Drive Select Swap Mod

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The Atari ST is unable to boot from an external floppy drive. Once implemented, this modification will provide a switch that swaps the drive select signals allowing the ablilty to toggle Drive A between the internal and external unit.

This is particularly useful if you have invested in a HxC floppy emulator and you would like the ability to have it externally connected to you ST and optionally boot from it.

Tools you will need

Phillips Screwdrivers

Soldering iron


Vacuum Pump

Heat Shrink (Optional)

Small electronics-style side cutters

ESD protection

Drill or handheld hobby tool

Helping Hands (Optional)

IC extraction tool (if your sound chip is socket mounted)

Parts list

DPDT switch


The DS0/DS1 signal mod inside the ST

1. Open it all up and locate the Yamaha sound chip (U202 on STE):

Yamaha Sound Chip.jpg

2. Mine is in a socket however you may not be so lucky - If yours is mounted, then you will need to cut the legs 19 and 20. I have extracted mine using an IC extraction tool and very gently bent the legs so they are sticking out:

Extracted Yamaha Sound Chip with Pins 19-20 bent.jpg

3. We now need a good line on the board for DS0 and DS1. These can be found at W301 (DS0) and W300 (DS1) handily located near the sound chip. Flip the board over and remove the solder from pin 1 at each of these points using a vacuum pump:

W300 and W301 with solder removed.jpg

4. Now sort the switch out. You must use a DPDT switch! I have used Yellow wire for the DS0 line and White for DS1. Solder as shown, the middle pins will be going to W300 and W301 and the right pins will be going to the sound chip. run a piece of heat shrink sleeve down the lines destined for the chip, we will use them later and it serves as a reminder as to what lines go where:

DPDT Switch wired for DS0-DS1 Mod.jpg

5. Solder the wires coming from the centre pins of the switch to W301 (DS0 - Yellow Wire) and W300 (DS1 - White Wire). Solder from the underside of the board and make sure you cut the excess wire using a small pair of electronics side cutters as flush as possible to the solder joint:

DS0-DS1 Mod - wiring switch to W300 and W301.jpg

6. Make a hole for your switch:

Hole for Switch near Serial Port.jpg

7. Reassemble the shielding making sure you don't trap the wires anywhere, I used the gap near the serial port for this. You can now reassemble the bottom half of the ST and mount the switch:

Running signals from motherboard to additional switch.jpg

8. Now solder the pins on the chip: Pin20=DS0 (Yellow wire) and Pin19=DS1 (White Wire):

DS0-DS1 Mod - Soldering lines onto Yamaha Chip.jpg

9. Apply the heat shrink over the pins, re-seat the socket (if required) and route the wires as neat as you care to:

DS0-DS1 mod complete.jpg

The DS0/DS1 signal mod inside the ST - without pin bending

My YM chip was not socketed and I dislike the idea of bending pins. So here's a version of the mod that doesn't need it :

  • Open your STe and locate W301
  • pins 2 and 3 of W301 are linked together by a small track on the PCB. Cut this one (carefully).
  • Cut wire 10 on the internal floppy cable, close to the motherboard side.

Now wire the switch as follows :

W301.1 ---- 1[ ]4 ---- W301.3
W301.2 ---- 2[ ]5 ---- > to floppy drive pin 10
W301.3 ---- 3[ ]6 ---- W301.1

The result is the same, and it is a bit easier to do.

Additional information

For reference, the wiring of W301 is (as found in Atari STe schematics, page 3) :

  • pin 1 : DS0
  • pin 2 : to external floppy drive select
  • pin 3 : DS1

As you can see, the switch will be connecting either DS0 to internal drive and DS1 to external (as usual), or DS1 to internal and DS0 to external. Proceed to Connecting an HxC Floppy Emulator to an ST

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