Atari Stacy

4Mb Memory Upgrade

©Copyright 2000 - Geir Øyvind Vælidalo

The Story Behind...  
A one meg Stacy is, to say the least, practicaly unusable for most tasks these days. At least for me... Therefore I tried, with luck I might add, to upgrade my Stacy to 4Mb. So in case someone is crazy enough to try, this page gives a relatively crude description on how to upgrade an Atari Stacy from 1Mb to 4Mb using a 72-pin PS/2 SIMM.

The description is, as mentioned, rather crude, but even people with mediocre skills, like myself, should without bigger problems be able to go through with this upgrade.

I must mention that the Stacy was donated to me by Jo Even Skarstein :-) I really appreciated that!

Good luck upgrading!

This upgarde isn't actually very complex, but because the memory is such a vital part of the Stacy, it's very important to have a minimum of knowledge about computer architecture, memory and electronics in general. The reason is that with so many wires and solder-points, it's almost bound to be some sort of problems, and without a minimum of understanding, one might give up too easily.
This upgrade also requires some rather basic soldering/desoldering skills and some elementary knowledge about the use of multimeters.

Bottom line: You do this at your own risk!

What you'll need
There are a few things which are nesessary to do this upgrade, and there are a few things that are helpful.

Nesessary thing:

  • One 1Mb Stacy
  • 4Mb 72-pin PS/2 SIMM - FastPage, NOT EDO!!!! (I actually used an 8Mb SIMM)
  • More than 35 wires - approx. 10 cm long
  • Soldering iron
  • Solder
  • Electro-tape
  • Different screwdrivers - to open up the Stacy

Useful things:

  • A 72-pin SIMM socket - Recommended
  • Multimeter
  • Wirecutter

Open up the Stacy
I will not explain to you how to open up the Stacy, partly because that's boring work, and partly because I happen to own a secondhand Stacy which doesn't have all the shielding inside. So there you're at your own.

Remove the screen, the shielding and the keyboard. There's probably no need for you to remove the hard-drive, floppy or hd-controller.

You should now have something like this in front of you:

The memory-/rom-/keyboard-controller-card is the card you have in the front looking like this:

Take out the 3 screws that hold this card and pull the card off the motherboard. It's much easier to work with it that way, as you don't have to drag the whole Stacy around.

Note: This is in fact a 2/4Mb card. I had to borrow this picture from Jo Even.

Disable the original ram
This part is rather tricky! You have to disable the original ram. It's located on the bottom-side of the memory-card. You can see the J3-connector at the top of this picture, and the row of sixteen 33 Ohm resistors below that. The original ram-chips are the eight 20-pin chips you can see.
I simply desoldered all of the eight ram-chip, but that's not very advisable unless you have nerves of steel. You should definitively try to find other ways of disabling the ram. I ruined at least ten traces during this massacre, none of which are unrepairable, but it would cost some considerable amount of work to put them back again. There could be solutions which simply involves some trace-cuts or similiar, but I chose to simply remove them all ;-)

A good place to start looking for alternative solutions would be this place:

PS/2 SIMM <-> Memorycard - The connections
These are the connections that have to be done in order to get this thing up and working. Actually not that scary, or what? J3 is located at the left side of the memory-card (See red marking)
NOTE: The pins are numbered this way, when seen from the solderpoint-side of J3:

