DMM before soldering to the board).
I even used a razor to ensure a gap between the ugly solder blobs, just in case.

Here are the pinouts I am using: Chip Pinouts.PDF (I made this)
And here are some more pictures: http://imgur.com/a/TDtyk

Unless anyone has an idea, I think my next step is to solder the socket directly to the board ...

Reprogramming should be easier than rewiring, so try flashing a few other things first:
* Tepples's Holy Diver Batman! for MMC1
* Galaxian (8KiB PRG) or nocash's Magic Floor (4KiB) (just duplicate them to fill your EEPROM).

Is there any chance the wires between the pcb and socket have broken?

Does your multimeter have a logic test mode? If so, does it indicate that all the lines on the ROM are toggling as the NES tries to play the game?

lidnariq wrote:Reprogramming should be easier than rewiring, so try flashing a few other things first:
* Tepples's Holy Diver Batman! for MMC1
* Galaxian (8KiB PRG) or nocash's Magic Floor (4KiB) (just duplicate them to fill your EEPROM).

Is there any chance the wires between the pcb and socket have broken?

Does your multimeter have a logic test mode? If so, does it indicate that all the lines on the ROM are toggling as the NES tries to play the game?

I checked continuity of everything before soldering it to the board. I doubt they broke since. I tried burning Holy Diver to my chip and same thing- black screen... I don't get it.

I triple checked my wiring- everything is exactly how it should be according to the PRG-ROM pinouts and the chip pinouts.

Maybe I am not burning the ROM correctly. I don't know. I've tried several ROMs and they all give a black screen, no 1Hz flash or anything.
The NES cart worked prior to modifying.
I am generally good with ESD practices.
I did not cut any traces.
My NES works fine with other games still.

UGH.