Having trouble using 27C512 on a TSROM board.

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BrokenYugo
Posts: 7
Joined: Sun Sep 27, 2020 7:19 pm

Having trouble using 27C512 on a TSROM board.

Post by BrokenYugo » Sun Sep 27, 2020 8:26 pm

Donor is Bill Elliot's Nascar Challenge, rom is Loopy's MMC3 hack of SMB2J. Rom was split into 64KB PRG and 16KB CHR, CHR was then quadrupled (and sanity checked in a hex editor to make sure I did it right, first 16KB match, both end the same), PRG and CHR burned to OTP AT27C512R 70ns on a genuine TL-866II plus. Initially I mistakenly soldered them both in directly with chip pin 1 in board hole 3 (see attached pic) as I thought I had read you could do that and hole 30/chip pin 28 (VCC) was tied to the 5 volt rail on both chip positions. This yielded nothing but that green/brown screen you sometimes see when a cart is dirty (but solid instead of blinking), with further thought I lifted the OE# pins of both chips and tied them to ground and got the attached mess. It actually plays (so long as one can without seeing much anyhow), so I think the problem is on the CHR side? Per this page:https://wiki.nesdev.com/w/index.php/Mask_ROM_pinout I think everything's hooked up right but it clearly isn't. It must be something stupid, I could probably track it down myself if I could find a schematic for the TSROM-08 board. Solder job isn't aerospace grade (I'm out of flux, my very old Radio Shack solder is sorta marginal) but it all looks ok, push come to shove I'll reflow it Tuesday when I have more flux. I've made a couple carts successfully before but they were both UNROM.
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IMG_20200927_215232771.jpg
IMG_20200927_211147178.jpg

lidnariq
Posts: 10260
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Having trouble using 27C512 on a TSROM board.

Post by lidnariq » Sun Sep 27, 2020 8:28 pm

You must connect both PPU A13 and PPU /RD to the CHR ROM (/CE and /OE respectively). By tying that pin highlow, you're producing bus conflicts all over the place.
Last edited by lidnariq on Sun Sep 27, 2020 8:54 pm, edited 1 time in total.
Reason: braino

BrokenYugo
Posts: 7
Joined: Sun Sep 27, 2020 7:19 pm

Re: Having trouble using 27C512 on a TSROM board.

Post by BrokenYugo » Sun Sep 27, 2020 8:46 pm

Thanks, bus conflicts are to be avoided, I do know that! So just to be clear remove jumper I made from CHR /OE to ground and connect PPU A13 (cart edge 65) to CHR /CE and PPU /RD (cart edge 21) to CHR /OE? Leave PRG chip as is?

lidnariq
Posts: 10260
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Having trouble using 27C512 on a TSROM board.

Post by lidnariq » Sun Sep 27, 2020 8:54 pm

Yes, that should be correct.

BrokenYugo
Posts: 7
Joined: Sun Sep 27, 2020 7:19 pm

Re: Having trouble using 27C512 on a TSROM board.

Post by BrokenYugo » Mon Sep 28, 2020 10:15 am

Works great now, looking again today at the mask rom pinout wiki page it's obvious what I did wrong. Pic of correct bodge wiring attached for posterity.
Attachments
IMG_20200928_130147513.jpg

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