On Tue, 24 Apr 2001 23:52:14 -0700 (PDT), you wrote:
>* I have implemented Gary's 7400 array of gates for the !CE signal to the
>EPROM
Er... this might be your problem, then. That gate array (Joe's two-NOR
arrangement would work as well, BTW) generates a HIGH only when A=0 and
B=1, but I don't think that's what you really want. Remember that CE* is
an active-LOW signal, so regardless of which way you wired the inputs:
A=A15 B=WR* O=CE* A=WR* B=A15 O=CE*
--------------------- ---------------------
0 0 0 0 0 0
0 1 1 0 1 1
1 0 0 1 0 0
1 1 0 1 1 0
...you've got a situation where the EPROM's chip-select signal is LOW
at times you probably don't want it to be. (The same would apply if you're
running the RD* signal to the gates instead of WR*, by the way.) To
chip-select the EPROM, I think you're going to need something more complex,
since you not only want the EPROM to be selected when both A15 and RD* are
low, but you have to stay out of the way of any RAM or memory-mapped I/O
devices in the memory map.
Let me go get some coffee and dig up an ASTEROIDS DELUXE schematic, and
get back to you. :) (BTW - Neil, feel free to contact me directly so we
don't continue to flood the list with pages of ASCII art, unless the list
finds the discussion of interest...?)
-----------------------------------------------------------------------------
"There is no virtue in suffering fools gladly, for it only encourages them to
persist in their foolishness." --Kehlog Albran
-----------------------------------------------------------------------------
solarfox@DON'TMESSWITHtexas.net (Gary Akins jr.)
http://lonestar.texas.net/~solarfox
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Received on Wed Apr 25 09:22:00 2001
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