The Disk II Floppy Disk Subsystem, often spelled as Disk ][, was a 5¼-inch floppy disk drive designed by Steve Wozniak and manufactured by Apple Computer. It was first introduced in 1978 at a retail price of US$495 for pre-order; it was later sold for $595 including the controller card (which can control up to two drives) and cable. The Disk II was designed specifically for use with the Apple II personal computer family to replace the slower cassette tape storage and cannot be used with any Macintosh computer without an Apple IIe Card as doing so will damage the drive and/or controller.
Apple produced at least six variants of the basic 5¼-inch Disk II concept over the course of the Apple II series' lifetime: The Disk II, the Disk III, the DuoDisk, the Disk IIc, the UniDisk 5.25" and the Apple 5.25 Drive. While all of these drives look different and they use four different connector types, they're all electronically extremely similar, can all use the same low-level disk format, and are all interchangeable with the use of simple adapters, consisting of no more than two plugs and some wires between them. Most DuoDisk drives, the Disk IIc, the UniDisk 5.25" and the AppleDisk 5.25" even use the same 19-pin D-Sub connector, so they are directly interchangeable. The only 5.25" drive Apple sold aside from the Disk II family was a 360k MFM unit made to allow Mac IIs and SEs to read PC floppy disks.
This is not the case with Apple's 3.5" drives, which use several different disk formats and several different interfaces, electronically quite dissimilar even in models using the same connector, and are not generally interchangeable.
Disk II 
Apple did not originally offer a disk drive for the Apple II, which used data cassette storage like other microcomputers of the time. Mike Markkula asked fellow cofounder Steve Wozniak to design a drive system for the computer after finding that a checkbook-balancing program Markkula had written took too long to load from tape. The first Disk II drives (A2M0003) sold were built using parts from Shugart Drives, but to reduce costs Apple switched to Alps Electric Co. of Japan who built them for half the cost. Early production at Apple was handled by two people, and they produced about thirty drives a day.
Normal storage capacity per disk side was 113.75kiB with Apple DOS 3.2.1 and earlier (256 bytes per sector, 13 sectors per track, 35 tracks per side), or 140kiB with DOS 3.3 and all later Apple II operating systems, and the accompanying ROM update for the controller card (16 sectors per track). The 16-sector upgrade was introduced in 1980, it modified only the software and the controller card firmware to use a more efficient GCR code. Neither the drive itself, nor the controller logic, nor the physical bit density was changed.
Since the Disk II controller was completely software-operated, the user had total control over the encoding and format so long as it was within the physical limits of the drive mechanism and media. Microsoft introduced a CP/M card for the Apple II in 1981 and because of this ability, it was possible for users to read any 5.25" single sided CP/M-formatted floppy disks (including Osborne, Kaypro, and Morrow) despite them using FM/MFM instead of the Apple's standard GCR format. This also allowed software companies to use all sorts of ingenious copy protection schemes.
The Disk II (and all Apple 5.25" drives except the A9M0110) was a single-sided unit with the read/write head on the bottom. However, it was common for users to manually flip the disk to utilize the opposite side, after cutting a second notch on the diskette's protective shell to allow write-access. Most commercial software using more than one disk side was shipped on such "flippy" disks as well. Only one side could be accessed at once, but it did essentially double the capacity of each floppy diskette, an important consideration especially in the early years, when the media was still quite expensive.
In the Disk II, the full-height drive mechanism shipped inside a beige painted metal case and connected to the controller card via a 20-pin ribbon cable; the controller card was plugged into one of the bus slots on the Apple's mainboard. The connector is very easy to misalign on the controller card, which will short out a certain IC in the drive; if later connected correctly, a drive damaged this way will delete any disk inserted into it as soon as it starts spinning, even write-protected disks such as those used to distribute commercial software. This problem was unpleasant both for the users and for Apple, who got many complaints from angry users and had to do a lot of servicing. It was the reasons why Apple soon started printing several fat-print warnings about checking connector alignment in its manuals and used different connectors that could not be misaligned in its later drives. DB-19 adapters for the original Disk II were eventually available for use with Apple's later connector standard.
Up to 14 drives could be attached to one Apple II or Apple IIe computer - two drives per controller card, one card per slot, and there were seven usable slots per computer. While the DOS and ProDOS operating systems worked equally well with the card in any of the normal slots (i.e. all except slot 0 of the Apple II/II+ or the special memory expansion slots of the later models), Apple's printed manuals suggested using slot 6 for the first controller card; most Apple II software expects this slot to be used for the main 5.25" disk drive and fails otherwise. A Bell & Howell version of the Disk II was also manufactured by Apple in a black painted case, which matched the color of the Bell & Howell version of the Apple II Plus, which Apple was already manufacturing.
Disk III 
In 1978, Apple intended to develop its own "FileWare" drive mechanism for use in the new Apple /// and Lisa business computers then being developed. They quickly ran into difficulties with the mechanisms, which precluded them from being incorporated in the Apple ///. That machine thus continued to use the same Shugart design as the Disk II.
