exxos's Atari, Amiga & retro forum

Hey, I own a Mega 4 and it was purchased from Germany and hence it ahs German TOS in it.
I need to replace this TOS with an English language version, and ideally update the machine to the latest useful TOS.

I have read a number of threads on this, and its somewhat confusing.

Currently the machine has 2 ROM sockets, and I understand this means each ROM chip holds 128kb out of a total 256kb ROM
I have read that the Mega 4 can use TOS 2.06, but its a little unclear HOW to get the 256kb image onto 2 ROMS

I have seen there is an adapter board to allow for use of a PLCC, which I assume can hold 512kb of data, as I read that it is a dual TOS solution (jumper switches ROM image).
I note it says it can switch between TOS 1.2/ Rainbow TOS 1.4, but not 2.06?

Are there any other solutions out there? I was assuming I could use two M27C1001 Eproms (128kb)
with an adapter PCB, to get its 32pins to work in the existing 28pin sockets.
Is this possible? I ask, ask I already have these EPROMS.

Alternatively, can I instead opt to install 4 more sockets in my machine, and instead go
for splitting the 256kb across 6 roms? This seems like an odd fit, as that would be 42kb per ROM...er....
With this being the case, I see that there are TOS versions that come as 192kb images,
so that would work out better at 32kb per EPROM.

Anyway, over to you guys...

Have a look here: https://www.exxosforum.co.uk/forum/vie ... ilit=ROMs

Sorry, I'm being dumb here, let me walk through your jumper settings

ROM	W2	W3	W4
256k	1-2	1-2	jumper
1Mx2	2-3	2-3	open
1Mx6	2-3	2-3	jumper

So, the first row, it says 256k, but it does not say if that is the EPROM size in KiloBits or... Does this mean 256Kbit x 6 = 192kb?
2nd row says 1M(bit?) x 2, which would be 2 x 128k EROMS = 256kb ROM
3rd row says 1M(bit?) x 6, which would be 6 x 128k EPROMS = 768KB

Something I'm not getting here, sorry, just trying to get this clear.

Anyway, lets assum row 2 means 2 x 1024KiloBit, which would be 128KB EPROMS x 2, if so, which 128KB EPROMS would fit in these sockets?

Retronaut wrote: 09 Apr 2025 17:32 Are there any other solutions out there?

You might want to take a look at the SidecarTridge TOS Emulator: https://sidecartridge.com/products/side ... -atari-st/

Retronaut wrote: 09 Apr 2025 23:15 Sorry, I'm being dumb here, let me walk through your jumper settings

ROM	W2	W3	W4
256k	1-2	1-2	jumper
1Mx2	2-3	2-3	open
1Mx6	2-3	2-3	jumper

So, the first row, it says 256k, but it does not say if that is the EPROM size in KiloBits or... Does this mean 256Kbit x 6 = 192kb?
2nd row says 1M(bit?) x 2, which would be 2 x 128k EROMS = 256kb ROM
3rd row says 1M(bit?) x 6, which would be 6 x 128k EPROMS = 768KB

Something I'm not getting here, sorry, just trying to get this clear.

Anyway, lets assum row 2 means 2 x 1024KiloBit, which would be 128KB EPROMS x 2, if so, which 128KB EPROMS would fit in these sockets?

From my post just before the table:

"The first thing to understand, is that like the STF/M, there are 3 jumpers that need to be set in order to specify just which ROMs you're going to use, as per this table:"

So yes, it's ROM size. You're not really going to be interested in any ROM other than 27C256, so you just need to follow the jumper settings in row 1 of the table and as in my pictures.

Jumper pins 1&2 of W2 and W3 and put a link in W4.

Make sure you read through my post, some people have found the 74LS11 (U12) needs to be removed, but that wasn't the case for me.

Ok, so the 27C256 is 32kb EPROM, so I am assuming its 6x of these to give me 192kb of ROM space... (TOS 1.04 - Rainbow)
I see that the 27C256 is a 28pin EPROM, so these would just fit, no adapters required.
OK, cool, thats a plan B Thanks!

Back to rows 2/3 on your table....

ROM	W2	W3	W4
1Mx2	2-3	2-3	open
1Mx6	2-3	2-3	jumper

Row 2 is 2 x 1Mbit Eproms, giving 256KB of ROM space, which would allow for a 2.06 TOS
However, I beleive all 1Mbit EPROMS come (these days) in 32pin packages, requiring an adapter to fit in the 28pin sockets
Do you have any insights into this?

And Row 3, not sure what this relates to?
6 x 1MBit= 768KB of ROM space, not sure what would require this, as its 3x the size of even the "large" 2.06 TOS ROM

I have read your article, and its clear on the jumper positions etc. Its rather WHAT EPROMs + Adapters I would ALSO need for each configuration.

