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Disk II drives

The Disk II Floppy Disk Subsystem, often rendered as Disk ][, is a 5 +1⁄4-inch floppy disk drive designed by Steve Wozniak at the recommendation of Mike Markkula, and manufactured by Apple Computer. It went on sale in June 1978 at a retail price of US$495 for pre-order; it was later sold for $595 (equivalent to $2,870 in 2024) including the controller card (which can control up to two drives) and cable. The Disk II was designed specifically for use with the 1977 Apple II personal computer to replace the slower cassette tape storage.

Apple produced at least six variants of the basic 5 +1⁄4-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 use four different connector types, they're all electronically extremely similar. They 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 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 +1⁄4" 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 +1⁄2-inch drives, which use several different disk formats and several different interfaces, electronically quite dissimilar even in models using the same connector; they are not generally interchangeable.

History

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Disk II

[edit]
A selection of Apple IIe software, intended to be read through the Disk II system
Disk II atop an original box for the Apple 5.25 Drive

Apple did not originally offer a disk drive for the Apple II, which used data cassette storage like other microcomputers of the time. Apple early investor and executive Mike Markkula asked 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.[1][2] Wozniak knew nothing about disk controllers, but while at Hewlett-Packard he had designed a simple, five-chip circuit to operate a Shugart Associates drive.[1]

The Apple II's lack of a disk drive was "a glaring weakness" in what was otherwise intended to be a polished, professional product. Speaking later, Osborne 1 designer Lee Felsenstein stated, "The difference between cassette and disk systems was the difference between hobbyist devices and a computer. You couldn't have expected, say, VisiCalc, to run on a cassette system." Recognizing that the II needed a disk drive to be taken seriously, Apple set out to develop a disk drive and a DOS to run it. Wozniak spent the 1977 Christmas holidays adapting his controller design, which reduced the number of chips used by a factor of 10 compared to existing controllers. Still lacking a DOS, and with Wozniak inexperienced in operating system design, Steve Jobs approached Shepardson Microsystems with the project. On April 10, 1978 Apple signed a contract for $13,000 with Shepardson to develop the DOS.[3]

Shortly after the disk drive project began in late 1977, Steve Jobs made several trips to Shugart's offices announcing that he wanted a disk drive that would cost just $100. After Wozniak finished studying IBM disk controller designs, Jobs then demanded that Shugart sell them a stripped disk drive that had no controller board, index sensor, load solenoids, or track zero sensor. Although puzzled by this request, Shugart complied and provided Apple with 25 drive mechanisms that they could use as prototypes in developing a disk system for the Apple II. The prototypes received the designation of SA-390.

Wozniak studied North Star Computers and others' more complex floppy controllers. He believed that his simpler design lacked their features, but realized that they were less sophisticated; for example, his could use soft-sectored disks. Following the Shugart controller's manual, Wozniak attempted to develop an FM-type controller with 10 sector per track storage, but realized that Group Coded Recording could fit 13 sectors per track. Wozniak called the resultant Disk II system "my most incredible experience at Apple and the finest job I did", and credited it and VisiCalc with the Apple II's success.[1]

Steve Wozniak found out that some of the SA-390 drives didn't work without adjustments while others simply wouldn't work at all.[citation needed] Fellow engineer Cliff Huston came up with several procedures for resuscitating the faulty drives on the assembly line. When Apple sent an order into Shugart for more SA-390s, a Shugart engineer admitted that the disk drive manufacturer had played a trick on Apple and that the SA-390s were actually rejected SA-400s that had failed to pass factory inspection. The idea was that Apple couldn't get the drives to work and would be forced to go back and purchase more expensive SA-400s.

The Disk II was very successful for Apple, being the cheapest floppy disk system ever sold up to that point and immensely profitable for the company, in addition to having nearly 20% more storage space than standard FM drives. For a while, the only direct competitor in the microcomputer industry was the TRS-80 Model I, which had used only standard FM storage for 85k.[4] The Atari 8-bit computer's disk drives' throughputs were much slower than the Disk II's 15 KB/s, seriously affecting their ability to compete in the business market. However, the advantage of Wozniak's design was somewhat nullified when the cost of double-density MFM controllers dropped only a year after the Disk II's introduction.

The initial Disk II drives (A2M0003) were modifications of the Shugart SA-400, which was the first commercially available 5+1⁄4-inch diskette drive. Apple purchased only the bare drive mechanisms without the standard SA-400 controller board, replaced it with Wozniak's board design, and then stamped the Apple rainbow logo onto the faceplate. Early production at Apple was handled by two people, and they assembled 30 drives a day. By 1982, Apple switched to Alps drives for cost reasons.[5][6]

Normal storage capacity per disk side was 113.75 KiB with Apple DOS 3.2.1 and earlier (256 bytes per sector, 13 sectors per track, 35 tracks per side), or 140 KiB with DOS 3.3 and ProDOS (256 bytes per sector, 16 sectors per track, 35 tracks per side). The 16-sector hardware upgrade introduced in 1980 for use with DOS 3.3 modified only the controller card firmware to use a more efficient GCR code called "6 and 2 encoding". Neither the drive itself nor the physical bit density was changed. This update had the disadvantage of not automatically booting older 13 sector Apple II software.

The stock SA-400 drives had a head load solenoid that would lift the read/write heads from contact with the disk when it was not being accessed; this was a feature inherited from 8" drives which had a continuously-running stepper motor powered off the A/C line, causing the disk to be rotating nonstop as long as the drive door was closed. Wozniak decided this was unnecessary as the SA-400 mechanism did not operate the motor except during disk access and there was minimal risk of wear to the disk media. As the Disk II proved that head load solenoids were not needed on 5.25" drives, most drive manufacturers subsequently omitted them.