The first variation of the Disk II introduced for the Apple ///, called the Disk /// (A3M0004), used the identical drive mechanism inside a modified plastic case with a proprietary connector. With some modification both drives are interchangeable. Though Apple sought to force the purchase of new drives with the Apple ///, many former Apple II users quickly devised a way to adapt their existing and cheaper Disk II drives, however only one external Disk II was supported in this manner. The Disk /// was the first to allow daisy chaining of up to three additional drives to the single 26 pin ribbon cable connector on the Apple ///, for a total of 4 floppy disk drives – the Apple /// was the first Apple to contain a built-in drive mechanism. The Apple III Plus changed its 26-pin connector to a DB-25 connector, which required an adapter for use with the Disk III.
In 1983 Apple finally announced the 860ki "FileWare" drives (UniFile and DuoFile) as a replacement for the Disk II & III, but due to the still-unresolved reliability problems of the underlying Apple-built so-called "Twiggy" drive mechanisms, which saw their only actual use in early Apple Lisas, the products never shipped.
In 1983, along with the introduction of the Apple IIe, Apple initially offered a combination of two, two third-height, 140ki Disk II drive mechanisms side-by-side in a single plastic case, called the DuoDisk 5.25 (A9M0108), which could not be daisy chained. The unit was designed to be stacked on top of the computer, and beneath the monitor. Each unit required its own disk controller card (as each card could still control only two drives) and the number of units was thus limited to the number of available slots; in practice, few uses of the Apple II computer can make good use of more than two 5.25" drives, so this limitation mattered little. Originally released with a DB-25 connector, to match that of the Apple III Plus, it was the first to adopt Apple's standard DB-19 floppy drive connector.
Disk IIc 
The Disk IIc (A2M4050) was a half-height 5.25-inch floppy disk drive introduced by Apple Computer in 1984 styled for use alongside the Apple IIc personal computer, the only Apple II to contain a built-in disk drive mechanism. The disk port on the original IIc was only designed to control one additional, external 5.25-inch disk drive, and as such, this particular drive omitted a daisy-chain port in back. It was possible to use it on other Apple II models, so long as it came last in the chain of drive devices (due to lacking a daisy-chain port); but since the Disk IIc was sold without a controller card, the Apple IIc compter needing none, it had to be adapted to an existing Disk II controller card in this case. Essentially the same as the full-height Disk II, it offered slightly faster access time (which didn't help unless software was specifically written to make use of it; most wasn't). Apple sold the Disk IIc for US$329, and other companies later sold similar drives for less.
In 1984, Apple had opted for the more modern, Sony-designed 3.5" floppy disk form factor in late-model Lisas and the new Apple Macintosh. Accordingly, they attempted to introduce a new 3.5" 800k floppy disk format for the Apple II series as well, to eventually replace the 140k Disk II format. However, the external UniDisk 3.5 drive required a ROM upgrade (for existing Apple IIc machines; new ones shipped after this time had it from the factory) or a new kind of disk controller card (the so-called "Liron Card", for the Apple IIe) to be used. The much larger capacity and higher bitrate of the 3.5" drives made it impractical to use the software-driven Disk II controller because the 1Mhz 6502 CPU in the Apple II line was too slow to be able to read them. Thus a new and much more advanced (and correspondingly expensive) hardware floppy controller had to be used. And many original Apple IIs could not use the new controller card at all without further upgrades. Also, almost all commercial software for the Apple II series continued to be published on 5.25" disks which had a much larger installed base. All of these reasons added up to one thing: the 3.5" format was not widely accepted by Apple II users. The Apple 3.5 Drive used the same 800k format as the UniDisk 3.5", but it did away with the internal computer, which made it cheaper. Unlike all earlier Apple II drives, it was designed to work with the Macintosh too, and among Apple II models, it was compatible only with the Apple IIGS and the Apple IIc+ models, which both had a faster main CPU. On the Apple IIGS, whose improved audiovisual capacities really demanded a higher-capacity disk format as well, the 3.5" format was accepted by users and became the standard format. Though Apple eventually offered a 1.44MB Superdrive with matching controller card for the Apple II series as well, the 5.25" Disk II format drives continued to be offered alongside the newer 3.5" drives and remained the standard on the non-IIgs models until the platform was discontinued in 1993.
Officially, the following 3.5" drives could be used on the Apple II:
- Apple 3.5" External (A9M0106) - Designed for Apple IIs with the Liron or Superdrive controller or all Macintoshes with an external 19-pin floppy port (Mac 512s must be booted from the internal 400k drive with the HD20 INIT, which provides HFS file system support - the Mac 128 will not work with this). The drive can be daisy chained, however this feature is not supported on the Macintosh.
