exxos's Atari, Amiga & retro forum

Hi all, I'm reaching out fo a bit of help with a memory expander I've acquired.

It came with this mainboard: viewtopic.php?f=41&t=466 which came to me via the most excellent Mr John Cove.

It's fairly standard fare, with an adaptor that clips on top of the MMU:


An adaptor that fits under the shifter:


And a SIMM module:


The MMU adapter is marked GDT & AT RAM or ATRAM,and looks very similar to a Marpet add on.

Does anyone have any information on this upgrade at all, or pointers to where documentation might be found?

I'm trying to resurrect it to expand one of my ST's, but it's not 100% clear on how some aspects of the upgrade.

For instance, on this mainboard, R59 has been snipped, and connected to 5v, I presume to disable the on board ram, but I'm wondering, since on other adaptors, such as this: https://www.exxosforum.co.uk/atari/last ... /index.htm, R59 through to R61 all need to be lifted and connected to 5v.

I will try this, but I'm also not clear on the orientation of the MMU adapter. I know what position it was in when it came, but that doesn't necessarily mean it was in the correct position.

Thanks in advance, I'll be very grateful of any assistance I can get at this point, since it will theoretically allow me to upgrade 2 ST's if I can get it working! (One with the RAM chips I can recover and put in another machine, and the other with this add on)

rubber_jonnie wrote: 21 Jan 2018 16:55 For instance, on this mainboard, R59 has been snipped, and connected to 5v, I presume to disable the on board ram, but I'm wondering, since on other adaptors, such as this: https://www.exxosforum.co.uk/atari/last/4MBMMU/index.htm, R59 through to R61 all need to be lifted and connected to 5v.

I think you can just lift one, once one of the strobe lines is isolated then the RAM isn't going to get selected anyway. I did a few resistors, mostly as was in a hurry to do the write up and never checked which resistors were for what, so I isolated all the control ones and had done with it :)
Of course it was only for initial testing anyway and "old ram" should be removed once new RAM tested and verified working.

rubber_jonnie wrote: 21 Jan 2018 16:55 I will try this, but I'm also not clear on the orientation of the MMU adapter. I know what position it was in when it came, but that doesn't necessarily mean it was in the correct position.

It will be the same as my MMU board. Though you have to check where pin 1 is.. my MMU board is labeled which pins are what, so once you work out which way my board looks to fit, the board you have will be the same.

exxos wrote: 21 Jan 2018 21:18
I think you can just lift one, once one of the strobe lines is isolated then the RAM isn't going to get selected anyway. I did a few resistors, mostly as was in a hurry to do the write up and never checked which resistors were for what, so I isolated all the control ones and had done with it :)
Of course it was only for initial testing anyway and "old ram" should be removed once new RAM tested and verified working.

It will be the same as my MMU board. Though you have to check where pin 1 is.. my MMU board is labeled which pins are what, so once you work out which way my board looks to fit, the board you have will be the same.

Thanks for the info exxos, as it happens, it would seem orientation of the MMU board was the key.

Also, I lifted only a single resistor, R59 and connected to 5v.

Referring to my spare CO70789-001 Rev D, I realised that the C103088 REV.5 has the MMU oriented 180 degrees opposite to the CO70789-001 Rev D.

The MMU adapter had however been plugged in in the orientation you'd expect for the CO70789-001 Rev D, so 180 degrees out. I removed and rotated it, and it all works just fine. I'll get a pen to mark the MMU adapter with the pin locations, but the long and short of it is, the orientation is exactly as per your MMU adapter.

I'll get some pics and document on the forum so that other folks can see how it's supposed to fit. To be clear, it worked happily on my CO70789-001 Rev D, and the C103088 REV.5, so as long as you have a socketed shifter (Soldered in would be nasty), and an MMU in a PLCC socket, this upgrade would work just fine I think.

It works with 1MB RAM removed from an STE, but none of the sets of 4MB simms I have, which have all been tested in my STE, work, I just get random crashes. The main difference seems to be that the RAM that works is 100ns, whilst the others are 80/70ns.

I don't think this is necessarily an issue, as it seems really sensitive in terms of grounding, and during my testing, the grounding was a bit crappy, so I will do some more tests when I have a bit more time.

I may even write an installation manual :)

:bravo:

Yeah those things are unstable at the best of times... adding a decent cap on the 5V rail simm board maybe help :shrug:

exxos wrote: 21 Jan 2018 23:20 :bravo:

Yeah those things are unstable at the best of times... adding a decent cap on the 5V rail simm board maybe help :shrug:

Any suggestions as to the size of cap gratefully received :)

rubber_jonnie wrote: 21 Jan 2018 23:28 Any suggestions as to the size of cap gratefully received :)

I mean low ESR, 1500uF - 4700uF kinda values.

exxos wrote: 21 Jan 2018 23:32
I mean low ESR, 1500uF - 4700uF kinda values.

Wicked, I'll see what I can do about getting one, will need to go and order one, so will make sure I get a decent make and low ESR, thanks again.

rubber_jonnie wrote: 21 Jan 2018 23:35 Wicked, I'll see what I can do about getting one, will need to go and order one, so will make sure I get a decent make and low ESR, thanks again.

I can send you a couple.. But probably won't be until the end of the week..

exxos wrote: 22 Jan 2018 11:37 I can send you a couple.. But probably won't be until the end of the week..

That would be very generous, thank you.