More RAM for your 1541
What is itAdd 24 KB of SRAM to your 1541 to be used for several purposes.
I needed more RAM for my 1541-II for several reasons. One reason was the 1541IDE project. But lately I decided to rebuild a 1541 as well and to use two extra 6522's. As I have a working EPROM emulator now, I didn't need the 62 KB used for the 1541-II.
The 24 KB SRAMThe 1541 originally only has 2 KB of SRAM onboard. The 24 KB add is done by piggybacking a 32K*8 SRAM like the 57256 on top of the original 2 KB SRAM (most probably a 2016 or 6116):
* pin 3 of the 28 pin 32KB SRAM is soldered to pin 1 of the 24 pins 2KB SRAM. If you aligned the ICs correctly then pin 14 is connected to 12, 25 to 23 and so on. Don't solder the pins 20, 22, 23 and 26 of the 32 KB SRAM.
* Connect pin 1 to pin 24 of the 6502 (A14), 2 to 22 (A12), 26 to 23 (A13) and 23 to 20 (A11).
* Connect pin 28 to pin 24 of the 2 KB SRAM and 27 to 21.
* Take a wire and connect pin 20, 22 and 28 with each other.
The above addition as so doesn't have any influence on the drive. You can check this by turning the drive on and simply use it. If the drive doesn't work anymore, you most probably short-circuit something.
The first 8 KB of memory is mirrored 3 times up to $8000. So the next step is to de-mirror this range. The I/O and original SRAM are selected using a 74LS42. address lines A14 and A13 aren't used in this process and that is what we are going to change by adding a 74LS139.
* Between the 42 and Ucx, a 74LS00, you'll find a line running from pin 10 under the 42 to pin 12. Check this with an Ohmmeter. Double check: this line comes from pin 15, A15, of the 6502. Cut the line between both ICs.
* Piggyback a 139 on top of the 42 and only connect the pins 8, 12 and 16 of both the ICs.
* Connect pin 13 of the 139 to pin 23 of the 6502 (A13), 14 to 24 (A14) and 15 plus 1 to 25 (A15).
* Apply power to the 1541, check if the drive light goes off and use it to test it a bit more.
Now we prepare the electronics to select the 32 KB SRAM.
* Connect pin 12 of the 139 with pin 2 and 3. * Remove the wire connecting pin 20, 22 and 28 with each other. * Connect pin 7 of the 139 with pin 20 and 22 of the 32 KB SRAM. * Apply power to the 1541, check if the drive light goes off and use it to test it a bit more.
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