exxos's Atari, Amiga & retro forum

Can someone point to the best solution for a 4 meg upgrade on a Revision D 512 K 520 ST motherboard?

I have a pro working on this one , he is willing to spend 4 - 8 hours on my behalf to do a 4 meg upgrade on a Revision D Atari STFM motherboard, just need pointing in the right direction. He has lots of different Dram modules available, can take the time to do the research but is also relying on me finding out information on what to do and what not to do, I thought I would ask you Exxos, as I am NOT doing any or it other than holding the motherboard for use of a heat gun for re-flow operations. What I need is a chip part number to look for in his vast collection, we need 32 of them. I don't want to upgrade to 1 meg as i already have a 1 meg system that is perfect and I don't intend to touch it at all, new case , etc , new keyboard pins in a header strip, and the back of the 520 case drilled properly so that the TV switch and TV connector will fit properly. (TV switch and TV Output were backwards on the new case), the new hole was so professionally done you can't tell which is the original or the drilled hole.

Here is a picture of the back of my new (old) case , to replace the shattered case from a 6 foot fall (from a shelf in my closet, bottom case shattered beyond repair, top case OK).
Anyways any help from either Exxos or Rubber Johnny would be appreciated, or anyone else.

Russ

Ok, so I have said my piece on FB too, but I'll add it here as well for a bit more permanence.

It isn't as simple to do this as you think.

I think you would be better off just upgrading to 1MB by adding another 512K RAM to the empty banks. The reasons why I will detail below:

1. You probably won't find much software that requires more than 1MB for the ST, plus if you can get your STE sorted then you will have a machine that can be asily upgraded with SIMM/SIPP modules to 4MB anyway.

2. Personally, I haven't attempted to do a chip only 4MB upgrade, only to 1MB (Successfully, at least twice, I might add!) but you would need to have replacement chips that are completely pin and voltage compatible with what's in there, which will be 41256 chips or equivalent. You'd be better off getting a plug in MMU type upgrade (Like this:https://www.exxosforum.co.uk/atari/store2/#0035 and having it installed by your friend if you really feel the need to go to 4MB. It's safer and more reliable.

3. If it doesn't work, you will risk killing another machine, which is a bad thing.

To explain a little, the RAM chips installed to get 512K into a 520ST/F/M are generally marked 41256. what is important is the 1256 part.

This means that they are 256k (262,144-Bit) x 1-bit. Since 8 bits makes 1 byte, then you need 8 for a memory bank that is 256KB x 8-bits (1 byte). Therefore to get an ST with 512KB x 8-bits (1 byte) you need 16. Double that to 32 and you have 1MB x 8-bits (1 byte).

Therefore in order to take the machine to 4MB using 32 chips, they would need to have 4 x the capacity i.e. 256k (262,144-Bit) x 4-bit.

Doing a little quick research, these are available, but the all of the packages I found, generally using the nomenclature 514256 or similar, come in a 20 pin package and not 16 pin as the originally fitted chips do, so they physically wouldn't fit anyway. Not only that, but the pin assignments are different too.

Therefore, unless you can find 4 bit RAM chips that meet the following criteria:

1. Are a 16 pin DIP package.
2. Are FULLY pin compatible with the existing chips.
3. Are also 5v packages, and not 3V.

Then this will not work and you will need to go back to the starting point and either put the old RAM back in and add in another 512KB in the bank that is currently empty, or leave the old RAM out and add an MMU type memory adapter.

I've attached a 41256 datasheet which shows the pinouts clearly. You'll need to remove the .txt from the end after downloading, as it's actually a PDF file.

FYI the de-coupling capacitors will likely be something like this: https://www.ebay.co.uk/itm/50-Pcs-Min-D ... 212d52649c, so .1uF.

Two final points. One, if you're adding RAM in the empty spaces, use sockets (I think you said you were, but just in case you didn't), and two, if you were considering doing a piggy-back RAM expansion,don't! I had to recover an ST from one of these recently, they are fiddly and open to problems, better off to get an MMU expansion instead!

As RJ says https://www.exxosforum.co.uk/atari/store2/#0035 is the best solutions as it only needs work doing to the shifter area mostly.

Though just to add some notes about adding more DRAM to the ST, a lot of ST's have failed going that route, I don't know why. But the ST PSU isn't to great in supplying power hungry DRAM banks. My MMU kit is a lot lower power and will work better than trying to add banks of DRAM to the ST. The old DRAM will need to be removed also if fitting my kit.

exxos wrote: ↑Fri Jan 25, 2019 11:06 am As RJ says https://www.exxosforum.co.uk/atari/store2/#0035 is the best solutions as it only needs work doing to the shifter area mostly.

Though just to add some notes about adding more DRAM to the ST, a lot of ST's have failed going that route, I don't know why. But the ST PSU isn't to great in supplying power hungry DRAM banks. My MMU kit is a lot lower power and will work better than trying to add banks of DRAM to the ST. The old DRAM will need to be removed also if fitting my kit.

To add to this, some of the machines I've upgraded to 1MB, there has been a problem with the traces, in that though they look physically connected, even when checking with a magnifying glass close up, some of them don't actually connect up properly even with brand new sockets soldered in. On one board I buzzed out the lines and found that the A7 line was missing between two chips, and had to bridge it with a bodge wire to get it running.

In short, you may have problems just adding an extra 256KB RAM, let alone what you're trying to do.

BTW, I have a conspiracy theory that Atari tested all the mainboard traces, and on the ones where bank 3 & 4 didn't work properly,they just put in 512KB. If they worked fine, they were good for 1MB, and became 1040STF/Ms.