I am trying to make a repro cartridge and I had a basic question. I am using a SLROM donor board and for the PRG rom and I am using a 28 pin Dip 27c512. The mask rom and the eprom are nearly pin compatiable with the only different being pin 22. The eprom has this as /CE (active low?) and the mask rom has pin 22 listed as A16.
My question is what am I supposed to do at this point. Will this work as is or do I have to do something? I have seen the pin conversion for the eproms, but those refer to 32 pin eproms and I am using a 28 pin so I am confused about using the right chip or not.
If the eprom and board signals don't line up/match you have to rewire it. The guidelines in that document can be used to figure out what you need to do.
What about the mapper? All you are doing is replacing the ROM. It isn't magic, it's dead simple. Take out the maskrom and put in an eprom. If the signals don't match, rewire to match. If you have the pinouts you are fine. If not, find them or figure out how to discover what each pin is for yourself.
So, where is pin 22 going? To the ground? Or to the slot? Or to the mapper?
If pin 22 is /CE, it should go into hole 20, which according to
this document is /CE. But what is pin 20 in the EPROM? You'll probably have to reroute it too, as well as the pin for A16, which will have to go to hole 22.
There's no secret really, just bend up the pins that don't match and wire them to the correct holes. All you need is the documentation for the 2 pinouts.
Oops, I meant pin 22 on my m27c512 is listed as /oe and the prg rom has A16. The m27c512 only has a0-a15 so what do I do in this case?
lordloss wrote:
Oops, I meant pin 22 on my m27c512 is listed as /oe and the prg rom has A16.
Well, I suppose you can just connect the /OE pin to GND. Since the mask ROM doesn't have an /OE line I imagine that output should always be enabled.
Quote:
The m27c512 only has a0-a15 so what do I do in this case?
Well, this means that your chip has only 64KB of memory. I don't think there are many MMC1 games this small. The chip is OK if you want to try your own programs that are 64KB or smaller though, in which case you can just not connect anything to hole 22.
Wow, this actually clears up a major misconception I had. I feel really stupid, but i'm glad I have learned from this.
In hopes of helping someone in the future who might come across this, here is what I was thinking:
I thought when it said M27c512, that it was 512Kb, but instead it was actually 512Kbits. So the 512kbits is only actually 64Kb of actual memory.
Thank you tokumaru for helping me through this misconception i was having.
tokumaru wrote:
I don't think there are many MMC1 games this small.
I can think of at least Tetris (32 KB PRG + 16 KB CHR) and Dr. Mario (32 KB PRG + 32 KB CHR).