8K RAM expansion for the 1541

Important note

This modification requires advanced soldering skills. Use it at your own risk! And, of course, it voids your 1541 warranty (guess you don't have it anyway, but now you can't say I didn't tell you ;-))

What you need

1 4364, 6264 or similar 8x8K SRAM (IC1)
1 74LS42 BCD-to-decimal decoder (IC2)
2 40-pin DIL precision sockets (highly recommended)
1 28-pin DIL socket (recommended)
1 16-pin DIL socket (optional)
A small piece of pre-drilled circuit board (5x7cm)
A good soldering iron/station
Some wire. Important: Try to keep the wires as short as possible (better shorter than on the pictures below!) to maintain reasonable signal quality.

What will the drive look like afterwards?

(Click to see 760x576 images, approx. 128K each)

Top view

PCB# 251830, Rev A. With XP1541 cable, 8/9 switch (blue and yellow cable on the left) and 8K RAM expansion board.

6502 sandwich

There are 26 lines to be connected to UC4 (6502) (8 data bus, 16 address bus, +5V, 0V). These lines are soldered to the upper 40-pin precision DIL socket's pins. This socket is then plugged into another socket. It's important to press them hard together so that each pin has good contact. (The colored cables in the background have nothing to do with this expansion, they belong to the XP1541 cable)

RAM expansion board

With 4364 and the 74LS42.

How it works

The additional 8K memory will be mapped at $8000-$9FFF. The stock 1541 mirrors the $C000-$DFFF ROM here, so the RAM expansion must provide the *CS (chip select) signal for UB3 (pin 20). Though not really necessary, it also creates *CS for UB4 ($E000-$FFFF ROM) because

there's no extra cost for it
it cleans up the memory map
it make the addition of another 8K expansion at $A000-$BFFF very easy

All needed signals (D0-7, A0-15, +5V, GND) are grabbed from UC4 (6502), except R/*W, which is taken from UB2 (2016). IC2 (74LS42) divides the upper 32K address space of the 1541 into four 8K sized blocks:

$8000-$9FFF IC1, 8K RAM expansion

$A000-$BFFF unmapped (future expansion)

$C000-$DFFF UB3, ROM0

$E000-$FFFF UB4, ROM1

Using the RAM expansion

I'm not aware of any software except Maverick that makes use of the additional RAM. However, I hope that there will be a .g64 ripper for cbm4linux soon.

Schematics download

*** schematic bug (2002-06-01) *** (finally fixed 2004-05-20)
The VCC label at UC4, pin 1 (processor socket) is incorrect. A convenient and correct place to take VCC from is pin 8 of UC4.

8k-expansion.png (31 KB)
8k-expansion.ps.gz (33 KB)
8k-expansion.sch (19 KB). This format is used by the EAGLE layout editor, Copyright CadSoft (there's also a freeware-license for non-profit projects).

Feedback

Please contact me if you have any corrections (can't say I'm an electronics expert ;-)), suggestions or, most important, software for this piece of silicon. Thanks in advance.

Page maintained by Michael Klein, last updated Thu May 20 11:18:42 CEST 2004

$8000-$9FFF	`IC1`, 8K RAM expansion
$A000-$BFFF	unmapped (future expansion)
$C000-$DFFF	`UB3`, ROM0
$E000-$FFFF	`UB4`, ROM1