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. This also allowed software companies to use all sorts of ingenious copy protection schemes.

The Shugart SA-400, from which the Disk II was adapted, was a single-sided, 35-track drive. 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 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 data from 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 resulted in numerous customer complaints and repairs, which led to Apple printing warning messages in their user's manuals to explain how to properly install the connector. They used different connectors that could not be misaligned in 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+1⁄4-inch 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

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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.[7]

The first variation of the Disk II introduced for the Apple ///, called the Disk III (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 III 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.

FileWare

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In 1983, Apple finally announced a single and dual external drive (UniFile and DuoFile) implementing the 871-kilobyte "FileWare" mechanism used in the original Apple Lisa, as a replacement for the Disk II & /// drives. However, due to the reliability problems of the Apple-built "Twiggy" drive mechanisms, the products never shipped.[8][9]

DuoDisk

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In 1984, shortly after the introduction of the Apple IIe the previous year, Apple offered a combination of two, two third-height, 140-kilobyte Disk II drive mechanisms side-by-side in a single plastic case, called the DuoDisk (A9M0108), which can not be daisy-chained. The unit was designed to be stacked on top of the computer, and beneath the monitor. Each unit requires its own disk controller card (as each card can still control only two drives) and the number of units is 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+1⁄4-inch drives, so this limitation matters 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

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Disk IIc drive

The Disk IIc (A2M4050) was a half-height 5+1⁄4-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 5+1⁄4-inch built-in disk drive mechanism. The disk port on the original IIc was only designed to control one additional, external 5+1⁄4-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 computer 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, Apple sold the Disk IIc for US$329, and other companies later sold similar drives for less.[10]

UniDisk and Apple 5.25 Drive

[edit]

Just over a year after the release of the DuoDisk, Apple introduced the UniDisk (A9M0104) in a plastic case whose styling was similar to that of the DuoDisk. After the release of the UniDisk 3.5 a few months later, the UniDisk was often informally referred to as the "UniDisk 5.25" to distinguish the similarly-named drives.

Since the UniDisk could fully replace the Disk II in all its uses, the original Disk II was discontinued at this point.

In 1986 a Platinum-gray version of the drive named the Apple 5.25 Drive (A9M0107) was introduced alongside the first Platinum-colored computer, the Apple IIGS. The drive's name was similar to that of the Apple 3.5 Drive, also released with the IIGS.

Like the DuoDisk and Disk IIc before them, the UniDisk and Apple 5.25 Drive were half-height disk mechanisms inside an individual drive enclosure. Both drives featured a daisy chain pass-through port. While the drives were essentially interchangeable among Apple II computers, both with each other and with the earlier drives, minor electrical differences dictated that only the Apple 5.25 Drive could be used with the later (ROM 3) version of the Apple IIGS and with the Apple IIe Card on a Macintosh LC.

3+1⁄2-inch drive

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In 1984, Apple had opted for the more modern, Sony-designed 3+1⁄2-inch floppy disk in late-model Lisas and the new Apple Macintosh. Accordingly, they attempted to introduce a new 3+1⁄2-inch 800-kilobyte floppy disk format for the Apple II series as well, to eventually replace the 140-kilobyte 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+1⁄2-inch drives made it impractical to use the software-driven Disk II controller because the 1-megahertz 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+1⁄4-inch disks which had a much larger installed base. For these reasons the 3+1⁄2-inch format was not widely accepted by Apple II users. The Apple 3.5 Drive used the same 800-kilobyte format as the UniDisk 3.5", but it did away with the internal controller, 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+1⁄2-inch format was accepted by users and became the standard format. Though Apple eventually offered a 1.44-megabyte SuperDrive with matching controller card for the Apple II series as well, the 5+1⁄4-inch Disk II format drives continued to be offered alongside the newer 3+1⁄2-inch drives and remained the standard on the non-IIGS models until the platform was discontinued in 1993.

Officially, the following 3+1⁄2-inch 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 400-kilobyte drive with the HD20 INIT, which provides HFS file system support – the Macintosh 128K 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 720-kilobyte/1.44-megabyte MFM floppy disks in addition to 800-kilobyte GCR. Designed for Apple IIs and Macs with the Superdrive controller, but will also work on machines with the older 800-kilobyte controller (as an 800-kilobyte drive – note that the G7287 is not compatible with the Mac 128/512)

The 400-kilobyte and 800-kilobyte Macintosh external drives (M0130 and M0131) are incompatible with standard Apple II controllers as they do not support the drives' automatic disk-eject feature, although they could be used with third-party controllers.

Apple PC 5.25" Drive

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Apple PC 5.25" Drive

There is one 5+1⁄4-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+1⁄4-inch drive manufactured by Apple that can be used by the Macintosh.

This drive was for use with industry standard double-sided 5+1⁄4-inch 360-kilobyte 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 formats, among others.[11] The drive is incompatible with all Apple II computers and the Apple IIe Card for the Macintosh LC as well; it also does not allow a Macintosh to read from or write to 5+1⁄4-inch Apple II-formatted disks.

This drive was made obsolete by the industry-wide adoption of 3+1⁄2-inch disks and was replaced by the 3+1⁄2-inch Apple FDHD Drive, which could read and write every existing Macintosh, DOS and Windows format, and the Apple II ProDOS format as well.

