3.2.3) What is the Atari XF551 Disk Drive?

The Atari XF551 disk drive was introduced in June 1987 as a replacement for
the 1050 disk drive.  The XF551 is fully backward compatible with the 1050,
with the addition of two new modes of operation: a true "double density" mode
offering 180KiB of data storage per (single-sided) diskette (fully compatible
with 3rd party double density drives for the Atari), and a "double-sided
double density" mode offering 360KiB of data storage per (2-sided) diskette.

Unlike the 810 or 1050, the XF551 drive has two drive heads, so it can
read/write to both sides of a 2-sided floppy disk (in double-sided double
density 360KiB disk mode).  The 360KiB mode of the XF551 writes "backwards" to
the second side of the disk in comparison to the way data would be stored on
the reverse side of a disk, inserted into the drive upside-down, in single-
sided double density 180KiB mode.

When working with double-sided double density 360KiB disks, the XF551 is also
capable of communicating with the computer at about twice data rate of the
earlier Atari 1050 or 810 drives.  This XF551 high speed mode is supported by:
DOS XE, SpartaDOS X, XDOS, MyPicoDos, Micro-SpartaDOS, SuperDOS, Turbo-DOS,
BW-DOS.  A patch was also developed for SpartaDOS 3.2d.

The front of the XF551 features a "Disk busy" indicator light.  (Notably,
there is no power indicator light on the XF551.)  The rear of the unit
includes a power on/off switch, a Power jack, two SIO ("Peripheral") ports,
and Drive Select DIP switches:
                      Left & Right down: Drive 1
                    Left down, Right up: Drive 2
                    Left up, Right down: Drive 3
                        Left & Right up: Drive 4

More features/specifications of the XF551 disk drive:
- 8040/8050 MPU.  One of:
      - Intel 8050 microcontroller (MCU) @ 8.3333MHz, C070235
      - Intel 8040 microcontroller (MCU) @ 8.3333MHz, CB101393
   - The 8050 has internal ROM space that is not used in the XF551.
   - (Initial Atari and media reports incorrectly indicated a 6507 MPU.)
   - (The DOS XE: XF551 manual incorrectly indicates 18.3333MHz.)
- Western Digital WD1772 Floppy Disk Controller (FDC) C026028
   - Late production units substituted: Western Digital WD1770
- SIO Data rate (normal): 19,040 bit/s (NTSC), 18,688 bit/s (PAL)
            (high speed): 38,908 bit/s (NTSC), 38,553 bit/s (PAL)
   - High speed available in DS/DD 360KiB mode operation only
- Rotation rate: 300 RPM
  Virtually all other Atari-specific drives spin at 288 RPM.  This results in
  rare compatibility issues.  Specifically, these commercial disks do not load
  in, and can possibly be damaged (!) by the XF551:
      - Flight Simulator II (subLOGIC)
      - Blue Max (Synapse) (I have personally destroyed multiple Blue Max
        disks with the XF551 drive!  -mc)
      - Bank Street Writer (Broderbund).  Conflicting reports about this one.
      - Boulder Dash II (Databyte release?)
      - Inside (Spektra, 1990?--original Polish version?)
- Media used is the same as for the Atari 810.

Operating modes      Single    Dual    Double    Double
  Encoding Method      FM       MFM       MFM       MFM
  Number of sides       1         1         1         2
  Total tracks         40        40        40        80
  Sectors per track    18        26        18        18
  Bytes per sector    128       128       256       256    (total capacity)
  Total Bytes      92,160   133,120   184,320   368,640    (total capacity)

The XF551 drive features a horizontal slot with latch bar door mechanism.
To complete the insertion of a disk, turn the latch bar clockwise ninety
degrees.  To release the disk, turn the latch counterclockwise.

XF551 drives were manufactured for Atari in two versions, both made in Japan.
- Mitsumi XF551 (earlier/common production)
   - Disk Busy light is rectangular, and is located below the drive latch.
   - "Straight" drive latch
   - See: http://farm5.static.flickr.com/4052/4669794021_de721aa12d_b.jpg
   - When the drive latch is opened, an inserted disk stays in place until
     removed manually. 
   - Inserting a disk does not cause any drive activity.
   - Like the Atari 810 and 1050, the Mitsumi XF551 ignores the disk's index
     hole/timing hole when reading/writing, so the Mitsumi XF551 CAN read
     from, and write to, the reverse side of a disk in single-sided (90KiB,
     130KiB, or 180KiB) mode if the disk is inserted into the drive upside-
     down ("flippy" disk with second index hole/timing hole not needed).
- Chinon XF551 (later/uncommon production)
   - Disk Busy light is round, and is located directly to the left of the
     drive latch. 
   - "Non-straight" drive-latch
   - http://www.atarimax.com/jindroush.atari.org/data/hardpics/xf551__f.jpg
   - Opening the drive latch causes an inserted disk to be ejected.
   - When a disk is inserted, the drive motor briefly spins and the Disk Busy
     light flashes accordingly.
   - Unlike the Atari 810 and 1050, the Chinon XF551 utilizes the disk's index
     hole/timing hole when reading/writing, so the Chinon XF551 CANNOT read
     from, nor write to, the reverse side of a disk in single-sided (90KiB,
     130KiB, or 180KiB) mode if the disk is inserted into the drive upside-
     down (unless the disk is a "flippy" disk with a second index hole/timing
     hole corresponding to the reverse side).

