Adding a 1.44 meg floppy to ORIGINAL ST's (NOT E's)

Things you will need:
1 disk drive, cost approx. $60.
1 Floppy controller chip capable of running 16 mhz (AJAX chip).

     First let me explain the differences between the Atari (Epson SMD-340)
and some other drives. The Epson SMD-340 that Atari is currently selling
with the upgrade kit for the MEGA STE, DOES NOT have the capability to
provide an output of density on pin 2. This means that if you use this drive
you will have to use a switch to switch densities. As it comes from Atari,
it is jumpered as follows:

Pin 14-15 = Drive Select 0 (drive A)
Pin 5-13  = The mode (density) is switched by the HDI input (pin 2)
Pin 1-2   = The 2 meg mode (1.44) is set by the HDI input HIGH

Other jumper settings: 15-16=DS1, 6-7=DS2, 7-8=DS3, 2-3=2 meg mode set by
HDI input LOW, and 4-5=The mode is switched by the internal sensor.

     As you may have figured out by now there are a couple of ways to hook
up this drive. On mine, I removed the 5-13 jumper and installed 4-5. This
lets the drive switch modes to 1.44 when a disk with the 1.44 meg extra hole
is inserted. The reason I did it this way is so I can easily tell my 1.44
disk from my 720K disk. If you want to use disk without the extra hole for
both densities, then leave 5-13 jumpered and configure jumper pins 1-2 or
2-3 depending on how you want to control the density, with a high (+5) or
low (ground). You must then manually switch the density.

     Now you must also be able to switch the 8 mhz clock going to pin 18 of
the floppy controller chip to 16 mhz for HD (1.44) operation. I simply bent
the pin out and connected it to the center post of a spdt toggle switch and
connected the 8 mhz clock (from the empty pin 18 socket) to one side, and
connected the other side to 16 mhz (pin 39 of video shifter chip). You could
also have a switch to control an IC that would allow you to control both
clock rate and density select with one switch, or use a dpdt switch.

Drives that CAN output density select on pin 2.

     Using a drive that can OUTPUT density select on pin 2 will allow you to
make the operation of the drive automatic (without a manual switch). Set the
jumpers to output density on pin 2. Use this drive output to control an IC
that flip-flops the clock to pin 18 of the floppy controller. You MUST use
the proper disk, as they are what selects the density.

     Here is a circuit that assumes a HD internal drive as A (configured for
high output on pin 2 for HD disk), and an external SF314/354, using a
74xx153 TTL chip for control. Connect pins 1,8,15 to ground. Connect pin 16
to +5. Connect pin 14 to Drive Select 0 (pin 20 of sound chip/L26, or other
places). Connect pin 2 to pin 2 of HD drive. Connect pin 4 to 16 mhz.
Connect pins 3,5,6 to 8 mhz. Connect pin 7 to pin 18 of floppy controller.
Be sure to remove pin 18 from the motherboard.

     In the above circuit, the 153 chip will send the 16 mhz clock to the
floppy controller when a HD disk is inserted in drive A, and the drive
select for that drive goes low (selected). In all other cases, 8 mhz is sent
to the controller. This will allow the use of both drive types on the
system.

     There are many different ways to hook up drives, and this is just a
few. Make sure that you get the jumper settings with the drive that you get.
Life can be tough without them.

Update:

     Here is the circuit that I used. It is a little easier than the above. 
And it is a tested (and used) design. It assumes a HD internal drive as A 
(configured for high output on pin 2 for HD disk), and an external
SF314/354. It uses a 74LS157 TTL chip for control. Connect pin 8 to ground. 
Connect pin 16 to +5. Connect pin 1 to pin 2 of HD drive. Connect pin 14 to 
16 mhz. Connect pin 13 to 8 mhz. Connect pin 12 to pin 18 of the floppy 
controller. Be sure to remove pin 18 from the motherboard.

     In the above circuit, the 157 chip will send the 16 mhz clock to the
floppy controller when a HD disk is inserted in drive A, and the drive
select for that drive is selected. In all other cases, 8 mhz is sent
to the controller. This will allow the use of both drive types on the
system.

NOTE:

I see no need for more than 1 HD drive on the system, but if you want two,
then you will have to use another TTL chip for the pins 2 off the drives
to provide the proper signal to pin 1 of the 74LS157. This should be done
as to provide the strobe when either of the drives go into HD mode. As 
the drives I tested only went into HD mode when they had a HD disk inserted,
AND when they were selected (by Drive select), this should work fine, 
although NOT tested.

Disclaimer (to CMA):

All this information is provided free of charge, and I will not be held
responsible for any damage incured by anyone attempting this modification.
Simply do it at your own risk.
