STE TOS Upgrade

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Before You Begin

NOTE: Due to technical differences between the ST & STE, this guide only covers TOS upgrade of STE.

Required tools: Screwdriver, Pliers, Soldering Iron, IC removal tool.

Tools that may be useful:, Multimeter (To check for shorts & connections after soldering), drill

NOTE: Before you start, consider the dangers of static electricity. Remove all cables, and ground yourself. Be careful as to not destroy any components with a static discharge. Handle electronic components with care.

Removing the case

Turn your computer upside down, remove all screws, even the three holding the floppy. (The TOS sockets are located under the floppy). Note where the screws are located since they are of different length & dimensions. If you are not careful you'll end up with screws going through the plastic enclosure.

Turn the STE back. Now you can remove the upper part. Remove the keyboard, you don't have to disconnect it. Internally the STE is covered with shielding plates. It consists of 4 parts. Remove the one covering the floppy. To the left of the floppy there's a plate covering the simm sockets. Remove that if you are going to upgrade RAM. To the far left there is a plate covering the PSU. Remove the floppy. (You can do the upgrade from here, but it is recommended to completely remove the motherboard, especially if you are a novice at soldering).

Changing one Tos for another

Beneath the floppy you will find two 32 pin sockets. The one closest to the front is TOS-LO. A little bit to the right of TOS-HI you see three jumpers. Denomenated W102, W103 and W104. They look like resistors, but are in fact 0-ohm resistors. If you like you can replace them with pins and use standard jumper caps. Pin 1 is marked on the motherboard, should be the one closest to the TOS socket.

File:STETOS02.gif STETOS.gif

If you are lucky, you already got two 32 pin eproms seated in the sockets, then all you have to do is to exchange them for the new ones. But normally you have two 28 pin roms there. If so, looking at the figure above, you'll see that you'll have to rearrange the jumpers for your Atari to properly handle the new 32 pin TOS eproms. W102 & W104 need to jumpered between pin 1 and 2. W103 does not need to be changed. Insert the eproms in the right direction! Look at the motherboard, the eproms and the figure above. The eproms got a small jack that indicates direction, similar to the figure above. Also note the difference on how to insert 28 pin & 32 pin eproms in case you want to switch back in the future.

Other things of interest.

E0-E7 are jumpers that are activated by creating a connection between them. They are used to activate various functions in TOS. The only one of interest is E6. It activates formatting of 1.44 mb floppys in TOS 2. However, for it to be of any use, you actually have to have a 1.44mb floppy installed. (On STE it also requires aditional logic, as well as a floppy disk controller able to handle 1.44mb drives. (16 MHz))

Multi TOS

Multi TOS is a very useful upgrade that lets you have 2 (or more) different TOS's in your STE. This is useful because TOS 2.06 is not compatible with all software written with TOS 1.6x in mind. With a simple switch you can easily switch between them and always be able to run your favourite game or application.


First, follow the directions above in regard to removing the case to gain access to the TOS chips. There are two methods to add multiple TOS's to your machine.

Method 1 (Easy)

The simplest method if you already have your 32 pin TOS chips in your machine and some 32 pin TOS 2.06 eproms in your hand is to piggyback them on top of the the existing TOS 1.6x chips and connect them to a small toggle switch so that you can flip between the two versions.


1. On all four chips Pin 24 must be bent outwards before piggy backing the top chip on top of the lower chip.

2. Solder together all pins (Those coloured in Yellow) like for like ie. 1 to 1, 2 to 2, 13 to 13 etc. DO NOT CONNECT 24 TO 24 !!!!

3. Connect wiring diagram as shown below, put some heat shrink around all four pin 24 connections as this will prevent shorts when you come to re-installing the roms. Now find a suitable place on the rear of the STE to drill and mount the switch.


5. Select the Tos you want using the switch then boot the machine. DO NOT CHANGE FROM ONE TOS TO THE OTHER WHILST MACHINE IS ON.

Atari STE Tos 2.06 Upgrade Diagram.jpg

Method 2 (Advanced)

This method involves effectively burning two versions of TOS to a single chip. By using an eprom that is twice the size of a normal TOS eprom, you can divide it into two banks, and program it with 2 different TOS instead of one. Banks are selected by connecting a switch that either feeds ground, or +5v, to the highest adress pin of the eprom, which effectively selects which part of the eprom that you want to use. The principle easily adapts to even larger eproms, in theory letting you choose between 4 or even as many as 8 different versions of TOS. Even though it is of little interest to most people.

Now, carefully bend out pin 30 (A17) of both your DualTOS eproms, until they point straight up. It must not be in contact with the socket, since pin 30 on the socket is connected to +5v. Grounding that could lead to the destruction of your computer! The next step involves soldering. First you solder a wire between pin 30 on both EPROMs, and the middle connector of your switch. After that you solder a wire between +5V and on of the other connectors of your switch, and finally a wire between ground and the last connector on the switch.


In case you have a QuadTOS using 27C040 eproms, you simply do the same procedure (as in Dual Tos Method 2) with pin 31 (A18) with another switch. Then both switches in conjunction will decide which TOS (or bank) you want to use.

With the help of a drill, you make a hole at a suitable place in your STE's enclosure (behind the disk drive is a good spot), and secure your switch(es) there.

File:Quadtos02.gif Quadtos.gif

TOS switching can be done during reset, or when your computer is turned off. If you accidentally hit the switch when your computer is on, it should not spell disaster, but it will most likely crash and need resetting.

As always with these kind of modifications, it is a very good idea to check the connections, as well as possible shorts, after you're done using a multimeter or similar tool. Look carefully after dropped solder or anything else that can cause malfunction. Also make sure that the shielding doesn't cut into any wires, or accidentally shorts your switch if you put it close to any shielding.

In case you don't want to use +5v and ground from the eproms, a multimeter can also be used to find alternative points where you can extract these.

Some sections are copied from original PDF by Greenious

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