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?
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...
"Rebuilding" cartriges with 74xx, possible problems?
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1. NO BLATANT PIRACY. This includes reproducing homebrew less than 10 years old, with the exception of free software.
2. No advertising your reproductions, with the exception of free software.
3. Be nice. See RFC 1855 if you aren't sure what this means.
"Rebuilding" cartriges with 74xx, possible problems?
Last edited by Fisher on Mon Jun 19, 2017 3:29 am, edited 1 time in total.
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.
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?
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?
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.
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.
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...
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...