STFM C070789 6 CHIP TO 2 CHIP CONVERSION

[exxos]

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)

6conv1.jpg (203.45 KiB) Viewed 302 times

6conv2.jpg (267.27 KiB) Viewed 302 times

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.exxoshost.co.uk/atari/last/storenew/#0042

[troed]

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 .. )

[Petari]

What is best solution in case of ST is to have TOS switch with extra logic for TOS 2.06 - then can use older TOS 1.xx and 2.06 , And it can be more than 2 TOS version - like 1.02, 1.04 , 2.06 + some patched ... All it can fit in 2 (Flash)EPROM chips.
And to add that 2 ROM for 2.06 is not same as 2 ROM for 1.xx - in case of 2.06 size is 256KB instead 192KB, that's what allowed much better Desktop for it.