A20 to 74hc00 not working for 4mb donor on 2mb board? lost..

For anything related to cart (SRAM, SF2 mapper, audio, CD mode 1, ...)

Moderator: BigEvilCorporation

Post Reply
DeV78
Newbie
Posts: 5
Joined: Wed May 30, 2012 12:19 am

A20 to 74hc00 not working for 4mb donor on 2mb board? lost..

Post by DeV78 » Wed May 30, 2012 1:15 am

People of SpritesMind, I come to you in search of knowledge pertaining to Genesis reproductions. First off, I don't sell them and will be glad to share my ebay ID if it's a concern. I'm a member of several other forums including Sega Age and EmuParadise so fret not. Second, I'm brand new so if this is posted in the wrong area I'm very sorry.

My situation :

I've heard from several repro makers that if you want to drop a 4mb game in a 2mb donor with sram you have to connect the 32nd pin aka A20 of the 27c322 to the 5th leg of the 74hc00 in order for it to save. From what I can tell that's not the case for Duke Nukem 3D or at least with the donors I'm using. I'm using run of the mill donors like NFL95, NFL94, Pebble Beach Golf, College championship football 2, etc. They work with every 27c160 eprom and game I've thrown on them but they don't support 4mb games with s-ram or not. Even homebrew games like Rick Dangerous just don't want to work on those boards, even if I scratch b9 out and run A20 to it directly it still fails on those boards for some reason. I have had small successes but nothing 100% bug free.

Here are my findings, I would have more but documentation seems to be my weak point. This is out of memory so bare with me here.

The tests :

Donor : NFL95
Status : With A2 soldered to the 5th leg of the 74hc00 (at IC4) the game would not boot. The other 2 vital address pins were wired to b7 and b8 accordingly. If Pin 11 was lifted and connected to B18 the game would boot depending on the connection from A20 to the 74hc00. Game would always load jumbled and broken. A20 snapped, next eprom test.

Donor : NFL95
Status : A20 to 5th leg of 74hc00(at IC4) and wired directly to b9 with original traces cut. Pin 11 to B18, no need to cut trace as it's unused. Game loads dependent on what leg of 74hc00 is attached, some legs yield perfect video but ruined audio, some don't do anything but black screen. I ran another wire to the 74hc00 (at IC5) and had less glitches depending on the wiring. No mix of connections will properly save or boot 100% glitch free. I noticed that in cases the graphics were jumbled that the sound was fine. Game would play as intended even if you couldn't see the menu / HUD. Eventually my connection from b18 and pin 11 broke and the game was untestable, would boot but graphics were now jumbled when they previously weren't.

Donor : NBA Showdown 94 (EA)
Status : A20 to 5th leg of 74hc00 (only one on this board), game would not boot. Lifted pin 11, ran to b18, would boot but jumbled. Music sounds fine, graphics are a total mess from boot. Jumbled graphics were not static, if I removed the cartridge and tried it again the jumble would change. Some starts looked much cleaner than others but always broken.



Now, I want to acknowledge something first. I know Duke doesn't have sram info in the header but I've seen the PCB and it's sram w/ a CR32 battery. I have a picture, be it crappy, but you can get the IC information off of it. IIRC it's a 256K sram. It's also got 2 mask roms and no 74hc00 from what I can see. I can only see about 80% of one side of the board so it's not to say there isn't a 74hc00 but I didn't see it. I also know that the everdrive doesn't support Duke Nukem saves for whatever reason, if it did I'd just buy one.

I full expect to hear that it won't work, but I'd really like an explanation why. If you can give that, then talk all the smack you want about the game at least I'll have some closure.

Chilly Willy
Very interested
Posts: 2586
Joined: Fri Aug 17, 2007 9:33 pm

Post by Chilly Willy » Wed May 30, 2012 4:07 am

There are two images of D3D floating around - the original, which has the save ram info in the header (this is also the most common image found everywhere), and a second hacked image that was designed for use on repurposed carts without save ram, and hence has no save ram info in the header along with hacked save code.

If you want saving, use the image with the save info in the header. If the Everdrive still doesn't recognize it, go to the options and force the save on. The NeoFlash MD Myth handles the D3D save automatically.

User avatar
TmEE co.(TM)
Very interested
Posts: 2307
Joined: Tue Dec 05, 2006 1:37 pm
Location: Estonia, Rapla City
Contact:

Post by TmEE co.(TM) » Wed May 30, 2012 8:28 am

You cannot use existing logic on on 4MB chips on below 2MB boards... as soon as game accesses upper 2MB then SRAM is enabled and game will crash. Such games have a mapper that allows to enable SRAM or ROM in the upper 2 MBytes.
Mida sa loed ? Nagunii aru ei saa ;)
http://www.tmeeco.eu
Files of all broken links and images of mine are found here : http://www.tmeeco.eu/FileDen

DeV78
Newbie
Posts: 5
Joined: Wed May 30, 2012 12:19 am

Post by DeV78 » Thu May 31, 2012 4:00 pm

TmEE co.(TM),

Thank you for your reply. I very much appreciate it.

Now that it's established that 2mb boards just won't work, are there in fact 3-4mb donor games that will support this? I know Phantasy Star 4 is a 3mb but it's pricey, would those hypothetically work -or- is Duke Nukem's mapper that unique?

User avatar
TmEE co.(TM)
Very interested
Posts: 2307
Joined: Tue Dec 05, 2006 1:37 pm
Location: Estonia, Rapla City
Contact:

Post by TmEE co.(TM) » Thu May 31, 2012 4:05 pm

You could use any game larger than 2MB. The mapper is not unique.
The mapper is simpler if game has split ROM e.g 2+2MB. Making single 4MB work is more hassle.

http://www.fileden.com/files/2008/4/21/ ... eFront.jpg
http://www.fileden.com/files/2008/4/21/ ... keBack.jpg
Mida sa loed ? Nagunii aru ei saa ;)
http://www.tmeeco.eu
Files of all broken links and images of mine are found here : http://www.tmeeco.eu/FileDen

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest