EPROM Chip Question

[Dr. Morbis | 2,088]

I don't know if this is the right section for this topic, but here goes:

So I'm making myself a Lan Master cart since I'm a puzzle game guy and I really dig the presentation of this particular game.  It's a 40k ROM which I split into a 32k PRG file and an 8k CHR file.  It's a standard NROM game, so simple stuff, but my question is, when I go to burn these files, do I have to use two 27C256 chips or can I use one of those and one 2764 since the second chip only needs to hold 8k?  I just don't know if an NES needs to have both chips be the same type or if it doesn't care.  If I had room in a NES cart to throw in two sockets so I could simply test and re-burn if necessary, I would, but sadly that's not an option.

Any help in answering this question is greatly appreciated.

[Deadeye | 1,596]

Maybe @SNESNESCUBE64 knows

[SNESNESCUBE64 | 539]

I believe using a 2764 would work. Personally, I use whatever I have on hand. The nintendo doesn't care how big the chip's capacity is, just so long as it can address what it needs. That said, they are both super common and super cheap. 

What will NOT work are the cheaper and easier to program 28c256/28c64 EEPROMs, they are not pin compatible.

Edited November 12, 2020 by SNESNESCUBE64

[Dr. Morbis | 2,088]

Thanks for the heads up.  I also want to use what I've got on hand rather than pay shipping for a cheap chip and then having to wait for it to show up, and since I collect arcade games, I've got tons of old chips lying around, of which one is a 2764 EPROM from a Mr. Do! board.  So I'll give it a try and solder it in and keep my fingers crossed!

Thanks again for the reply.

[guitarzombie | 924]

Yeah it'll work with either a 32k or 8k.  Once your cross that 64k threshold, then the socket size gets too big and wiring is necessary.  So at the 28 pin chip level, anything'll work you just have to fill it and drop it in.  Of course as long as its the JEDEC standard pinout, which is mostly the standard for a majority of the eproms.

*EDIT* Its been a long time but I just wanted to add that I think even a 28 pinout chip will still work fine in a 32 pinout slot without wiring, as long as the chip is set correctly towards the back of the slot.

Edited November 19, 2020 by guitarzombie

[Dr. Morbis | 2,088]

Just for the sake of closure, yeah, the game did work flawlessly, but my burner (TL866II Plus) only goes up to 18 volts for the programming voltage, and the chip I was using was supposed to get 21 volts, so I had to re-burn it over and over for like an hour before I finally got a 100% match.  So that's one more thing to pay attention to next time I pick up chips.

But in the end it did work, so all's well that ends well

[guitarzombie | 924]

To add one thing in here, you can 'over erase' your chips, if they're left too long in the UV light.  I had an issue with a chip like that.  It'll burn and verify 100% but when you put it in, didn't work.  Forgot how that works but i was pulling my hair out for like a week.

[TylerBarnes | 184]

Yeah, it is better to use the correct capacity PROM for the mapper you're using.

However, it should also be noted that you can still use larger capacity chips in place of a smaller one, but you generally have to copy your data to fill out the whole chip. Generally you just do this in your batch file when compiling with the copy command and the /b argument to designate you are concatenating a binary file. 

You can do this like this directly in the command prompt, but in a build .bat is better. 
 

C:\folder>copy /b input_file.chr+input_file.chr output_file.chr