Atari ST Drive Swap

Introduction

This page describes how to add a drive swap switch to your Atari ST or STE. This switch will swap drives A & B so the external drive becomes drive A and the internal drive becomes drive B. The reasons why you would do that are:

• Games on two floppies that do not support loading from a second drive. With the switch you only have to toggle the switch instead of swapping floppies.
• Booting from your external drive. For example if you want to boot from your external 5 1/4" drive.
• You are the proud owner of a HxC floppy drive emulator. If you don't want to replace the internal drive with the floppy emulator, you can connect it to the external floppy port. By swapping the drives, you can boot from the floppy emulator.

Of course, doing this modification is totally on your own risk!

Parts required

• Double-pole double-throw (DPDT) switch.

Schematics + build description

Drive swap switch schematic.

The drives are selected by bit 1 & 2 of port A of the YM2149 soundchip. These are pin 20 & 19 on the soundchip. So by swapping these signals the external drive becomes drive A and the internal drive becomes drive B. The switching of the signals is done by a double-pole double-throw (DPDT) switch. Below is a step-by-step description of you to perform the modification. If the description is not clear, refer to the schematic above.

• First, pin 20 & 19 of the soundchip have to be disconnected from the floppy connectors. This can be done by cutting the pins as close as possible to the motherboard. Then bend the pins up. Desolder the parts of the cut pins that are still in the motherboard¹.
  
• Then solder a wire between the first "throw" of the switch common connector and the motherboard where you cut pin 20 loose.
  Also solder a wire between the second "throw" of the switch common connector and the motherboard where you cut pin 19 loose.
  
• Next solder a wire from the bend-up pin 20 of the soundchip to the first side of the first "throw" of the switch and to the opposite side of the second "throw" throw of switch.
  Also solder a wire from the bend-up pin 19 of the soundchip to the first side of the second "throw" of the switch and to the opposite side of the first "throw" of the switch.
  
• Give the switch a nice place on the computer's casing.
  
• Now you are finished and can test the switch. Place a floppy in drive A and one in drive B and turn on the computer. The computer must now boot from one of the drives. Turn off the computer and toggle the switch. Then turn on the computer and now the computer must boot from the other drive.

¹ If you don't want to cut the pins of the soundchip, it is also posssible to cut the two traces running from pin 20 & 19 on the motherboard and solder the wires at the proper places on the moterboard. But you need to be carefull to cut the traces at the right places. If you cut the traces too far from the chip it might be possible you swapped the signals to only one of the floppy connectors (internal & external) instead of both floppy connectors.