Mega ST ROM swap - 2 chip to 6 chip.

[rubber_jonnie]

When I added the Beta IDE interface to my Mega ST 4, the installed UK 6 chip version of TOS 1.04 became spare, since the IDE board also comtains a dual ROM with 1.04 and 2.06 on board.

Since I had also recently acquired a Mega ST 1, but with German TOS 1.04, I thought it would be useful to recycle the spare 6 chip UK set I had.

There was a slight bump in the road however, it turned out the Mega 1 had a 2 chip ROM set. So here's how to change it if you decide you want to do the same, and in fact this will also work for the other way too.

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:

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

Unlike the STF/M, these are not solder pads, but a mixture of jumpers which use zero ohm resistors as links like the STE, and a single wire link for W4. Below is a reference picture from my Mega ST 4 showing the positions required for 6 chip TOS with 256k EPROMS:

Mega4_JMP_REF.jpg

Looking at my Mega ST 1, they were set as below:

Mega1_JMP_REF.jpg

At this point I'd already removed the zero ohm links and replaced them with proper jumpers. As you can see, W4 is unlinked at the point.

I then added the W4 link and swapped over the W2 & W3 jumpers to position 1-2:

Mega1_JMP_After.jpg

So stage 1 was completed, with the Jumpers set for 6x256k EPROMS.


Next was the extra sockets and capacitors. You'll need to add 4 100nF capacitors as below:

Caps.jpg

I used components recovered from a dead ST, but up to you where you source from. They are on large copper planes, so soldering is difficult, I did it from both sides to be sure.

Final thing was to fit the sockets and ROMs:

finished.jpg

After that a boot and test of everything, and all good to go.

If you want to go from 6 ROM TOS to 2 ROM TOS, then all you'll need to do is set W2, W3 & W4 appropriately.

[Roberto]

Hi rubber_jonnie, do you know if instead of putting the 6 ROMs it is possible to use two 28C256, then write the TOS there. In this way is it possible to update your Mega ST without making any changes other than just replacing the ROMs?

[rubber_jonnie]

Hi rubber_jonnie, do you know if instead of putting the 6 ROMs it is possible to use two 28C256, then write the TOS there. In this way is it possible to update your Mega ST without making any changes other than just replacing the ROMs?

I very much doubt it, you need 6 27C256 ROMs for TOS, so 2 ROMs of a different type but the same capacity won't be large enough to hold TOS.

[coonsgm]

Hi rubber_jonnie, do you know if instead of putting the 6 ROMs it is possible to use two 28C256, then write the TOS there. In this way is it possible to update your Mega ST without making any changes other than just replacing the ROMs?

I very much doubt it, you need 6 27C256 ROMs for TOS, so 2 ROMs of a different type but the same capacity won't be large enough to hold TOS.

So no need to remove the 74LS11? I'm working on a Mega 1 and don't want to remove the chip unless it's needed. Everything else is done and checks out good. However, with known good eproms in place....no joy. My other Mega ST4 has this chip removed and is running 6 chip roms delightfully.

Can anyone verify if the 74LS11 needs removal?

[coonsgm]

Nevermind, I got impatient (a whole 30 minutes) and removed the 74LS11 and now the 6 rom tos boots lovely.

[rubber_jonnie]

I very much doubt it, you need 6 27C256 ROMs for TOS, so 2 ROMs of a different type but the same capacity won't be large enough to hold TOS.

So no need to remove the 74LS11? I'm working on a Mega 1 and don't want to remove the chip unless it's needed. Everything else is done and checks out good. However, with known good eproms in place....no joy. My other Mega ST4 has this chip removed and is running 6 chip roms delightfully.

Can anyone verify if the 74LS11 needs removal?

No, you don't need to remove the 74LS11 you just need to set W2, W3 and W4 correctly for a 6 ROM setup. That being said AFAIK from the schematics, if you are using a 6 ROM configuration, the LS11 is not actually in use, but the jumpers disable it. Simpler to use the jumpers than remove the LS11 though.

[coonsgm]

So weird....I installed these jumpers with the 74LS11 in and TOS wouldn't even begin to initialize....just a white screen. Remove the 74LS11 and bingo, perfect boot with 6 ROM's.

IMG_1621.jpg

[exxos]

Is better to look at the schematic on how things are wired up...

rom.PNG

[rubber_jonnie]

So weird....I installed these jumpers with the 74LS11 in and TOS wouldn't even begin to initialize....just a white screen. Remove the 74LS11 and bingo, perfect boot with 6 ROM's.
IMG_1621.jpg

Well you can see in my images that the LS11 is still in place. Not sure why it wouldn't work for yous as your W2&W3 jumpers are both on 1&2 like mine and W4 is connected.

It could be that you have a bad jumper connection on W2 and/or W3, only way to really test is with a multimeter in continuity mode to check it all out.

Below is the setting table for the jumpers:

Screenshot 2024-09-10 at 13.05.47.png

And the top row is what's correct for 6x256KB ROMs. It's possible that this is suggesting that you don't use the LS11/U12 for 6 ROM configurations, but again, it works for me. Typical...

[coonsgm]

I'm not too concerned...not a whole lot of reason to ever use two ROM TOS. Working with 6 ROM's is what's needed.