SIMM   Memory card
Pin Code Description Code Pin J3
1 VSS Ground GND 49
2-3 DQ0 & 16 Data 0 D0 31*
4-5 DQ1 & 17 Data 1 D1 32*
6-7 DQ2 & 18 Data 2 D2 33*
8-9 DQ3 & 19 Data 3 D3 34*
10 VCC +5 VDC VCC 1
12 A0 Address 0 MAD 0 15
13 A1 Address 1 MAD 1 16
14 A2 Address 2 MAD 2 17
15 A3 Address 3 MAD 3 18
16 A4 Address 4 MAD 4 19
17 A5 Address 5 MAD 5 20
18 A6 Address 6 MAD 6 21
20-21 DQ4 & 20 Data 4 D4 35*
22-23 DQ5 & 21 Data 5 D5 36*
24-25 DQ6 & 22 Data 6 D6 37*
26-27 DQ7 & 23 Data 7 D7 38*
28 A7 Address 7 MAD 7 22
31 A8 Address 8 MAD 8 23
32 A9 Address 9 MAD 9 24
34 !RAS2 Row Address Strobe 2 !RAS1 29
40 !CAS0 Column Address Strobe 0 !CAS0L 9
41 !CAS2 Column Address Strobe 2 !CAS1L 27
42 !CAS3 Column Address Strobe 3 !CAS1H 28
43 !CAS1 Column Address Strobe 1 !CAS0H 10
44 !RAS0 Row Address Strobe 0 !RAS0 11
47 !WE Write Enable !WE 12
49-50 DQ8 & 24 Data 8 D8 39*
51-52 DQ9 & 25 Data 9 D9 40*
53-54 DQ10 & 26 Data 10 D10 41*
55-56 DQ11 & 27 Data 11 D11 42*
57-58 DQ12 & 28 Data 12 D12 43*
60-61 DQ29 & 13 Data 13 D13 44*
62-63 DQ30 & 14 Data 14 D14 45*
64-65 DQ31& 15 Data 15 D15 46*

* The 16 data lines must not be soldered directly on to the J3, but instead they should be soldered on to the other end of the sixteen 33 Ohm resistors on the memorycard (See photo).

You could use a multimeter to find the corresponding resistor for the given pin on J3, but I think it's pretty forward to follow the picture. D0 is to be connected to the rightmost resistor, and D15 to the leftmost. You must of course solder on the resistor-end closest to you when holding the card like on the photo. The reason is that there must be a 33 Ohm resistance between the given pin on J3 and the given pin on the SIMM.
The pinout of the SIMM is pretty straightforward. All SIMMs have a tag on one of the sides which tells where pin 1 is.
I strongly advise you to use a SIMM-socket. That way you can easily replace and test your SIMMs! I first tried without a socket, and it turned out that the SIMM I used was uncompatible (most likely EDO)! I then sawed a SIMM-socket from an old 486-motherboard and used that instead. That way I could test all my SIMMs, and it turned out that approx. 75% of all the SIMMs I had where EDO! It's a pity it's so hard to tell the difference between a FastPage-SIMM and an EDO-SIMM...

The finished product
This is the finished module:

Nice, isn't it?

Well... Remember to tape the module good to prevent short circuiting the machine! Put the Stacy back again... The module should fit nicely between the memory-card and the PSU-card. Turn the Stacy on and hope for the best!
You should now have a working 4Mb Stacy!

These are the most likely causes for trouble:
  • Wrong type of SIMM
  • Broken SIMM
  • Bad soldering
  • too little GND or +5V to SIMM
  • Bad assembly of Stacy
  • Switched wires
  • Soldered to the wrong pin

So what can you do?

  • Check that you have a correct SIMM-type, and that it actually works. Use your imagination on this one.
  • Check the connections for bad soldering and short-circuits.
  • Check if you've soldered to the correct pins.
  • Question my information
The data lines can be found at the Shifter-chip (See picture). Pin 3-10 is D0-D7 and pin 12-19 is D8-15. Check that these corresponds with the data pins on the SIMM. Remember that there should be a 33 Ohm resistanse between these points!
The address lines can be found at the MMU (C025912-38 - See picture). Pin 59-54 is MAD0-MAD5 and pin 60,62,63,64(or whas it 66?) is MAD6-MAD9. Check that these corresponds with the address pins on the SIMM. Remember that there should be a 33 Ohm resistanse on these lines too!

The write enable line can be found at pin 23 on the MMU. Check this pin against pin 47 on the SIMM. Again 33 Ohm.
The CAS0L, CAS0H, CAS1L, CAS1H, RAS0 and RAS1 originates from pin 7,6,21,22,8 and 18 on the MMU, but goes through a bus driver-chip (74LS244 or similar) located nearby the MMU (See picture). The signal from this chip is fed through six 33 Ohm resistors before they reaches J3 (a.k.a J25 on the motherboard).
74LC244 pinout:

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