> Three month ago I made a simple 32 KB-expansion for someone piggy-backing a
> 62256 (32*8KB SRAM) on top of the BASIC-ROM. A 74LS21 (dual 4-input AND)
> took care of ANDing the RAM and BLK-lines (not BLK5 !).
[..]
> there is interest, I'll make a SCH in GIF and check it with Pasi to see if
> this is what he had in mind.
Not quite, because for a cartridge you need to generate A13 and A14,
because they are not available in the cartridge port. My thought was
to use quad 2-input AND to generate the address bits and the chip
select for the RAM. Okay, I'll sketch the connection here..
_
+---| \
| | )-------------- A13ram
| +-|_/
H | | _
/B1 --o o-+---+-| \
H | | )----+--------- A14ram
/B2 --o o-+-----|_/ | _
H | _ +--| \
/B3 --o o-+-+---| \ | )--- /CSram
H | )-------|_/
/B5 --o o-+-----|_/
VR/W ---o H
o-+--- R/Wram
CR/W ---o
H is a pullup resistor, 100-470kOhm should work fine.
--o o-- is a place for a jumper block or a switch.
You should also add ~100nF capacitor between VDDram and GNDram.
You can disable all blocks separately and write-protect the whole
thing by removing jumpers.
If you use this style, it would be usable for both internal and
external expansion.
-Pasi
--
/His hands almost went to his ears in a strangely human gesture. Like a
boy he looked, almost lost. God, that our immortal bodies could be such
varied prisons for us, that our immortal faces should be such masks for
our true souls./ -- Lestat in "The Vampire Lestat"
-
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