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Original 520ST 2 to 6 tos rom chips


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I'm new here and also i'm new to ST computers. I recently found an Atari 520ST and external floppy drive with ac adaptors in the wild for 5$! It is in working condition, i purchased an Atari monitor locally for 5$ also.


This is the first model with no RF output, only 512k of RAM, no internal floppy and only 2 of 6 rom chips inside to boot TOS from floppy.


I want to install TOS 1.04 with 6 rom chips instead of booting from a floppy and using too much memory doing so.


I have 6 TOS rom chips that i installed in the right position but i am only getting a grey screen. I think i may have to solder some diodes i am not sure.


I found a picture of the same motherboard revision that i have with 6 chips installed and there is diodes installed where there is none on my ST.


Here is the picture in question:




And this is my motherboard:




What modification must i do to make this work?



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You have the TOS BOOT ROMs, that is rare but pointless with only 512k RAM! They called this machine the 260ST in Europe! It should work with no jumper mods with a 6 chip TOS, I upgraded one of mine this way. Make sure the ROMs were burned correctly and installed in the right sockets.

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Looks like the ROMs are not installed correctly..


Number TOS chip Chip Mega ST Old ST New ST
Chip 1: C026160-001 H2 U3 U2 U48
Chip 2: C026161-001 H1 U6 U3 U59
Chip 3: C026162-001 H0 U9 U4 U63
Chip 4: C026163-001 L2 U4 U5 U53
Chip 5: C026164-001 L1 U7 U6 U53
Chip 6: C026165-001 L0 U10 U7 U67


Edited by tjlazer
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It looks that ROMs are inserted in proper sockets. Really don't see why tjlazer says that not, despite giving same ROM order example.

I checked in my 520 ST (what was shipped with TOS 1.02, from 1987) and has same component layout - that's the proper order. Difference is that on that later revision beside chip numbers - which are same Lo 0-2 and Hi 0-2 is printed too .


Well it is working perfectly with only the 2 rom chips. I get a nice rainbow effect background and a window that tells me to insert system disk.

But i don't want to load the TOS that way, i want to load it from Roms.

That's clear from beginning. You need to add those capacitors. And what I said about electrolyte ones was to make it more stable and reliable. Not just "working" . Components degrade by time.


And it may be that actual problem is that chip order is given in reversed order. So, it's worth to just swap Hi-2 with Hi-0 and Lo-2 with Lo-0 and test so .

I say it because by Glue chip order is reversed: ROM3 and 4 go to cartridge port, and they are lower addresses than ROM2-0 lines which go to TOS ROMs .

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Look at the notches on my diagram and on his motherboard the chips are not installed correctly. He has Hi-2 where Lo-2 needs to go at U2, etc. No need to mess with jumpers as he has TOS boot ROMs. Just install 6 chip TOS and go.




Edited by tjlazer
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"Should be the same for all" - it is not the case. And by Atari it is even not sure that 00 means lowest address ROM chip, as explained above.

Order in my 520 ST from 1987, from top to bottom is: Hi-2, Hi-1, Hi-0, Lo-2, Lo-1 & Lo-0 . And it is printed on PCB .

I looked some photos of revisions - https://www.google.hu/search?q=atari+st+motherboard+revisions&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwj5tbqivrLdAhVOLVAKHaXcCM8QsAR6BAgDEAE&biw=1411&bih=853

That order, with Lo at top and Hi lower is in case of STFM - later model, with integrated floppy http://scacom.bplaced.net/Collection/1040/10409.jpg


What must not mean that there is no such order by some 520 STs (no internal floppy) .

Replacing chips and testing is not big deal. Only dangerous situation is when chip is inserted improperly in socket.

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EDIT: on mine I did not need to mess with jumpers. I had the two ROM TOS BOOT ROM setup. All I needed to do is put in the proper 6 chip TOS 1.0 and it worked.

Maybe this post will help:


Edited by tjlazer
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  • 1 year later...

I've recently came across a 520ST two chip with the colorful splash that asks you to insert a system disk just like the original poster.   I just want to share my experience in case it helps any other ST enthusiasts in the future.  This post in particular was very helpful and I began trying combinations of TOS chips from my 1040STf in the hopes I could have TOS in Rom.  Each time I tried I would get a grey screen and nothing else.  I finally noticed my external floppy drive was making noises during the startup with the ROMs in numerical order from top to bottom (see diagram above listed as "Typical 520 ST") and my keyboard would make noises when hitting random keys as well.  I unplugged my external floppy and reset the unit. I immediately was greeted with the GEM Desktop after doing so.  I plugged the floppy drive back into the ST and rebooted again.  This time I waited patiently in the hopes the ST was looking to the floppy first as a boot option and had to fail over to ROM.  This was the case.  What is odd however is that the time it would take for the ST to give up trying to boot from the drive varied.   In one instance it took 45 seconds and another time it 1 minute and 10 seconds.  I don't know why this varies for my particular 520ST and it may indicate some other problem I may have but I can confirm successfully swapping in 6 chip TOS.  I've attached a picture of the 6 TOS chips on my board.  My board is C070243 REV A.  Hope this helps!


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That time when Desktop appears after failed floppy read is not set by some accurate timer. TOS just tries to read from disk and determine it's parameters - so, tries to read first boot sector, there is number of retries, and attempts take not same time. Then, if boot sector finally reads need to read ROOT DIR and possibly FAT - that would be more time case if fails. So, there is no other problem, except now really not rare unreliable floppy work.

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Has anyone dumped the boot-disk splash screen ROMs?  I tried searching but the oldest I can find is TOS 1.0.  


Yeah I know a 520ST with TOS loaded from disk would be mostly useless.  But hey, a 520ST with TOS in ROM but without any memory expansion isn't exactly useful either.


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What is TOS 1.00 too - that's what is in header.

There is lot of SW what can run with this early TOS:  games which don't use TOS functions for file access, and there is plenty of it: Carrier Command, Turrican ...

And with 520 ST, normal TOS 1.00,  you can run some 80% of games.  And even many utilities. Or most of (99%) of SW made before 1987 .

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Thanks for the TOS boot disk.


It looks like the BOOT ROM is available here: http://www.atari-forum.com/viewtopic.php?t=18983


So I can use the ROMSPLIT tool from PLM's website to split this into HI and LO, burn it to a pair of 2764 ROMs and insert them into the ST right?


I don't have my ST open right now, but it looks like sockets U4 and U7 from the picture in the first post.

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  • 3 years later...

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