Re: DVG Multigame

On Fri, 10 Mar 2000 03:33:58 -0800 (PST), you wrote:

>> (Personally, if there's already some kind of Memory-Write-Enable signal
>> available, I would just look for A15 to go high while MEMW* goes low, and
>> have that condition generate a positive-going edge to the 'LS377 latch,
>> thusly: <<warning - bad ASCII art ahead! :) >>
>
>There isn't an A15/!MEMW qualifier unfortunately. A15 isn't used at all in
>production.

        The two XOR gates in my diagram form the logic for the A15 & MEMW*
condition we're looking for. Actually, they're both acting as inverters;
A15 is inverted and fed to the '377 latch's G* (active-low enable),
allowing it to count on the next low-to-high transition of CLK. Then,
presumably, when MEMW* goes low, it too gets inverted and fed to the latch,
where it clocks the data on D0-D1 through to the ROM addressing lines.

        There would almost certainly have to be a Memory-Write-Enable
_somewhere_ on that board, and A15 can be picked directly off the CPU if
necessary (I thought you said there was a pad on the PCB for it, though?).

>> I know, it's not as sexy as a PAL, but I'm a great believer in keeping
>> things simple and straightforward. :)
>
>Oh, me too. I just don't know any better. ;-)

        ::tsk:: What _are_ they teaching in the schools these days... :)
Don't get me wrong, PALs _do_ have their uses - especially if board space
is at a premium, or if you're trying to obscure the internal workings of
something to prevent the design from being stolen. I just dislike the
"black box" aspect of them; they make the board that much harder to
maintain and troubleshoot later.

>But how would we handle the 2 to 4 address decoder/enable on the vector
>generator? Two bits going in to select which part of the 1K area the
>vector generator owns and 4 not bits coming out. We need to do a !OE
>on 3 of the 4... wait a sec... an inverter would work here.
>
>Thanks for the * - Lemme go rethink this...

        Hm - looks like I'd better go download a copy of the schematics, so I
have a clearer idea of what you're up to...
-----------------------------------------------------------------------------
BREAK UP MICROSOFT!!
(Preferably with a wrecking ball and some dynamite.)
---------------------------------------------------------------------
solarfox@DON'TMESSWITHtexas.net (Gary Akins jr.)
http://lonestar.texas.net/~solarfox
---------------------------------------------------------------------
---------------------------------------------------------------------------
** To UNSUBSCRIBE from vectorlist, send a message with "UNSUBSCRIBE" in the
** message body to vectorlist-request@synthcom.com. Please direct other
** questions, comments, or problems to neil@synthcom.com.
Received on Fri Mar 10 08:25:49 2000