The Atari 1040 STe can have a simple modification to switch between multiple versions of TOS quite easily. I will install the following TOS versions:
The mod requires the installation of new chips to replace the 2 LO & HI roms that contain the TOS, along with switches to allow the choice of TOS to use.
Parts/ tools required
2 x 27C040 EPROMS (or equivalent)
2 x single pole on-on switches
Preparing the TOS ROMS
There are plenty of good sites around where you can download TOS files. For example:
For my Atari STE I need to split the files into LO & HI files. To do this I used a program ROMSPLIT from http://atari.8bitchip.info/astopensw.php
To help anybody out I have attached split files for the TOS versions listed above (zip file). TOS_SPLIT
The EPROM I am using can fit 4 different TOS versions. So the 4 LO files need to be combined, along with the 4 HI files.
The easiest way to do this is using the copy /b command from a Windows command window. Make sure all the files that need to be combined are in the same directory. Using the files in the ZIP archive as an example, the following command will combine all the HI files, and create a new file named QUADTOS_HI.ROM
Repeat the same steps to create the LO file. Ensure you combine the files in the same order for the LO & HI ROMS!
Now the ROMS are ready for programming onto replacement chips. 27C040 512kb eproms are suitable.
As the files have already been combined, there is no need to prepare the ROMS with start/ end addresses in the programming software. Any programmer which can handle 27C040 chips will do. My personal programmer is a VP-280.
Preparing the LO & HI ROM chips for insertion into the Atari STe
Some physical changes are required to the chips before they can be inserted into the Atari. This is to allow switching between the different TOS versions.
Notice the notch at one end of the chip. This is used to identify which side the first (lower left) and last (top left) pins are located.
Carefully bend up pins 30 (A17) and 31 (A18) on each chip. Do it very slowly, they can be easy to snap off. These pins need to not have any contact with the socket.
Now have a look at the switches. VCC and Ground are required either side of each switch, and the A17/ A18 pins from both switches need to be connected together and connected to the middle switch (both A17 pins connected to one switch, both A18 pins to the other).
When preparing wires to connect the chips to the switches, keep in mind to leave enough length to comfortably route the wires from the TOS sockets to the casing where the switches will be mounted. About 25-30cm each wire should do it.
On each chip, solder ground wires to pin 16, and VCC wires to pin 32. Ensure the solder does not flow down the length of the pins, as it may make it difficult to insert the chips into sockets later. Solder these wires to the corresponding legs of each switch. Now connect the A17 wires to the bent of chip legs. It is a good idea to place some heat shrink tubing over the pins as well to insulate them. Connect both A17 wires to the middle leg of one of the switches. Repeat for pin A18. The new TOS chips & switches are now ready for installation.
Preparing the Atari STe for the new TOS chips
Installing the new TOS chips does require some preparation in the Atari STe. Firstly we need to pull the unit apart.
Flip the Atari STe over, and remove the screws as indicated. Note this also removes the screws which hold the floppy drive in place. Some of these screws are different, so it is a good idea to store these in jiffy bags during the process.
Remove the keyboard by unplugging the connection to the motherboard.
Remove the 2 screws from the floppy drive housing frame.
Now that the frame is removed, unplug the ribbon cable and power connector. The floppy drive can be removed now providing the screws securing the floppy drive to the case were previously removed.
Notice the LO & HI TOS chips, plus a number of resistors used as jumpers. These are actually zero ohm resistors. These resistors are used to determine which type of chips are used for the TOS ROMS. Refer to the following diagram, which I have taken from http://www.scribd.com/doc/19238728/Atari-STE-TOS-upgrade-v2#scribd. This is a great document which contains most of the information I found useful for this project.
Notice the jumpers W102, W103 & W104. As I am replacing with 27C040 chips, the zero ohm resistors need to be changed to suit the 27C010 configuration above. I removed these resistors and replaced with header pins with a jumper to make it easier to change. To get better access to do this the Atari ST needs to be dismantled further.
Remove all the remaining screws from the metal motherboard casing to remove it.
Remove the motherboard from the plastic casing.
Remove the cable screws on the rear panel.
Next remove the rear shielding. This may be a bit fiddly to remove, especially around the RCA sockets.
Now remove the bottom metal casing, and the motherboard should be fully exposed.
Underneath the motherboard. Notice the position of the 3 zero ohm resistors. Remove them.
If required, clean up the holes with some solder braid.
Prepare the jumper pins. Simply use a sharp knife to cut off 3 sets of 3 header pins from a strip.
Solder the header pins in place as seen above.
Now place jumpers on the pins as per the previous diagram.
Remove the existing ROMS, and replace with the new programmed TOS ROMS together with switches.
Prepare the case to install the switches by drilling a couple of holes. Ensure the holes are in a position which will not be affected by any case supports, and make sure they are not mounted too high due to the overlap from the top case.
Now while we are at it time to install a 4 MB RAM upgrade. These can be purchased from http://www.st-freakz.co.uk/. Cheap and easy to install (for the STe).
Above is 4 x 1MB RAM sticks.
Remove the existing RAM and replace with the new sticks – easy!
Now having put everything back together, the switches for the multi TOS can be installed. Carefully position the wires so they don’t end up where the top case supports need to go.
Make some labels to identify the different switch configs for each TOS.Each switch can be left or right, so that gives 4 different configurations.
And thats it!