Roland TR909 Quad Mod

YouTube Uploaded by sequentix on Nov 30, 2010

"Demo of a TR909 with a quad memory expansion.
I fitted a 32k FRAM chip in place of the original 8k SRAM, so no battery is needed either.
A rotary selector switch chooses which of four banks of 8k the 909 sees.
I was worried it might crash if the switch was moved without powering-off, but as it turns out, you can change bank mid-pattern and it doesn't miss a beat.

NB Modifying a 909 CPU board is not recommended unless you know exactly what you're doing. The RAM and ROM chips are soldered to the PCB, which has very delicate tracks and through hole plating. There is no space to use a socket.
You can easily damage the board.
If you *do* know what you're doing, a 32k SRAM will fit in an 8k's place with minor changes. The rotary switch selects the 4 possible combinations of the 2 extra address lines with the help of some diodes and pull-up resistors.
FRAM needs CE toggled on every access, which the 909 circuit doesn't do. You need to hack the ALE line into a spare select line on the address decoder to make it work. A 32k battery-backed SRAM wouldn't need that."