exxos's Atari, Amiga & retro forum

On later motherboards where the TOS ROM's are in a row as shown on the images below, these motherboards can be converted to 2 chip TOS. Generally 6 chip was was V1.00 & TOS1.02 and later versions of TOS were generally 2 chip with TOS1.04 (but also TOS1.04 can be 6 chip!) . There are many combinations of TOS and 2 or 6 chip, but that is a general rule to keep things simple at this point.

I would recommend TOS1.04 be installed in all STFM machines. While other TOS versions can be made to work, it is not a simple ROM chip change as for example, TOS2.06 needs some address decoding as its ROM address is different than TOS1.04. Also generally I would advise against upgrading TOS higher than TOS1.04 if you want to keep compatibility with games and older software in general.

In anycase, here is how to convert 6 chip to 2 chip.. (note I took images of 2 different motherboards to show what needs to be done to save time on myself actually doing the mod)

Notice the location of where the 2 chip ROM's are installed. Also notice the HI and LO printed on the motherboard. The jumper pads are changed to 1M. That is basically it!

I would recommend if you upgrade to TOS1.04, that you install one of my Fast-Rom kits. As if you decide to do a CPU upgrade, fast-roms will be needed.

Also consider fitting STFM DUAL TOS kit which gives you a easy solution for TOS1.02 & TOS1.04.
https://www.exxosforum.co.uk/atari/store2/#0042

Just to make it even more clear - if there's no chip at U68 you need to install a 74LS11 there - it's what combines the chip selects that were for six chips down to two.

(I've seen more than one person buy a 2 chip replacement only to find out they have to order the 74LS11 as well once they've already begun doing the change .. )