3.2.4) What were the Atari 815, XF521, and XF351 Disk Drives?

The 815 Dual Disk Drive was introduced by Atari in June 1980 at the Summer
Consumer Electronics Show (CES) in Chicago.  The unit would feature two
single-sided, double density (MFM encoding) disk drives.  The 815 would not
have a single density (810-compatible) operation mode (a shortcoming that
would inspire Percom to invent the "Percom configuration block" as a standard
for density-configurable Atari disk drives).
- Total storage capacity per diskette:
    40 tracks  x  18 sectors/track  =  720 sectors/disk
    720 sectors  x  256 bytes/sector  =  184,320 bytes/disk  (180KiB)
- While most 815 sample drives utilized the same Tandon drive mechanism found
  in late-production Atari 810 drives, there were also 815 sample drives built
  that used the MPI drive mechanism found in earlier 810 drives (including all
  810 drives manufactured while the 815 was under development).
- Manual: Atari 815 Operator's Manual C016377 (CO16377)
- The 815 disk drive has a built-in power supply (plugs directly into the
  wall, 120V AC).
- 815 accessories from Atari:
   o CX8201 Master Diskette II
     (DOS II version 2.0D, or DOS 2.0D; version of DOS II supporting both 815
     and 810 disk drives; prototype box exists, but never shipped)
   o CX8202 5 Blank Diskettes
     Box contains five CX8202 Atari 810/815 Blank Diskettes (certified for
     double density use).
     Minimal production, see: http://www.myatari.com/ebay/815dt.html
- Software titles announced by Atari to require the 815:
   o CX401 The Atari Accountant: General Accounting System (prototypes exist)
   o CX402 The Atari Accountant: Accounts Receivable System (never shipped)
   o CX403 The Atari Accountant: Inventory Control System (never shipped)
     The Atari Accountant flyer at AtariMania:
   o CX408 Atari Word Processor (Atari 815 Version; never shipped)
- Atari continued to promote the 815 through the summer of 1981 (see Company
  Store price list for 8-24-81 for a late reference); it was dropped from
  the product line by October 1981.  Prototype units exist (according to
  Atarimuseum.com: 60 units were produced), but the 815 never shipped.

The XF521 5.25" disk drive was previewed by Atari in January 1985 at the
Winter CES in Las Vegas.  The XF521 was to be a 130KiB, XE-styled, feature-
equivalent, and gradual replacement for the Atari 1050.  The XF521 never
shipped, and no more than a handful of prototype units may exist.
Pictures: http://www.atari-computermuseum.de/pics/zubehoer/xe/xf521.jpeg

The XF351 3.5" single-sided double density disk drive was first previewed by
Atari in March 1986 at CeBIT in Hanover, West Germany.  The XF351 was
subsequently previewed at the 1986 Spring COMDEX show in Atlanta and at the
January 1987 Winter CES in Las Vegas.  With ADOS (would ship as: DOS XE), the
XF351 was reported to provide 325KiB (6/1986 report) or 320KiB (9/1986 report)
of storage (formatted) per disk.  The XF351 never shipped, and no more than a
handful of prototype units exist.
Picture: http://www.atari-computermuseum.de/pics/zubehoer/xe/xf351.jpg
Discussion: http://atariage.com/forums/topic/231674-xf351-for-sale-on-ebay/
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