Disk II

The Disk II Floppy Disk Subsystem, often rendered as Disk ][, is a 5¼-inch floppy disk drive designed by Steve Wozniak 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 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. These floppy drives cannot be used with any Macintosh computer without an Apple IIe Card as doing so will damage the drive or the 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.

Apple did not originally offer a disk drive for the Apple II, which used data cassette storage like other microcomputers of the time. Apple 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. Wozniak knew nothing about disk controllers, but while at Hewlett-Packard had designed a simple, five-chip circuit to operate a Shugart Associates drive. Wozniak began studying existing floppy controllers, including North Star and IBM models. Standard floppy controller boards were large devices that used dozens of TTL chips. In addition to being expensive, they were also much too big to fit into the Apple II's small case. Initially, Wozniak attempted to develop an FM-type controller with a smaller chip count and by making improvements to the FM encoding scheme, he was able to squeeze out 10 sector per track storage. However, he soon decided he could do even better and switched to another, completely different "5 and 3 encoding" method known as "GCR" (Group Coded Recording) which was a modified version of recording schemes used on tape storage. This allowed still more storage, up to 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.

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