Disk II cable pinout

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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.[12]

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
x 9 Violet EXINT* External interrupt

NOTES:

  • 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

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Disk II

View on Grokipedia
from Grokipedia
The Disk II was a 5.25-inch floppy disk drive and controller interface developed by Apple Computer specifically for the Apple II personal computer, serving as its primary mass storage solution after the initial cassette tape system. Introduced in 1978, it replaced the slow and unreliable audio cassette storage that limited the Apple II's practicality for software distribution and data handling. Designed by Steve Wozniak with assistance from Randy Wigginton, the system featured an external drive connected via a single expansion slot card, enabling formatted capacities of 113 kilobytes per single-sided disk—23 kilobytes more than competing drives at the time. Development of the Disk II began in December 1977 at the urging of Apple co-founder , who recognized the need for faster storage to expand the Apple II's market beyond hobbyists. Wozniak completed the prototype by January 1978, leveraging a SA-400 drive mechanism initially, which was later replaced by cheaper Alps Electric clones to reduce costs from $495 at launch. The controller card's innovative design used only eight integrated circuits, including a single 8-bit for soft-sectored formatting, which allowed flexible track layouts without dedicated hardware for hard sectors. Technically, the Disk II employed Group Code Recording (GCR), a five-bit encoding scheme that increased sectors per track from the standard 10 to 13, enhancing data efficiency on the 5.25-inch disks rotating at 300 RPM. This minimalistic approach not only minimized manufacturing costs—totaling around $140 in components—but also set a benchmark for engineering elegance in early personal computing peripherals. Over the Apple II's lifespan, the Disk II evolved into several variants, with later successors including 3.5-inch UniDisk models, but the original remained iconic for its role in enabling DOS 3.1 and subsequent operating systems. The Disk II's introduction was pivotal in transforming the Apple II from a niche hobbyist machine into a dominant force in the personal computer market, driving sales surges and establishing Apple's high-margin peripheral strategy. By providing reliable, high-capacity storage at an accessible price, it facilitated the growth of commercial software ecosystems, including productivity tools and games, and helped Apple achieve temporary market leadership in the late 1970s and early 1980s.

Design and Technology

Controller Card

The Disk II controller card, designed by in late 1977, was a groundbreaking hardware solution that interfaced the Apple II computer with 5.25-inch drives using minimal components for cost efficiency and performance. Unlike contemporary controllers that relied on dozens of chips for full hardware-based encoding, decoding, and , Wozniak's employed just eight off-the-shelf integrated circuits: two 256-byte PROMs (one for bootstrap code and one for the state machine), a 74LS174 hex flip-flop, a 74LS323 eight-bit , a 74LS259 addressable , a 556 dual timer, a 74LS05 hex inverter, and a 74LS132 quad . This architecture shifted much of the data processing burden to software running on the Apple II's 6502 processor, enabling reliable operation at a of one bit every 4 microseconds while using the Group Code Recording (GCR) format for higher storage density—initially 5-and-3 encoding for 13 sectors per track ( 3.2) and later upgraded to 6-and-2 for 16 sectors ( 3.3, yielding 140 KB per disk). The design's ingenuity lay in its software-driven approach, where the CPU handled tasks like head stepping and data framing, outperforming more expensive competitors that required dedicated hardware for these functions. The controller card connected to the Apple II motherboard via a standard 50-pin edge connector in one of the expansion slots (typically Slot 6), accessing the system's data bus (pins P42-P49 for D0-D7), address bus (pins P2-P17 for A0-A15), and control signals such as I/O strobe (P20), read/write (P18), and clock phases (P38 for PHI1, P40 for PHI0) to synchronize operations. For drive interfacing, it featured two 20-pin ribbon cable connectors (for up to two drives), with signal handling optimized for direct control of the Shugart SA-400 drive mechanism. Key signals included power distribution (+12V on pins 7-8, +5V on pins 6 and 16, -12V on pin 5, and grounds on pins 1-4 and 9-10), read data (pin 18, RDDATA for raw flux transitions from disk rotation), write data (pin 19, WRDATA for serialized output), write protect (pin 20, WRPROT), and drive select (pin 14, active low to enable a specific drive). Head movement was managed via stepper motor phase signals on pins 11-14 (PH0-PH3), which the controller generated as precise pulses to position the read/write head across 35 tracks. These assignments ensured low-latency signal propagation without additional buffering, leveraging the card's custom logic to interpret bus commands and drive the peripherals directly. Central to the controller's operation was its role in generating and synchronizing timing signals for disk and head movement, achieved through the 556 dual timer and state machine . Disk , driven by a at 300 RPM, produced transitions on the media that the controller captured via the read head as asynchronous pulses every 32 microseconds on average (4 µs per bit cell). The state machine framed these into bytes by detecting gaps longer than 6 µs (inserting logical zeros) and aligning data within 4 µs windows, feeding serialized bits to the CPU for GCR decoding without a dedicated . For head movement, the controller output phased pulses on PH0-PH3 to the stepper motor coils, typically in a 4-phase (e.g., , , 0110, 0011) at 2-4 ms intervals per step, allowing track seeking in under 200 ms across the full 35 tracks. Write operations reversed this , with the CPU providing GCR-encoded bytes that the serialized into timed pulses, ensuring precise placement during . This hardware-software synergy minimized and errors, supporting reliable data rates up to 250 Kbps. To facilitate and further reduce costs, Wozniak modified the initial by stripping control electronics from the Shugart SA-400 drive unit, relocating motor and head logic to the controller card itself—a move that eliminated redundant components and dropped the per-unit price below $150. Custom logic in the PROMs and TTL gates handled drive selection and power sequencing without extra chips, while the addressable latch managed auxiliary signals like motor-on (tied to +12V enable). A key production tweak involved reprogramming the state machine PROM to permit two consecutive zero bits in the GCR stream, enabling the denser 6-and-2 encoding for DOS 3.3 and boosting capacity by 23% without hardware changes. These optimizations, combined with the use of inexpensive TTL parts available from distributors like , made the Disk II subsystem affordable for consumers and scalable for Apple's early market dominance.

