exxos's Atari, Amiga & retro forum

Hi everybody, im installing the exxos 4mb upgrade following every single instruction i found online but when it comes to the part of isolating the factory ram, i don't know how to proceed because my board looks different to all the ones in the tutorials and my electronics knowledge is just very very basic. Anyone can help me to sort out which resistors i have to lift and where to extract the 5 volts? Can't figure this out. I'm attaching a photo of the board

:?:

Hello, I cant find a schematic for this version but try looking for the two resistors that are connected to pins 6 and 7 on the MMU.

@llanquin

Ok, so based on the instructions for the STFM, you need to lift R59, R60 and R61 on the MMU end and tie to 5v.

The mappings are as follows:

R59 - RAS0 - MMU pin 8
R60 - CAS0L - MMU pin 7
R61 - CAS0H - MMU pin 6

So taking this through to the the 520ST schematic we get the following mappings:

No Resistor - RAS0 - MMU pin 8
R134 - CAS0L - MMU pin 7
R133 - CAS0H - MMU pin 6

I'd suggest that for the RAS0 line where there is no resistor you'd need to cut the trace along the way somewhere sensible and easy to repair if you need to, and tie that to 5v, but you'd probably want to also add in a 68K resistor to be safe.

Alternatively, trace the RAS0 line from the RAM back to the MMU and see if there is a resistor along the way, as that'd be better than cutting a track.

The best way to really disable is to remove the RAM, but I can understand why you wouldn't want to do that.

hi, so i could not find those pins (R134 or R135) . Is there any other way? Im attaching more detailed images of the board in case i missed them and anybody can spot them. The piccs go from left to right and the last 2 ones are a detail of the area with the metal enclosure which i still haven't removed since i want to be sure first about what i need to do. I would like to eventually physically remove the ram but that's something i will do once i get a heat gun. In the meantime i really just want to isolate the ram but i haven't seen examples done with my board. Any more clues?

I was looking into this problem today in fact. I've had to mark it on my site that rev D and earlier boards do not have the resistors. So other than tracing and cutting tracks as @rubber_jonnie mentioned , I don't see there is any simple test solution. Both my boards are rev H. So I can't even trace where the tracks are.

So you will have to just remove the old dram. You need to do this after the install anyway. So IMO it's just not worth the trouble in trying to isolate the old RAM.

You may have took it out already, but do you realise you're missing the shifter chip?

Remembering back to when I upgraded my own ST, which was a long time ago, It's possible it didn't have resistors either, but I'm not 100% as I ended up removing all my old RAM anyway.

The only other way, if you don't plan on removing the old ICs is to trace the RAS0, CAS0L and CAS0H lines from the MMU to the RAM. The pins I've indicated (6,7 & 8) are correct for the MMU, you should have RAS on pin 4 of your RAM, and CAS on pin 15, but double check the datasheet for your RAM to be 100% sure.

As @exxos has said, you are better off removing the old RAM in the long run.

thanks for the quick responses!
i think atm i just want to isolate the ram instead of removing it until i get proper tools.

located pins 4,6,7 and 15 (see image )
but cant seem to find pin 8

i apologise for my ignorance but what does it means to have RAS on pin 4 of your RAM, and CAS on pin 15

i would appreciate if i can get a more basic explicit explanation of what i have to do since i know very little about electronics and i dont have a datasheet for the ram, im not even quite sure if i know what that means :D .The upgrade im trying to install is the 4mb one sold by exxos.

You mention that you're trying to install a 4mb kit from Exxos, but you realise he sells two different types? One is the MMU piggy back and the other is the direct soldered type where you remove the RAM and solder the PCB in. I assume you're doing the MMU kit since you mention not wanting to remove the RAM? Sorry for the basic question but I think it's always good to start off knowing the full situation for certain.

Steve wrote: 29 Mar 2023 07:31 You mention that you're trying to install a 4mb kit from Exxos, but you realise he sells two different types? One is the MMU piggy back and the other is the direct soldered type where you remove the RAM and solder the PCB in. I assume you're doing the MMU kit since you mention not wanting to remove the RAM? Sorry for the basic question but I think it's always good to start off knowing the full situation for certain.

The solder-in board doesn't fit this motherboard, so it will be the plug-in.