Disk II
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.
[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.
Still lacking a DOS, and with Wozniak inexperienced in operating system design, Steve Jobs approached Shepardson Microsystems with the project.
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.
He believed that his simpler design lacked their features, but realized that they were less sophisticated; for example, his could use soft-sectored disks.
[citation needed] Fellow engineer Cliff Huston came up with several procedures for resuscitating the faulty drives on the assembly line.
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 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".
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.
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.
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.
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.
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.
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.
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.
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.
