On Tue, 15 Jun 1999 gentlben@xxxxxxxxxxxx wrote:
> okay, one more question and im done for the day...does anyone out there
> know where i could find schematics for cartridges made for the 2600,
> which show the basic layout of how it accesses the rom hardware-wise?
Well, for a simple 2k or 4k game, it's extremely simple. Address lines A0
through A11 from the 6507 connect directly to the corresponding pins on the
rom.
Remember that for the roms used, the chip enable line (CE) and the output
enable line (OE) are active when they are high. For a standard eprom,
(a 2716, for example) these lines, /CE and /OE are active low.
With this in mind, A12 connects directly to the rom's CE pin. Or, if an eprom
is used, A12 runs through an inverter to connect to the /CE pin. The output
buffers are permanently enabled for both versions (CE = VCC or /CE = GND).
And that's it!
For bank switched games, it's a little more complex. Everything above still
applies. But there are extra address pins on the rom, A12 through A(n).
An address decoder is used to detect an access to a "hot address". When a
hot address is detected, the decoder sets the extra address pins to the
appropriate values, based on the bankswitching method being used.
I can go into more detail on bankswitching if needed...just ask! But everyone
should read Kevin Hortons Bankswitching Document first. It's available in
several places on the 'net.
I hope this helps!
-Chris
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