Modifying A PC-XT "Clone" Controller For Single-Density Reading

      Got a bunch of Xerox 820 diskettes that you'd love to be able to read?
      Sold your Osborne I at a swap meet and need your correspondence  files
      from 1981?  Too bad, your PC can't handle single-density diskettes...

      Or can it?

      Well, it can be done with a little patience, a few bits of wire, and a
      small  soldering  iron.    The result is a controller that remains PC-
      compatible as far as any DOS software can tell,  but has an  extra  FM
      mode in addition.

      Do  not  attempt  this modification unless you really know what you're
      doing and have worked with this kind of equipment before.   This is no
      place for a novice.

      First, you need to determine the type of controller you're using.  The
      original  IBM PC and PC/XT controllers are NOT readily adaptable,  but
      most inexpensive clones are.

      The important thing to locate on  the  controller  card  is  the  data
      separator chip; it's an 8-pin DIP.  It looks something like this:

                             +--U--+
                     Pin 1   |     | Pin 8
                     Pin 2   |     | Pin 7
                     Pin 3   |     | Pin 6
                     Pin 4   |     | Pin 5
                             +-----+

                           (Top View)

      This DIP (Dual Inline Package) will have pins 4,  5 and 6 grounded and
      pin 8 connected to the +5 supply trace.    Numbering  of  this  device
      varies  --  Western Digital (the prime source) has it as a WD 9216;  a
      NEC part number is UM 8326.   But it's all the same as  far  as  we're
      concerned.

      Identify  the  diskette  controller  chip on your board -- this is the
      large 40 pin DIP  labeled  uPD765A  (NEC),  Z765A  (Zilog),  or  8272A
      (Intel).   Note that pin 26 is a no-connection on your card -- this is
      the MFM/FM mode output pin.

      The last thing you need to find is an unused inverter section.    This
      is the most difficult part of this whole modification.  Most cards use
      a  SN7404 or 'S04 as an oscillator (look near the crystal).   Usually,
      not all sections are used.   Or look for a section of an 'LS00 gate or
      some  such  thing that can be wired as an inverter (Hint: tie both in-
      puts together).

      You need to lift pin 5  of  the  8-pin  data  separator  (above)  from
      ground.  Depending on how your card is laid out, this can be quick and
      simple (cut the ground trace) or very messy (unsolder the whole chip).
      Connect  a  wire  (wire-wrap wire is great) between this pin 5 and the
      output of the inverter which you located.   Connect a second wire  be-
      tween  pin 26 of the floppy disk controller (uPD 765) and the input of
      the inverter.

      That's it -- put the card back into your PC and  make  sure  it  still
      handles regular DOS diskettes okay.

      If  you've got an AT-style controller or a Western Digital 'Fox' dual-
      density XT controller,  you can even drive SD and DD 8" diskettes with
      this modification.

      Note  that  this  simple  modification will enable you to read single-
      density diskettes,  but not to write them.   If you're  really  adven-
      turous,  an  XT-style controller can be made to write diskettes if the
      write-precompensation circuitry is bypassed.   We do it  here  with  a
      section of a 74LS157 driven from the MFM pin of the '765.  A schematic
      of your controller is a must for this kind of work.