- Unidisk 3.5" Drive (A2M2053) - Designed for Apple IIs with the Liron or Superdrive controller (not compatible with Macintoshes) Recommended only for 8-bit Apple IIs as the A9M0106 operates faster on the IIgs
- Apple FDHD External (G7287) - Supports 720k/1.44MB MFM floppy disks in addition to 800k GCR. Designed for Apple IIs and Macs with the Superdrive controller, but will also work on machines with the older 800k controller (as an 800k drive - note that the G7287 is not compatible with the Mac 128/512)
The 400k and 800k Macintosh external drives (M0130 and M0131) are incompatible with standard Apple II controllers as they do not support their automatic disk eject feature, although they could be used with third-party controllers.
UniDisk 5.25" and Apple 5.25 Drive 
Along with the UniDisk 3.5", Apple introduced the UniDisk 5.25 (A9M0104) in a plastic case, which modernized the appearance of the Disk II to better match the Apple IIe. Since the UniDisk 5.25" could fully replace the Disk II in all its uses, the original Disk II was canceled at this point. This was followed in 1986 by a Platinum-gray version which was renamed Apple 5.25 Drive (A9M0107), companion to the Apple 3.5 Drive, and introduced alongside the first Platinum-colored computer, the Apple IIGS. Essentially these were both single half-height Disk II mechanisms inside an individual drive enclosure, just like the Disk IIc had been. All of these drives introduced a daisy chain pass-through port. While the drives are essentially interchangeable among Apple II computers, both with each other and with the earlier drives, only the Apple 5.25 Drive can be used with the Apple IIe Card on a Macintosh LC.
Apple PC 5.25" Drive 
There is one 5.25 inch drive made by Apple that is completely incompatible with all the drives named above. In 1987 Apple sought to better compete in the IBM dominated business market by offering a means of cross-compatibility. Alongside the release of the Macintosh SE & Macintosh II, Apple released the Apple PC 5.25" Drive which required a separate custom PC 5.25 Floppy Disk Controller Card, different for each Mac model. It is the only 5.25" drive manufactured by Apple exclusively for, and that can be used by, the Macintosh. This drive was for use with industry standard double-sided 5.25" 360K formatted flexible disks. It was similar in appearance to the Disk IIc. Through the use of a special Macintosh Apple File Exchange utility shipped with it, the drive could read files from, and write files to, floppy disks in MS-DOS formats. Software "translators" could convert documents between WordStar and MacWrite, among others. Unfortunately, this drive is not just incompatible with all Apple II computers, but also with the Apple IIe Card for the Macintosh LC; it cannot be used to allow a Macintosh to read from or write to 5.25" Apple II formatted disks either.
This drive was made obsolete by the industry-wide wide adoption of 3.5" disks and was replaced by the 3.5" Apple FDHD Drive, which could not only read and write all then-existing native Macintosh, DOS and Windows formats, but the Apple II ProDOS format as well.
Disk II Cable Pinout 
This table shows the pinout of the original 1979 Disk II controller and newer 1983 Uni/Duo Disk I/O controller (655-0101).
The circuitry of these two controllers are identical. The Disk II header pin numbering is per the Disk II controller card silkscreen and the circuit schematic given in the DOS 3.3 manual. The Uni/Duo Disk D-19 pinout is taken from the Apple //c Reference Manual, Volume 1.
|Disk II Header Pin||DuoDisk/Disk IIc/UniDisk/Apple Disk D-19 Pin||DuoDisk/UniDisk/Apple Disk Controller Card Cable Color||Signal Name||Description|
|1,3,5,7||1,2,3,4||Brown,Orange,Green,Violet||GND||Ground reference and supply|
|2||11||Red||SEEKPH0||Phase 0 stepper motor signal|
|4||12||Yellow||SEEKPH1||Phase 1 stepper motor signal|
|6||13||Blue||SEEKPH2||Phase 2 stepper motor signal|
|8||14||Grey||SEEKPH3||Phase 3 stepper motor signal|
|9||5||White||-12V||-12 volt supply|
|10||15||Black||WRREQ*||Write request signal|
|11,12||6,16||Brown,Red||+5V||+5 volt supply|
|13,15,17,19||7,8||Orange,Green||+12V||+12 volt supply|
|14||17||Yellow||DR2* (ENABLE*)||Drive 2 select/Drive enable signal|
|16||18||Blue||RDDATA||Read data signal|
|18||19||Grey||WRDATA||Write data signal|
|20||10||White||WRPROT||Write protect signal|
- Active low signals are suffixed with a "*"
- Since most signals are shared with both drive 1 and drive 2, the logic in each drive uses the ENABLE* signal to activate appropriately.
- Pin 14 for Disk II drive 1 and drive 2 have separate enable signals (14a and 14b)
- Pin 17 for Uni/Duo Disk is chained to first drive (drive 1) and second drive (drive 2) is enabled via other logic in the first drive.
- The EXTINT* signal is not present on the Disk II controller card. In the Apple //c computer, it is routed to the DSR* signal of the internal 6551 ACIA (UART) chip.
See also 
- Macintosh External Disk Drive
- Apple ProFile External hard disk
- List of Apple drives
- List of products discontinued by Apple Computer
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