Unlike the Atari 810 and 1050, the XF551 (both Mitsumi and Chinon) utilizes
the disk's index hole/timing hole when formatting the disk.  As a result, the
XF551 CANNOT format the reverse side of a disk in single-sided (90KiB, 130KiB,
or 180KiB) mode if the disk is inserted into the drive upside-down (unless the
disk is a "flippy" disk with a second index hole/timing hole corresponding to
the reverse side).

The XF551 disk drive is used with an external 9 volt AC transformer power
supply rated for at least 50 watts; shipped with Atari CA017964.

The XF551 shipped with DOS 2.5 (1987-1988) or DOS XE (from 1989).

- Atari DOS 2.5: XF551 Disk Drive Owner's Manual C033537
- Atari DOS XE: XF551 Disk Drive Owner's Manual C300557

The key engineer/designer of the XF551 was Jose Valdes at Atari.

Third-party upgrades for the XF551:

XF35 Kit, by Innovative Concepts, 1988-11-11
- Developed by Bob Woolley
- A standard 3.5" 720KiB drive mechanism (not included with Kit) may replace
   the 5.25" drive of the XF551, or it may be added to the drive making it
   possible to select one or the other using a toggle switch.
- Dual drive option requires several additional parts, not included with Kit
- Kit also includes replacement drive ROM: (thanks Bob Woolley for details)
    - Changes the number of tracks to 80,
    - Tweaks the track seek timing to match a typical 3.5,
    - Adds (or deletes) spin-up delay that improves write reliability, and
    - Provides for density retry.
- XF551 high speed remains available

XF Single Drive Upgrade, by Computer Software Services (CSS), 1991
-- Replace the 5.25" drive mechanism with a 3.5" drive mechanism
-- US Doubler compatible/UltraSpeed capable
-- Shipped with IBM/ST Transfer Utility disk, containing IBMREAD.COM

XF Dual Drive Upgrade, by Computer Software Services (CSS), 1991
- Same as XF Single Drive Upgrade, except keep using the XF551 5.25" drive as
   well.  Result is two disk drives; the new 3.5" drive assumes the next drive
   number higher than the 5.25" drive.
- Shipped with ST/IBM Disk Reader disk, containing IBMREAD.COM

XF551 Enhancer
- By Computer Software Services (CSS)
- Provides a switch that allows the XF551 to read/write/format, in single-
   sided mode, the back side of a disk that has only one index hole/
   timing hole corresponding to the front side of the disk.
- No need for the disk to have a second write protect hole.
- A write protect disable is built in.
- 60Hz version for North America or 50Hz version for Europe

XF Update
- By Computer Software Services (CSS), 1992
- Replacement drive operating system (ROM)
- "Runs more commercial software"
- Changes density properly
- US Doubler compatible/UltraSpeed capable
- Improved/optimized interleave in high speed formats
- Shipped with IBM/ST Transfer Utility disk, containing IBMREAD.COM

XF Speedy
- (c) 1994 Klaus Peters Elektronik + Software
- Converts drive to a Speedy 1050 clone with a DSDD 360KiB mode
- Replaces the 8040 MPU with a 65C02 + ROM + Memory

Hyper-XF  (or HyperXF)
- (c) 1998 Stefan Dorndorf
- Replacement drive operating system (ROM)
- Hyper-XF 1.0A is for original XF551 with 5.25" drive
- Hyper-XF 1.0B is for the XF551 with replacement 3.5" drive installed
- US Doubler compatible/UltraSpeed capable
- "Hyper Speed" is an UltraSpeed support routine in ROM
    - Allows the drive to support UltraSpeed using DOS versions that
      otherwise lack such support
- XF551 standard high speed is removed
- Can use disk partitions (2 on 5.25", 4 partitions on 3.5") with mixed
   Densities (S/E/D) or standard 360KiB (5.25") / 720KiB (3.5")
- Can theoretically read/write ST/PC 720KiB disks (software is missing!)
- Improved double density detection

Reimaged XF551 Disk Drive Controler
- By Bits of the Past (Dropcheck / Lenore Underwood), 2014
- With Modern Power Supply or with Original Power Supply
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