Sorry about all the questions, its just I will be making a video about this and I am trying to be thorough about all the different options

Retronaut wrote: 10 Apr 2025 14:37 Ok, so the 27C256 is 32kb EPROM, so I am assuming its 6x of these to give me 192kb of ROM space... (TOS 1.04 - Rainbow)
I see that the 27C256 is a 28pin EPROM, so these would just fit, no adapters required.
OK, cool, thats a plan B Thanks!

If all you want is English TOS 1.04, I'd make it your plan A, the 27C256 ROMs will drop straight in with no issues at all, doing anything else will complicate matters and you'll find the other ROMs mentioned difficult to get hold of, hence why I always go for 27C256's.

Retronaut wrote: 10 Apr 2025 14:37 Back to rows 2/3 on your table....

ROM	W2	W3	W4
1Mx2	2-3	2-3	open
1Mx6	2-3	2-3	jumper

It's not really my table, rather from the Atari schematic, just reproduced for information.

Retronaut wrote: 10 Apr 2025 14:37 Row 2 is 2 x 1Mbit Eproms, giving 256KB of ROM space, which would allow for a 2.06 TOS
However, I beleive all 1Mbit EPROMS come (these days) in 32pin packages, requiring an adapter to fit in the 28pin sockets
Do you have any insights into this?

Yes you'd need adapters to change 32 pin to 28 pin ROMs, not worth the trouble IMHO, which is why I always use 27C256's as they are a drop in fit with no messing around.

There are 32 pin to 28 pin adapters available, but I can't comment on them because I don't like them and don't use them.

As for TOS 2.06, you can't just install the ROMs and have it work. It uses additional decoding that a standard ST or Mega can't do without installing additional hardware: TOS decoder board

Retronaut wrote: 10 Apr 2025 14:37 And Row 3, not sure what this relates to?:
6 x 1MBit= 768KB of ROM space, not sure what would require this, as its 3x the size of even the "large" 2.06 TOS ROM

I have read your article, and its clear on the jumper positions etc. Its rather WHAT EPROMs + Adapters I would ALSO need for each configuration.

Sorry about all the questions, its just I will be making a video about this and I am trying to be thorough about all the different options

I'm not sure what Atari meant by that, but the same comments apply as for TOS 2.06 as I stated before. I've never seen a 6x1Mbit installation in any Atari ST, just 1Mbitx2 and 256Mbitx6. Personally I'd ignore that option.

If all you want is an English language TOS 1.04 then don't make it complicated by trying to do it the hard way. 4x27C256 ROMs, 4 new sockets and decoupling caps for the extra ROMs as per my thread. You'll probably struggle to find the right size ROMs anyway if you try for something 1Mbit in size.

If you really want to do something and end up with TOS 1.04 and TOS 2.06 then you'll need to get a suitable dual TOS adapter from the store and the decoder module to allow TOS 2.06 to work as expected. You could then use a switch to jump between TOS versions.

There is a Mega ST DualTOS adapter board in the store, but I don't know if it will work with the decoder, that would be an @exxos question.

Thanks for the extra info, very useful. So yeah, it does look like Plan B is going to be plan A :)

I took a look a the [0248] TOS206 ST DECODER GLUE PLUGIN and it states "This board is NOT suitable for the H4,H5,MEGA platforms"
So it looks like using 2.04 on my Mega is a dead end, so yeah, 6 ROMs and 1.04 Rainbow it is!

On another note, oddly I took a look at my Mega and its empty ROM sockets DO have capacitors already installed next to the empty pads
Seems strange, that Atari factory put in caps for socket pads that are not installed. Anyway, saves me a job :)

Thanks for your help again ;)

Retronaut wrote: 10 Apr 2025 16:52 Thanks for the extra info, very useful. So yeah, it does look like Plan B is going to be plan A :)

No problem.

Retronaut wrote: 10 Apr 2025 16:52 I took a look a the [0248] TOS206 ST DECODER GLUE PLUGIN and it states "This board is NOT suitable for the H4,H5,MEGA platforms"
So it looks like using 2.04 on my Mega is a dead end, so yeah, 6 ROMs and 1.04 Rainbow it is!

Ah, I hadn't noticed that. I do have a Mega 4 with TOS 2.06, but it uses a completely different interface, and it isn't available any more. I doubt you'll need more than 1.04.

Retronaut wrote: 10 Apr 2025 16:52 On another note, oddly I took a look at my Mega and its empty ROM sockets DO have capacitors already installed next to the empty pads
Seems strange, that Atari factory put in caps for socket pads that are not installed. Anyway, saves me a job :)

Thanks for your help again ;)

No problem.

It varies, some have them, some don't, no real rhyme or reason to it. Some have empty holes for the ROM sockets, some are filled with solder.

MEGA ST would need :

DECODER
https://www.exxosforum.co.uk/atari/store2/#0212

DUALTOS PCB
https://www.exxosforum.co.uk/atari/store2/#0008

ROM
https://www.exxosforum.co.uk/atari/store2/#0050

The plugin decoder doesn't fit under the PSU unfortunately on the MEGA Someone else tried and failed a few months ago.

You only need the decoder if you want to use TOS206. Otherwise your limited to TOS104.