If many of you recall, I'm working on the great atari development
system/game server. Currently, mainly on the game server.
I am having slight quirks, and maybe someone out there may know why.
The hardware:
Attached to the atari bus(the A+D line out of cartridge socket), is a
screensaver ROM(it is just an atari rom that John Harvey wrote for me),
and the output of a octal 3-state buffer.(I'm using a 74HCT541) The
address lines go both the the screensaver ROM, and into a MUX(3x 74HCT157.
The output of the address mux's and the input of the 3state buffer go to a
couple other roms(and later other stuff). It is configured so that only
the screensaver, or the buffer reacts to the atari's reads(atari A12).
When setup for the screensaver, runs fine. when setup for the other
roms(through the 3state buffer & MUX's), only 'mostly' runs. In this
state, the output of the 3 state is only enabled when the atari reads(and
in this state), and the mux's select the atari address input to be passed
on to the other roms.
What happens:
The ROMs are currently a pile of 'commerial' software. The problem is
that the games run improperly. All games at least start up. Some games
seem to run, and crash later. Others display wierd graphics. For
instance, Adventure, the power up screen looks like this:
#################
#x x#
#x x#
# #
# #
# 1 #
# #
# #
# #
# #
####### #######
where the x's are a patch of boarder that appears and dissapears. It
flickers on and off irratically. Boths sides flicker at exactly the same
time, but it does not appear to be periodic. The patch is exactly the
same width as the edge(2 playfield blocks, I think).
I believe the problem has to do with the 3 state buffer(the other rom's
are 150ns, and should be plenty fast enough). When changed to an ALS,
games by and large behave better, but still crash and have funny graphics
problems. My best guess is that because the atari doesn't give the
cartridge a real read signal, but instead just makes the cart(and hence my
buffer) base it's output on the address, that my buffer happens to either
hold the lines driven too long, or more likely not long enough.
Any ideas about timing?
Any ideas about what else may cause such software actions?
Mark De Smet
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