Switch to flip between 4mb and 1Mb

[GadgetUK]

Does anyone know which MMU pin you cut to chop 3Mb away from a 4Mb STFM? I read a couple of techie_alisons threads (very sadly shes no longer with us) and she mentioned pin 64 in one thread and pin 18, both refering to A9, I think one is on a SIMM and the other she might mean on the MMU (64?)? Does anyone know if cutting A9 on the MMU will leave 1Mb? And is it pin 64 on the STFM MMU?

 

Edit: Using Marpet - dont know if that helps?

Edited February 1, 2013 by GadgetUK

[GadgetUK]

A9 seems like an odd address line to use btw?

[ParanoidLittleMan]

Address lines are used 2x - because it goes multiplexed to RAM. Cutting A9 maybe is enough.

But you can set RAM to 1MB (or even to 512K) with SW:

 

http://atari.8bitchip.info/astopensw.php

 

See at bottom.

[GadgetUK]

Thanks but I really want a hardware solution as its likely some software won't work with the software solution.

 

I've put a switch on A9 from MMU to the Marpet board and on the first try it dropped to 1024Kb and tested all OK. I put it back together and retested it and found it just flips out a lot when set to the 1024Kb position and often will not do anything other than black screen, garbage on screen, start to boot and bomb, or boot, run the memory test program and then graphically glitch before bombing. If I re-enable A9 its fine and shows 4096Kb and everything works as normal.

 

I am puzzled as to whats going on here as it does occasionally work with A9 disconnected and it can run memory tests for half an hour with out issue?!? Yet reboot it and then its bombing, corruption on screen etc.

 

Does anyone else on here have experience of adding an A9 switch, and if so what type of memory upgrade was it used against? I am wondering if its something to do with the way the Marpet 4Mb SIMM board works and the fact i've cut A9 isnt stopping its interaction with lines that go to the video shifter chip? Or is its some wierd timing problem related to the SIMMS themselves and what I am doing by floating the A9 line?

 

=(

Edited February 2, 2013 by GadgetUK

[GadgetUK]

Edit: nvm, still does the same when running with 2 simms, instead of 4. Ie. toggle from 2mb to 512kb

Edited February 2, 2013 by GadgetUK

[ParanoidLittleMan]

SW solution is OK - no SW which can override it and see more RAM. Because it sets RAM controller chip (MMU) in state as it is there 1MB only. Hardly that there is some, which will write to MMU register - it must be done in special way.

 

And you must not left A9 at RAM side floating. Connect it to GND or +5V.

[GadgetUK]

Thank you VERY much! You've made my day, it works a treat when grounded - which on reflection makes complete sense now. Alison was right in that the MMU A9 can be floating as that's how it ships, but the SIMM needs A9 pulled to ground (or high as you mentioned) in order that it doesnt do strange things when the rest of the modules are being addressed on A0 - A8. It must be occasionally tripping into addressing on A9 which will cause the wierd behaviour I experienced.

 

I've also got to thank Alison as she pointed out the A9 address line in another thread, and in another of her threads she talks about the A9 pin on the MMU being pin 64. She also mentioned that its pin 18 on the SIMM, but not sure if that thread was dealing with 30 pins SIMMS - I know she's not here but her memory lives on, and she's helped me sort 3 things out now on my ST, you never know they might have internet access where she is =(

 

The simple solution for the Marpet is to find pin 64 on the MMU, theres a test point on the Marpet MMU header itself, so you can put meter on there, and then with the other end of the meter find which pin it goes to on the SIMM PCB connector - on my revision it was the very middle pin of the connector on the top side, so it was easy to test continuity. All I did is bend the pin back so it doesnt plug into the socket, connect the end of a toggle switch to the bent back pin, connect one side of the toggle to ground (I used the earth point on the Marpet PCB), and the final side of the switch into a pin which I removed from a pin header (just so its an exact fit) and plug that into the socket, then plug the socket back into the Marpet PCB. So it simply toggles between MMU 64 to ground, to MMU 64 to the Marpet as normal.

 

Now got an STFM running TOS 1.04, 4Mb of RAM switchable to 1Mb, with Blitter, HxC Floppy (I've mounted the buttons, just need to mount the LCD). Sweeeeet!!!

Edited February 2, 2013 by GadgetUK

[Charlie_]

Sound cool. Post pictures when your all done.

[+DarkLord]

Yep, we love pictures (and video).

[ParanoidLittleMan]

GadgetUK: you can DL schematics for almost all Atari machines. For start:

http://dev-docs.atariforge.org/

[GadgetUK]

Thanks! I found that site the other day but downloaded service manuals expecting a circuit diagram but it didnt have the main PCB. The Schematics section does seem to have one I noticed today.

[+rdemming]

SW solution is OK - no SW which can override it and see more RAM. Because it sets RAM controller chip (MMU) in state as it is there 1MB only. Hardly that there is some, which will write to MMU register - it must be done in special way.

 

My version of Super Neo Demo Show (TEX) never worked again after I upgraded my TOS 1.02 1040ST to 4MB. Not even with a memory schrinker that set the MMU registers that worked for all other programs that had problems with 4MB. I'm not sure if it is a 4MB problem. I still need the try the version on AtariMania to see if it has the same problems on my ST.

 

Robert

[GadgetUK]

Just a quick update to this thread:- I added a switch to my STe today to do the same thing. Pin 18 on the 30 pin SIMMS is A9, if you hold the SIMM facing you (chips facing you, connector edge downwards), pin 1 is on the left, count 18 pins right. From there I checked continuity to the resistors going towards the MMU chip (well, on STe its a combined chip, I forget which but I think its merged with Glu). Found the resistor carrying the A9 line to the MMU (which was R454 or something I think). Cut the end of the resistor connection nearest the SIMMS, put a switch to connect the SIMM end to either ground or back to the resistor - so effectively toggling the SIMM A9 point between ground and normal = 1024K or 4096K.

 

Works fine.

Edited February 14, 2013 by GadgetUK