1MB SRAM based memory extension for ATARI XL/XE
Tools required:
- Fine tip soldering station (60W)
- solder pump
- solder wick
- 2m of 28AWG wire (kynar)
- few centimeters of thicker wire
- xtaco knife
- JEDEC file for GAL and EPROM content
- a lot of patience
Start with desoldering EPROM for OS (U5), and BASIC ROM (U4)
remove U34, and second bank of DRAM (if present) (for 130XE) or three 0 ohm bridges occupying its socket.
Cut tacks marked RED on figure 1, and drill 9 holes (BLUE) (or one bigger).
(I've used a template made from piece of prototype board)
Separate pins 1, 28 and 27 form each other on EEPROM, and solder in OS
socket
Fig. 1 - cut marked tracks with xtaco knife on top layer
Fig. 2 - cut marked tracks with xtaco knife on bottom layer
Cut PB7 PB1 near MMU, and OS signal on bottom side of pcb (figure 2)
solder in memory adaptor pcb and the GAL socket, and populate the board
Rewire EXTMMU socket according to figure 3, plug in GAL, and turn on
computer - if everything is ok, it will come up showing basic or QMEG
Fig. 3 - wiring diagram for EXTMMU
Fig. 4 - wiring diagram for SRAM
At this time you have to wire 10 more connections as shown at figure 4. At this point it won't hurt to add shotky diode on EXTSEL line, to restore open-collector nature of this signal
When completed, your newly installed 1MB extension is ready for test run.
First - put it in compatibility mode (pin 11 of GAL connected to 5V) and try
to load bomb jack, getris, or crownland - if this runs, you may set
extension to mode 0 and test full 1MB.
Word of advice - kynar isolation is very prone to high temperature - it melts pretty easly and since there is many connections done with this wire crossing each other you may find that some of them have compromised isolators and are s/c or - even worse intermediate results will pop up on xram test by satantronic (hang, all banks dead, random content on screen)
Now its time to congratulate yourself
Enjoy!
Fig. 5 - finished extension