Drive Mechanism

The Disk II floppy drive unit was based on a customized version of the SA-390 mechanism, a variant of the SA-400 minifloppy drive adapted for Apple's needs by removing certain sensors to simplify integration with the custom controller card. This electromechanical assembly utilized single-sided 5.25-inch floppy disks with a formatted capacity of 140 KB per disk under standard Apple DOS 3.3 configuration. The drive employed a belt-driven DC spindle motor to rotate the disk at a constant 300 RPM, ensuring stable media speed without the need for servo feedback. Head positioning was handled by a 4-phase stepper motor mounted on a carriage assembly, which advanced the head in precise 2-step increments per track for accurate alignment across the disk surface; this servo-less design relied on open-loop control from the host system, with no dedicated track-zero sensor or closed-loop velocity feedback. Synchronization during read and write operations is managed entirely by software on the Apple II, relying on the disk's constant rotation speed of 300 RPM and self-clocking data patterns encoded on the media, without an index hole sensor. The read/write head was a single-element ceramic assembly with an integrated straddle erase feature, featuring a narrow 0.012-inch track width to support a soft-sectored layout of 35 concentric tracks (numbered 0–34) per side at 48 tracks per inch . The head moved linearly via the , allowing access to each track in approximately 40 milliseconds, while the soft-sectored format enabled flexible sectoring defined by software rather than physical holes. Power for the drive mechanism was drawn from the host Apple II's through the controller card interface, requiring +5 VDC (±0.25 V) at up to 0.9 A for logic and motor control, and +12 VDC (±0.6 V) at up to 1.8 A primarily for the spindle and motors. The enclosure consisted of a compact plastic cabinet measuring approximately 3.25 inches high, 5.75 inches wide, and 8 inches deep, housing the SA-390 mechanism on a base casting with a spring-loaded disk clamp and a front-loading slot protected by a metal flap. The drive connects to one of the controller card's two 20-pin ports, enabling a total of two drives.

Interface and Connectivity

The Disk II interface employs a 20-pin flat with insulation displacement connectors (IDC) to link the floppy drive to the controller card installed in an expansion slot. This cable carries power supplies, ground references, control signals, and bidirectional data lines essential for reading and writing operations. The controller card generates key control signals, such as motor on and head load, transmitted over this connection to manage drive activity. The following table details the standard pinout for the 20-pin connector, viewed from the cable side with pin 1 typically marked by a red stripe on the :
PinSignalDescription
1GNDGround reference
2Ph 0 phase 0
3GNDGround reference
4Ph 1 phase 1
5GNDGround reference
6Ph 2 phase 2
7GNDGround reference
8Ph 3 phase 3
9-12 VNegative 12-volt supply for head
10WRREQWrite request
11+5 VPositive 5-volt supply for logic
12+5 VPositive 5-volt supply for logic
13+12 VPositive 12-volt supply for motors
14DRVENDrive enable (active low)
15+12 VPositive 12-volt supply for motors
16RDDATARead data (TTL input to controller)
17+12 VPositive 12-volt supply for motors
18WRDATAWrite data (TTL output from controller)
19+12 VPositive 12-volt supply for motors
20WRPROTWrite protect sense (high if protected)
Power is supplied via multiple lines: +12 V (up to 1.5 A total from the Apple II power supply) powers the spindle and stepper motors, +5 V (logic and electronics), and -12 V supports head erasing functions. The Disk II controller card features two identical 20-pin connectors, enabling direct connection of up to two drives without true daisy-chaining through the drives themselves. Drive selection occurs via the DRVEN signal on pin 14, which the controller asserts low for the active drive based on software commands; no onboard jumpers or modifications are needed on the drives, as they function interchangeably. Control and data signals operate at TTL voltage levels (0 V low, 5 V high), ensuring compatibility with the Apple II's 6502-based . The read data line (RDDATA) delivers serialized flux transitions from the drive head, while the write data line (WRDATA) receives clocked pulses from the controller; timing aligns with the drive's 300 RPM rotation and Apple's Group Code Recording (GCR) scheme, yielding effective data transfers at approximately 156 kB/s without requiring user adjustment. Common connectivity problems arise from physical cable issues, such as bent or pushed-in pins on the IDC connectors, which disrupt power or signal paths and often require careful straightening or replacement. Incorrect cable orientation—failing to align the with pin 1—can invert signals and prevent operation, while occasional user attempts to twist the cable for drive identification (inspired by other systems) introduce errors, as selection is controller-managed.

