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"Rebuilding" cartriges with 74xx, possible problems?
http://forums.nesdev.com/viewtopic.php?f=28&t=16112
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Author:  Fisher [ Sun Jun 18, 2017 6:52 pm ]
Post subject:  "Rebuilding" cartriges with 74xx, possible problems?

Hi.
I just got some pirate and dead SNES cartridges that I'm rebuilding to myself.
Could the use of 74xx, in the case a 7400, become a problem in the future?
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Most of the time I find the epoxy ROM IC to be the problem.
Is it normal?
Maybe I'm not testing the support circuit properly...

Author:  lidnariq [ Sun Jun 18, 2017 7:00 pm ]
Post subject:  Re: "Rebuilding" cartriges with 74xx, possible problem

I would arbitrarily guess that the 74'00 is there serving as a demultiplexer, enabling one chip-on-board ROM or the other depending on cart /ROMSEL and probably A20 or A21.

Author:  Fisher [ Sun Jun 18, 2017 8:19 pm ]
Post subject:  Re: "Rebuilding" cartriges with 74xx, possible problem

Exactly that!
No problem in using a non ls/hc part?
I'm also a little confused with the demux, since I got a mixed bag, with some games with it on /RD, others on /Cart...
Even the ROM control pins plugged on the demux sometimes are /CE, others /OE...
What's the correct way of doing it?

Author:  lidnariq [ Sun Jun 18, 2017 8:29 pm ]
Post subject:  Re: "Rebuilding" cartriges with 74xx, possible problem

Each ROM already has two enables. They're usually called /CE (which "should" be connected to /ROMSEL) and /OE (which "should" be connected to /RD) but plenty of games, even from Nintendo themselves, swap them.

The ROM consumes power/generates heat, but doesn't do anything else, if only its /CE is low. It only drives the data bus if both /CE and /OE are low.

So it's not really any less functional to demux /RD depending on an extra address line.

Conceptually, however, the /ROMSEL signal is generated from A15 and A17..A23, because that's the "who" signal and /RD and /WR are the "doing what" signal. So it's conceptually clearer to demux /ROMSEL.

Author:  Fisher [ Sun Jun 18, 2017 8:46 pm ]
Post subject:  Re: "Rebuilding" cartriges with 74xx, possible problem

Thanks!
I've been "fighting" for weeks with a board I'm trying to rebuild with an 74ls139 and 4 ROMs.
I'll post the results as soon as I got something new.
This one was simpler. The demux was behaving weird, getting the outputs activated ramdomly. Looks like I was right about the problem...

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