Re: DVG Multigame

> There's half an LS139 that takes two address lines as inputs. There are 4
> outputs - 3 of them go to VROMs, one goes to VRAM. In the "real world" of
> the board, 4000-47ffh is vector RAM, and 4800-5fffh is vector ROM. At the
> basic level, I need to be able to map ROM over the top of the 4800-5fffh
> region but not enable the chip's output when it's at 4000-47ffh. Normally
> three of the outputs of the LS139 go to !OEs - vector ROMs. Somehow I have
> to take those three lines and turn them into one big !OE if any of them
> are low.

That would require a 3-input AND gate. You get 3 of these per chip on a
7410??? 74xx something.... Or you can replace that 139 with some form
of PLD that does the job in one chip (not your desired solution).
I assume if you're replacing the VROMs with a larger one (hence the need
to combine the !OEs) you'll be doing that on a daughter card that
plugs into the ROM sockets. That's an ideal place to put the extra
AND gate, and leave the board untouched.

SeeYa,

-- 
 ___   __   _   _  _
|   \ /  \ | | | || |       phkahler@oakland.edu     Engineer/Programmer
|  _/| || || |_| || |__     " What makes someone care so much?
|_|  |_||_| \___/ |____)      for things another man can just ignore. " -S.H.
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