Operation and Usage

Software Integration

The Disk II integrated seamlessly with , the primary operating system for the , by storing the DOS image across tracks 0 through 2 and utilizing a structured process initiated from track 0. Upon power-on or reset, the controller card's ROM automatically seeks to track 0 and reads sector 0 into at $0800, loading the initial loader (Boot 1); this routine then sequentially loads additional sectors from track 0 (sectors 1-9) and track 1 to bring in the Read/Write Track Sector (RWTS) routines and further code (Boot 2), ultimately relocating DOS into main starting at $9600 for execution. The (VTOC) and catalog structures reside on track 17, sector 0, mapping free sectors and listing file entries with their track/sector (T/S) lists for efficient allocation and retrieval. This integration enabled random-access storage, a significant advancement over the sequential, slow-loading cassette tapes previously used for data persistence, allowing direct addressing of any sector without rewinding or fast-forwarding. DOS 3.3 commands facilitated user-friendly disk operations, such as CATALOG to display the directory from the catalog track, and COPY to duplicate files by reading from a source track/sector list and writing to a destination via the routines. The Disk II's software support extended to third-party applications, most notably VisiCalc, the pioneering program released in 1979 that required the drive for file saving and loading, leveraging Apple DOS file management to handle worksheets up to 254 rows by 63 columns. Development tools, including assemblers like and Applesoft BASIC compilers, relied on the drive for storing source code, object files, and executables, with utilities such as the Apple Toolkit enabling disk-based assembly and linking processes. Low-level disk I/O was managed by routines embedded in the controller card's 256-byte ROM, located at addresses C600−C600-C6FA, which handled track seeking, sector reading/writing with verification, and phase control for the . Programs accessed these via the RWTS vector at $3D9 for sector operations (e.g., command 1 for read, 2 for write) and the entry at $3D6 for higher-level calls like OPEN, READ, and WRITE, using parameter blocks to specify track, sector, and buffer locations.

Data Storage and Formatting

The Disk II employed soft-sectored formatting, where sector boundaries were defined by software rather than physical markers on the disk media, allowing for flexible organization of data. Each sector contained 256 bytes of logical data, encoded using Group Code Recording (GCR) to achieve higher storage density on the 5.25-inch floppy disks. GCR converted 8-bit bytes into variable-length groups of bits (typically 5-and-3 or 6-and-2 encoding schemes), enabling efficient use of the disk surface while incorporating self-synchronizing properties for reliable reading. Tracks on a Disk II disk were organized into 35 concentric circles, numbered from 0 (outermost) to 34 (innermost), with each track divided into 16 sectors under DOS 3.3 formatting. The structure of each sector included an address field followed by a field, separated by gaps filled with self-sync bytes to aid in locating fields during rotation. The address field consisted of a (bytes D5,D5, AA, $96), identifiers for volume, track, and sector (each one byte), a byte (computed as the exclusive-OR of the three identifiers), and an (DE,DE, AA, EB);thisfield,encodedin4−and−4GCR,spanned512bytesondisk.Thesubsequent[data](/page/Data)fieldfeatureda[prologue](/page/Prologue)(EB); this field, encoded in 4-and-4 GCR, spanned 512 bytes on disk. The subsequent [data](/page/Data) field featured a [prologue](/page/Prologue) (D5, AA,AA, AD), 256 bytes of user encoded in 6-and-2 GCR (resulting in 342 bytes on disk), a two-byte (exclusive-OR of paired data bytes), and the same ; gaps between these elements varied by track position to optimize space. Disk initialization was performed using the INIT command in Apple DOS 3.3, which formatted the media by writing and fields to all tracks while progressively reducing gap sizes to fit the maximum number of sectors without overlap. This process began with oversized gaps and iteratively shrank them (down to a minimum of five self-sync bytes) until the track layout stabilized, ensuring seamless continuity as the disk rotated. detection relied on the checksums in both address and data fields, which verified integrity via exclusive-OR calculations; if a mismatch occurred, the system reported an error, prompting retries or manual intervention, though no built-in correction beyond detection was provided. The formatted capacity of a single side reached approximately 140 KB (precisely 143,360 bytes across 35 tracks and 16 sectors), though usable space was slightly less after accounting for DOS overhead on tracks 0–2, the Volume Table of Contents, and catalog. To enhance read efficiency during sequential access, sectors were interleaved on each track—for example, physical sectors 0, 8, 1, 9, and so on—allowing the drive head to position for the next sector while the disk rotated, minimizing wait times without requiring hardware changes. This interleaving, combined with GCR's density advantages, made the Disk II a efficient storage solution for its era.

Compatibility Considerations

The Disk II controller card requires installation in one of the Apple II's peripheral slots, with slot 6 serving as the standard and recommended location to ensure compatibility with the majority of software that assumes this configuration. The card functions in any slot from 1 to 7 on the original and , though the boot ROM defaults to probing slot 6 for initialization and startup. In the enhanced , revised firmware enables automatic configuration by scanning slots 7, 6, and 5 in sequence for a bootable upon power-on, improving flexibility without manual slot specification. Compatibility can be affected by add-on cards that share bus resources, potentially causing conflicts if installed in overlapping slots or with incompatible timing; for instance, placing the Disk II controller in slot 3 may interfere with an 80-column card, while slots 5 and 6 are preferred for disk controllers to minimize such issues. International variants like the Europlus require careful card installation to avoid bus contention, as the modified keyboard layout and connector can exacerbate signal interference with expansion cards in lower slots. For use with non-Apple systems, adapters such as the USB floppy controller enable connection of the Disk II drive to modern PCs, facilitating archival and transfer of 5.25-inch disks via flux imaging without native Apple hardware. Similarly, the KryoFlux interface supports reading and writing Disk II-compatible media on contemporary computers through specialized software that handles Group Code Recording (GCR) formats. Emulation in software like AppleWin replicates the Disk II interface fully, supporting files (.dsk, .2mg) for running original media on Windows systems without physical hardware. Backward compatibility with upgraded drives, such as the UniDisk 5.25 and AppleDisk 5.25, generally allows reading of original Disk II media due to shared 19-pin connectors and single-sided double-density formatting, though some aged disks may exhibit read errors from head alignment drift or media degradation not resolvable on newer mechanisms. In cases of persistent issues, original Disk II drives remain preferable for verifying and recovering early 13-sector formatted disks from earlier DOS versions such as 3.1 and 3.2.

