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 Post subject: how to SNROM to SXROM
PostPosted: Wed Sep 23, 2015 10:10 pm 
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Joined: Mon Jan 23, 2012 11:27 pm
Posts: 141
tried making repros of Dezaemon translated and PR8 using american SNROM carts. they run but seem unable to either write to sram or read back what was written. I followed this guide: http://hackitup.tumblr.com/post/1920173 ... o-sxrom-to

this is how I did the 32K ram:

pin 1 (a14) -> mmc pin 10
pin 20 (/CE) -> 7404 pin 2
pin 26 (a13) -> mmc pin 9

inverter:
pin 1 (1A) -> mmc pin 6
pin 2 (1Y) -> ram pin 20 (see above)
pin 7 -> prg pin 30 for 5v (convenience)
pin 14 -> prg pin 16 for gnd (convenience)


ram pin 1 is bent up so it's isolated. pretty sure I cut traces to ram pins 20 and 26 to isolate them rather than bending due to limited room. I checked for continuity and they should not be connected to the old pins anymore.

prg a18 is not hooked up but I think that's fine for both of the roms I tried, neither is 512K.

anyone see any issues with this?

mmc1 pinout for quick ref: http://wiki.nesdev.com/w/index.php/MMC1_pinout


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PostPosted: Tue Dec 22, 2015 1:02 pm 
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Joined: Mon Jan 23, 2012 11:27 pm
Posts: 141
I worked on this some more but I'm not getting anywhere.

so far:

make a socketed famicom sxrom test cart (final fantasy 1&2), standard prg rewiring for 512K support
put it on a gyromite adapter,
ran holy diver batman test on NES -> everything ok (no surprises)
ran Dezaemon T+Eng on this board, everything appears to work as intended. can edit/save/load ship graphics

took one of my previous NES SNROM conversions and pulled the prg
replaced prg with a socket with full 512K wiring support
ran holy diver batman -> all tests ok (screenshot attached)
did test, power cycle, test to do the battery check
- I didn't read the source, does this test save/load from every ram bank?
ran Dezaemon T+Eng, will not load any edits. Not sure if it fails to save, load, or both.

So, I'm pretty frustrated at this point. Whatever is causing this to not work eludes me. Wires too long causing timing issue? I dunno. Anyone have any ideas? I have another FC sxrom cart coming to disassemble to double check all the wiring for prg/sram/7404 so I don't have to undo my test cart. Can post pics and try to answer any questions anyone is curious about.


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PostPosted: Wed Dec 23, 2015 11:01 am 
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Joined: Mon Jan 23, 2012 11:27 pm
Posts: 141
It looks like the issue is the inverter chips I was using. I had built my conversions with LS series chips, where the original SXROM boards use HC series chips. I borrowed the HC chips from my test board and rewired my conversion and everything seems functional now. Pretty irritated I didn't make that connection before.

Also worth noting is that I moved the 5V connection from PRG to the ram chip and same for GND. I don't know how that would make a difference, but that's how it is on the SXROM boards so that's what I'll go with.


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PostPosted: Wed Dec 23, 2015 12:59 pm 
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Joined: Sun Apr 13, 2008 11:12 am
Posts: 6449
Location: UK (temporarily)
FrankWDoom wrote:
I moved the 5V connection from PRG to the ram chip and same for GND.
What does that mean? The PRG still needs to be connected to +5/gnd, but the PRG RAM "should" be using the battery backup version for SXROM ... I thought ...


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PostPosted: Wed Dec 23, 2015 2:59 pm 
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Joined: Mon Jan 23, 2012 11:27 pm
Posts: 141
You are right on both points.

I was initially tying Vcc and GND pins on the inverter to the respective pins on the PRG chip. In the process of swapping out the LS chip I had been using I also moved the inverter's connections from PRG to the ram chip. I'll double check the original sxrom tonight, but I'm pretty sure that board puts the inverter on the same traces the ram chip uses. Like I said I don't know if or why that makes a difference, but it does look like the inverter could be drawing from the battery.


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