HD module pinout:
-----------------

HD module BOM:
1  crystal oscillator 16MHz, DIL-14 size
1  74LS221
1  GAL16v8-25 or faster
2  C 100nF ceramic
1  C 15nF
1  C 6.8nF
1  R 2.2k
1  R 100k
1  resistor array, 7*10k, common pin
1  pot 270k vertical PT10V
1  SIL connector 10-pin

The disk drives must be jumpered to use pin 2 as a hight-density
OUTPUT. normal mode for PC drives is input on this pin!

       HD0: high-density select for drive 0 - connect to pin 2 of drive
       HD1: high-density select for drive 0 - connect to pin 2 of drive
            (3.5"), in case of 5.25" drive: add additional switch to ground
            to switch to HD mode. Tie to ground for DD drive.

      /DS0: Drive Select 0 - pin 10 at the drive, or pin 20 at the sound chip.

      /DS1: Drive Select 1 - pin 5 at the external 14-pin floppy jack, or
            pin 19 at the sound chip.

  40TRKsel: Connect switch to ground. Switch "open" switched drive 1 to
            40-track mode. Tie to ground for 3.5" drive.

    CLKout: Clock output for the FDC chip (AJAX or WD1772). Connect to pin
            18 of FDC after cutting the connection of the FDC pin to the
            main board.

     /STEP: Input, connect to FDC pin 16

  /STEPout: Step output for 5.25" disk drive, required for 40-track
            mode. Connect to pin 10 of the external 14-pin floppy jack, and
            cut the old connection from that pin to the main board /
            FDC. Leave open if 40-track mode is not required.

       +5V: connect to decoupling cap on the main board.
       GND: connect to decoupling cap on the main board.

The pot on the PCB adjusts the step rate in HD mode and the delay between the
two step pulses in 40-track mode. It should be a bit below 3mb (so that the
FDC is switched back to the right clock before the step is finished). This
can be easily adjusted "by ear" by making the disk drive seek to track
0->79->0 and adjusting the pot so that the audible step rate in HD mode is a
bit higher than in DD mode.

HD mode (ie. 16MHz clock) is out of spec for a WD1772/VLSI1772 FDC - the
only chip guaranteed to work is the AJAX FDC from Atari. However, HD mode
may work more or less depending on the chip. I had one WD1772-0202 that
worked flawlessly even after hours of use without pause.

In case of problems with a 1772 FDC, the following may help a bit: Connect a
100k resistor from the clock input to ground and another 100k resistor from
the clock input to 5V, and connect the new clock signal to the clock input
via a 10nF capacitor.

(C) 1993 Michael Schwingen