Historical Development

Origins and Introduction

The Disk II floppy disk drive was developed by in late 1977 as a direct response to the limitations of storage on the computer, which was frustratingly slow and unreliable for loading programs and data. Prompted by Apple co-founder , Wozniak prototyped the drive using minimal components, including adapted Shugart SA390 mechanisms, to achieve a production cost under $150 while maintaining high performance. His design emphasized simplicity and affordability, hand-crafting the initial controller card over just a few weeks to prepare for demonstration at the Consumer Electronics Show (CES) in January 1978. A key innovation was the software-driven controller, which offloaded much of the disk access logic to the Apple II's processor, drastically reducing the hardware parts count from around 50 integrated circuits in competing designs to just 7-8 ICs, including a low-cost and TTL chips. This approach not only cut costs but also introduced group code recording (GCR) and soft-sectoring, enabling higher data density and faster operation compared to standard Shugart drives. Early prototypes were rigorously tested with programs, allowing quick loading times that demonstrated the drive's potential to transform user workflows. The Disk II was officially released in June 1978, bundled with systems to enhance their appeal, and initially priced at $495 for pre-orders (later $595 including the controller card and cable). Its debut significantly boosted personal computing by enabling rapid program execution and , shifting the from a hobbyist tool to a viable platform for and applications.

Production and Market Impact

Mass production of the Disk II began in mid-1978, with initial assembly rates of approximately 30 drives per day handled by a small team at Apple's facilities in the Bay Area. The drive utilized modified Shugart SA390 mechanisms, later supplemented by lower-cost clones from Alps Electric, which helped reduce component expenses from around $140 to $80 per unit while maintaining retail prices of $495 for preorders and $595 thereafter, including the controller card. This high-margin strategy underscored Apple's early and facilitated broader . By the end of 1980, Disk II sales had aligned with Apple's shipment of more than 100,000 systems cumulatively since 1977, driving substantial revenue growth for the company. Cost efficiencies in production enabled the bundling of the Disk II with the , launched in 1979 with enhanced memory and built-in , creating more affordable complete systems that accelerated adoption in the home and educational computing markets. This integration transformed the Apple II from a hobbyist tool into a mainstream , contributing to Apple's sales doubling to 78,000 units in fiscal year 1980 alone. The Disk II significantly fueled the software industry boom around the Apple II ecosystem by providing reliable, random-access storage that supported the development and distribution of complex applications. Notable examples include the productivity-focused word processor AppleWriter, released in 1979, and entertainment software such as the action game Choplifter, released in 1982, which leveraged the drive's capabilities for immersive gameplay and data persistence. These advancements helped establish the Apple II as a platform for third-party innovation, expanding its user base beyond initial enthusiasts. Despite its successes, the Disk II drew criticisms for reliability concerns, particularly with head alignment in the original Shugart SA390 drives, which could lead to read/write failures and required manual adjustments using specialized procedures. Apple's support policies, including one-year warranties and engineer-developed guides, mitigated some issues but were often viewed as inadequate for non-technical users, prompting third-party service solutions in the early 1980s.

Evolution to Later Models

Following the initial success of the Disk II, Apple introduced revisions to the controller card in 1980 alongside DOS 3.3, which supported 16 sectors per track for increased capacity up to 140 KB per disk and enhanced error correction for greater reliability over the original 13-sector format. The Disk II's soft-sectored design allowed existing hardware to benefit from this improved performance via a simple ROM replacement on the controller, facilitating the transition from DOS 3.1 without requiring new drives. In the same year, Apple extended the Disk II interface principles to the Disk III controller, specifically designed for the business computer as an optional SASI-based hard disk interface supporting the 5 MB ProFile drive. This marked Apple's first foray into rigid media storage, using a parallel bus similar in logic to the Disk II's serial protocol but adapted for higher-capacity Winchester drives, enabling up to 5 MB of non-removable storage formatted under the Sophisticated Operating System (). In 1981, Apple bridged floppy and hard disk technologies with the ProFile hard drive, a 5 MB SASI device that leveraged the Disk III controller's interface logic for compatibility with the Apple II series and Lisa, providing a formatted capacity of approximately 4.98 MB and facilitating faster data access for professional applications. This evolution emphasized seamless integration within Apple's , using daisy-chainable parallel connections to support multiple units while maintaining with Disk II software environments through updated drivers. The progression culminated in the 1984 DuoDisk for the , featuring dual single-sided 5.25-inch drives with an enhanced controller card that supported stepping for up to 140 KB per disk, allowing a total of 280 KB across two disks. The new drive mechanism in the DuoDisk improved reliability by incorporating foolproof connectors to prevent damage from improper insertion and reduced operational noise compared to the original Disk II's louder design. Priced at $795, it streamlined workflows by allowing simultaneous read/write operations across two drives, solidifying the Disk II lineage as a foundational element of Apple's storage architecture before the shift to 3.5-inch formats.

