A 2.5MByte RAM Expansion for the Atari ST by Christopher Hicks
This document details the addition of an extra 2MBytes of RAM to
a standard 520/1040 STF/STFM. The motherboard I upgraded is
marked C070789-001 REV D (1040 STF), but this procedure should be
OK for other revisions, except that the IC numbers may be
The existing RAM consisted of thirty-two 256K by 1 DRAMs, mounted
under the power supply. This procedure replaces sixteen of these
with two 1MByte SIPs, giving a total of 2.5MBytes.
Although this procedure worked perfectly for me, I cannot
guarantee that anyone else can perform this upgrade without
damaging their computer. I therefore disclaim any responsibilty
for any damage that may occur as a result of attempting this
upgrade. It will also void any warranty on your computer.
On a more positive note, there is no reason why someone who is
experienced in wielding a soldering iron, and has done some
electronic construction/troubleshooting, should not be able to
perform this upgrade successfully.
The ST's memory management unit (MMU) can support two banks of
RAM (upper and lower). These may each contain 512KBytes or
2MBytes. In addition the upper bank may be empty. This arrange-
ment gives rise to the following memory configurations:
Lower Bank Upper Bank Total
========== ========== =====
1. 512 kB --- 512 kB
2. 512 kB 512 kB 1 MB
3. 2 MB --- 2 MB
4. 2 MB 512 kB 2.5 MB
5. 512 kB 2 MB 2.5 MB
6. 2 MB 2 MB 4 MB
This upgrade converts a machine from configuration 1 or 2 to
configuration 5. An extra address line is required for the 2 MB
bank. This is obtained from the MMU (U56) pin 64.
Device Pin Diagrams
1. 256kbit DRAM
A8 |1 16| GND
D |2 15| CAS
WE |3 14| Q
RAS |4 13| A6
A0 |5 12| A3
A2 |6 11| A4
A1 |7 10| A5
Vcc |8 9| A7
2. 1MByte SIP
1 Vcc 11 A4 21 WE
2 CAS 12 A5 22 GND
3 DQ0 13 DQ3 23 DQ6
4 A0 14 A6 24 NC
5 A1 15 A7 25 DQ7
6 DQ1 16 DQ4 26 Q8
7 A2 17 A8 27 RAS
8 A3 18 A9 28 CAS8
9 GND 19 NC 29 D8
10 DQ2 20 DQ5 30 Vcc
NB Pins 26, 28 and 29 refer to the ninth bit required for
IBM PC systems. This bit is not required on the ST upgrade.
| Chips mounted this side |
A0 - A9 Address inputs (row/column multiplexed)
CAS Column address strobe
RAS Row address strobe
D Data input
Q Data output
DQ0 - DQ7 Data combined inputs/outputs
WE Write Enable input
Vcc Power (+5V)
GND Ground (0V)
NC Not connected
1. Two 1M x 8, or 1M x 9 SIPs, 120ns or faster
2. Two 16 pin and two 14 pin IC sockets
3. A small piece of veroboard (big enough to mount the two SIPs)
4. A few dozen short bits of wire (a shredded scrap of ribbon
cable is ideal - lots of colours for easy identification!)
Total cost - about 80 pounds including VAT!
The motherboard and SIPs may be damaged by static electricity, so
don't work on this project wearing synthetic clothes and rubber
shoes on a nylon carpet etc... Seriously though, no damage
should occur if you are reasonably careful (use a grounded
soldering iron, and don't work on a highly waxed or polished
surface, for example).
1. Remove the motherboard from the computer, and identify the
RAM chips (U 3>6, 10>13, 18>25, 27>30, 34>37, 39>42, 44>47
on my 1040STF). On a 520 only half of these will be present,
and step 2 may be skipped.
2. Desolder and remove the upper bank of DRAMs. These are
scrap, so don't worry about damaging them, but do take care
not to damage any tracks on the motherboard. On my 1040
these were the right-hand 16 chips (nearest the FDD), though
I know that on some boards (marked REV 4 ?) it is the 16
nearest the rear of the computer. (The way to tell if you
are unsure is that all the pin 4s of each bank are connected
together, but the two banks are separate.)
When these DRAMs have been removed, the computer should work
as a 520ST. If it doesn't, the safest recourse is to buy
thirty-two 16-pin IC sockets, and sixteen 256k x 1 bit
DRAMS. Remove all the DRAMS from the motherboard, and
replace them with sockets. Then fit one of the new DRAMS in
each of sixteen sockets, and move them until you find a
combination that works. You then have a 512k lower bank, and
an empty upper bank as required.
3. Mount the four IC sockets on the veroboard, end to end, and
butted up together, so as to make two 30-pin SIP sockets.
The connections on these will, from now on, be refered to as
SIP1 pins 1 thru 30, and SIP2 pins 1 thru 30.
4. Break the veroboard tracks between the following pins on
SIPs 1 and 2: 2,3,6,10,13,16,20,23,25,26. This operation
separates the data and CAS lines on the two SIPs.
5. Connect together the following veroboard tracks: 22, 28, 29.
This disables bit 8 if present, leaving bits 0>7 for the ST.
6. Decide where the expansion board is to go. Mine is in the
space normally occupied by the TV modulator and associated
circuitry. Bear in mind that leads to the board should be no
more than about 6 inches long to avoid capacitance problems.
7. Back to the motherboard... Using a continuity tester you
should find that the CAS lines (pin 15) of the removed DRAMs
are connected together in two groups of eight. Take a short
wire from one of these groups of pin 15s to SIP1 pin 2, and
another from the other group of pin 15s to SIP2 pin 2.
8. Take wires from the pin 2s of the first group of eight DRAMs
to pins 3, 6, 10, 13, 16, 20, 23, 25 on SIP1.
Similarly connect the pin 2s of the second group of DRAMs to
pins 3, 6, 10, 13, 16, 20, 23, 25 on SIP2.
The databus is now connected up, and the complicated bit is
over. Note that it is essential that CAS (pin 15) and D
(pin 2) of each DRAM are taken to the same SIP.
9. Now wire A0 > A8, WE, Vcc, GND and RAS from the SIP board to
the similarly named pin of any of the vacated DRAM slots.
10. Lastly wire pin 18 of the SIP board to pin 64 of the MMU
(U56 on my board). To aid identification, the part no of my
MMU is C025912-38. Pin numbers are marked on the board.
11. Make sure there are no shorts, and check all the
connections. Insulate the back of the SIP board (eg by
taping a piece of card over it) and insert the SIPs.
12. Reassemble the computer and hey presto - a 2.5Meg ST.
13. Give a copy of this documentation to anyone you please, for
them to try the same upgrade.
14. When you run out of memory again, repeat exactly the same
procedure, replacing the other sixteen DRAMS, and you will
find that you have a 4 Meg ST - what could be simpler?
If you make use of this upgrade, please drop me a line at my
email address, and I will send you details of any other hardware
hacks I succeed with! Good luck...
P.S. I don't have transatlantic e-mail facility. So, if sent from the other
side of the Atlantic, it is most likely that I shall not be recieving it.
However, if you have any sugestion or criticism, please send to the address
M.A.Rahin@lut.ac.uk, who will redirect to me.
| The Final Value Theorem:
Christopher Hicks firstname.lastname@example.org | The answer you get is
email@example.com | always wrong...
Marko, Suomen Atari-sivut / ArkiSTo 2003