Variants and Successors

Internal and SASI-Based Models

The ProFile, Apple's inaugural hard drive kit released in 1981, provided 5 MB of storage capacity and was initially developed for the but adapted for the via a dedicated interface card. Adapting it for internal installation in the necessitated extensive disassembly of the computer's metal case to mount the drive and card, along with modifications to the original 40 W power supply to deliver additional +5 V and +12 V lines for the drive's operation, as the stock unit lacked sufficient output for such peripherals. Cooling was a critical concern, as the sealed mechanism generated significant heat in the confined Apple II ; users typically added muffin fans or ventilation upgrades to prevent overheating and ensure reliability. The ProFile's controller extended Disk II technology by emulating sector-based access patterns familiar to Apple DOS, enabling seamless integration with existing floppy software while leveraging the drive's superior performance, including a raw data transfer rate of 5 Mbit/s (approximately 625 KB/s), compared to the Disk II's 250 kbit/s (31.25 KB/s). In 1984, Apple introduced the Widget drive for the Lisa 2 series, a 10 MB 3.5-inch internal hard drive targeted at the Lisa platform. Integrated directly into the Lisa's chassis, the Widget used a custom controller that supported Disk II for and file operations, allowing legacy software to run without major rewrites on the new storage medium. Like the ProFile, its installation in upgraded systems involved power supply adjustments for stable voltage delivery and enhanced cooling to mitigate thermal buildup in the compact form factor. SASI-based models built on these foundations by adopting the System Interface for broader compatibility and scalability in systems. The SASI controller, launched around 1982, enabled 5 MB drives to connect via an internal slot card, requiring similar power and cooling modifications as earlier kits while offering plug-and-play sector emulation for Disk II-formatted volumes. By 1985, the First Class Peripherals Sider series popularized SASI for users, delivering 10 MB or 20 MB capacities with sustained transfer rates of up to 300 KB/s—far exceeding the Disk II—through efficient DMA transfers and ProDOS support, which preserved floppy compatibility. These systems prioritized internal card installation for minimal footprint, though full internal drive mounting still demanded case disassembly and PSU enhancements to handle the increased power draw (up to 20 W additional). Evolution from the original Disk II controller involved shifting to SASI's parallel protocol for higher throughput while retaining software-level .

Compact and Dual-Drive Versions

In 1984, Apple introduced the Disk IIc as the built-in 5.25-inch drive for the portable computer, marking a compact revision of the original Disk II design. This integrated drive featured a low-profile to accommodate the IIc's slim form factor while retaining the standard 140 KB storage capacity per single-sided disk. The Disk IIc connected directly to the IIc's onboard disk controller, enabling seamless operation without external cabling, and was compatible with existing Apple II software formats like DOS 3.3 and ProDOS. Also launched in 1984, the DuoDisk represented Apple's first dual-drive 5.25-inch external floppy system, building on the for enhanced storage efficiency in Apple IIe and IIc setups. Housed in a single beige enclosure, it incorporated two independent drives—designated as Drive 1 (left) and Drive 2 (right)—each offering 143 KB capacity on single-sided disks, for a combined total of up to 286 KB. The unit utilized a shared analog controller card with foolproof cabling to prevent common connection errors seen in earlier single-drive configurations, and it supported daisy-chaining up to four additional drives for expanded setups. Compatible with the broader family (excluding direct IIc connection without accessories), the DuoDisk shared mechanical components with the UniDisk 5.25 Drive, including stepper motors for precise head positioning. Priced at a suggested retail of $729, the DuoDisk was often bundled in business-oriented systems to support dual-disk operations for applications like Pascal programming and , thereby improving for professional users. An external variant of the Disk IIc was also available for $329, providing supplementary storage for IIc owners needing a second drive. Both models maintained with Disk II media and controllers, facilitating a smooth transition in mid-1980s Apple ecosystems.

3.5-Inch and PC-Compatible Drives

In the late 1980s, Apple shifted toward smaller form factors and cross-platform compatibility in its floppy drive lineup, culminating in 3.5-inch models and adaptations for PC ecosystems. The UniDisk 3.5, released in 1986 alongside the Apple IIGS, represented this transition as Apple's inaugural 3.5-inch drive for the Apple II series. This external, double-sided drive provided a formatted capacity of 800 kilobytes per disk, significantly surpassing the 140-kilobyte limit of earlier 5.25-inch Disk II media, while drawing power solely from the host computer. Designed for the IIGS's enhanced architecture, it incorporated a built-in 6502-based controller to interface via the SmartPort protocol, ensuring seamless integration without requiring additional cards on compatible systems like the IIc or IIe with accessory kits. The UniDisk 3.5 employed variable-speed motor control to support both Apple's Group Code Recording (GCR) encoding for high-density 800K disks and (MFM) for lower-density PC-compatible formats, enabling read/write operations across types at optimized data rates up to 500 kilobits per second. This adaptability addressed the IIGS's dual role in and emerging applications, where larger storage was essential for software like graphics tools and productivity suites. Access times were efficient, with track-to-track seeks at 6 milliseconds maximum and settle times at 30 milliseconds, facilitating faster file transfers compared to legacy 5.25-inch drives. Complementing the 3.5-inch shift, the Apple 5.25 Drive, introduced in 1988 for Macintosh SE and II systems, extended to Disk II-era media in a refined 5.25-inch package. This enhanced single- or double-sided drive supported Apple's GCR format at 140K or 280K capacities, allowing Macintosh users to access vast libraries of software and data without conversion, via a dedicated controller card. Its daisy-chainable design and 300 RPM constant speed maintained reliability for archival tasks, bridging the gap between Apple's proprietary ecosystems during the transition to 3.5-inch dominance. To foster interoperability with PC environments, Apple launched the Apple PC 5.25" Drive in the late 1980s, targeted at Macintosh users needing support. This 360K double-sided drive, paired with a custom PC 5.25 Floppy Disk Controller Card, enabled direct reading and writing of MFM-encoded PC disks, using a simplified controller architecture inspired by the bit-banged design of the original for efficient data handling on NuBus-equipped Macs. Priced accessibly at around $295, it facilitated file transfers in business settings, where mixed-platform workflows were common, without compromising Macintosh's native 3.5-inch operations. Supporting these format evolutions, Apple developed transition drivers and utilities to manage mixed media scenarios. The Apple II 3.5 Disk Controller Card, for instance, included software drivers for ProDOS and GS/OS, allowing Apple II systems to format and access both 800K GCR disks and 720K/1.44MB MFM disks interchangeably. Complementary tools like the Disk Copy utility and format initializers enabled users to initialize hybrid volumes, preserving during migrations from 5.25-inch Disk II stocks to 3.5-inch standards. These resources were crucial for educators and developers adapting legacy content to newer hardware.

References

  1. https://www.apple2history.org/history/ah05/
  2. https://www.storagenewsletter.com/2019/02/18/history-1977-apple-ii-gets-first-disk-drive/
  3. https://collections.museumsvictoria.com.au/articles/2787
  4. https://www.bigmessowires.com/2021/11/12/the-amazing-disk-ii-controller-card/
  5. https://mirrors.apple2.org.za/ftp.apple.asimov.net/documentation/hardware/misc/Apple%20II%20Family%20Pinouts.pdf
  6. https://computerhistory.org/blog/apple-ii-dos-source-code/
  7. http://www.bitsavers.org/pdf/shugart/SA400/54096-2_SA400_Service_Manual_Apr1979.pdf
  8. https://www.applefritter.com/content/disk-ii-rainbow-ribbon-cable-color-pinout
  9. https://mirrors.apple2.org.za/ftp.apple.asimov.net/documentation/hardware/misc/Apple%20II%20Disk%20Pinout.txt
  10. https://tinkerdifferent.com/threads/apple-disk-ii-woes.1547/
  11. https://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Books/Beneath%20Apple%20DOS.pdf
  12. https://www.computinghistory.org.uk/det/22208/VisiCalc/
  13. http://www.appleoldies.ca/anix/
  14. https://ciderpress2.com/formatdoc/Nibble-notes.html
  15. https://vintageapple.org/apple_ii/pdf/Apple_II_DOS_Users_Manual_1983.pdf
  16. http://justsolve.archiveteam.org/wiki/Apple_II_16_sector_disk
  17. https://wiki.reactivemicro.com/images/1/1a/Apple_IIe_Enhancement_Kit_V2.pdf
  18. https://www.bigmessowires.com/ys/yellowstone-instructions.pdf
  19. https://www.applefritter.com/content/logical-problem-apple-ii-europlus-error-and-random-freeze
  20. https://applesaucefdc.com/hardware/all-about-applesauce/
  21. https://forum.kryoflux.com/viewtopic.php?t=919
  22. https://github.com/AppleWin/AppleWin
  23. https://forum.vcfed.org/index.php?threads/disk-ii.32728/
  24. https://gswv.apple2.org.za/a2zine/faqs/Csa2FDRIVE.html
  25. https://archive.org/details/Apple_DOS_v3.3_1980
  26. https://paleotronic.com/2024/01/29/steve-wozniak-talks-disk/
  27. https://www.gamedeveloper.com/business/a-history-of-gaming-platforms-the-apple-ii
  28. https://www.apple2history.org/history/ah15/
  29. https://lowendmac.com/2015/apple-iii-chaos-apples-first-failure/
  30. http://bitsavers.org/pdf/apple/disk/profile/ProFile_Owners_Manual_1983.pdf
  31. https://www.apple2history.org/history/ah09/
  32. https://www.apple3.org/Documents/Technotes/1128.html
  33. https://wiki.reactivemicro.com/Universal_PSU_Kit
  34. https://comp.sys.apple2.narkive.com/TUJpcfaK/apple-ii-disk-drive-question
  35. https://512pixels.net/wp-content/uploads/2025/05/Lisa-DIY-Guide.pdf
  36. https://www.atarimagazines.com/compute/issue50/227_1_Evolutionary_To_The_Core_The_Apple_IIc_Heads_For_Home.php
  37. https://mirrors.apple2.org.za/ftp.apple.asimov.net/documentation/hardware/storage/disks/AppleService%20Technical%20Procedures%20-%20Apple%20DuoDisk.pdf
  38. https://www.cvxmelody.net/Apple%20IIe%20&%20IIc%20Products%20Confidential%20Dealer%20Price%20List%20-%20Nov%2015%2C%201984.pdf
  39. https://www.lowendmac.com/2015/apple-iic-and-iic-plus-compact-apples-with-internal-floppy-drives/
  40. https://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Peripherals/Disk%20Drives/Apple%20UniDisk%203.5/Manuals/Apple%20UniDisk%203.5%20Owner%27s%20Manual.pdf
  41. https://www.savagetaylor.com/TIL/TIL01307.pdf
  42. https://groups.google.com/g/comp.sys.apple2/c/2RLal6eZvgI
  43. https://www.applefritter.com/content/how-important-are-iwm-features-apple-iic
  44. https://lowendmac.com/2006/floppy-drive-observations-a-compleat-guide-to-mac-floppy-drives-and-disk-formats/
  45. https://oldcrap.org/2018/08/24/apple-disk-5-25/
  46. https://www.ithistory.org/db/hardware/apple-computer-inc/apple-pc-525-drive
  47. https://68kmla.org/bb/index.php?threads/all-about-apple-pc-5-25-drive-a9m0110.40785/
  48. https://www.podfeet.com/blog/2017/07/apple-e-disk-transfer-to-mac/
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