RK05

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The RK05 was a moving head magnetic disk drive manufactured by the Digital Equipment Corporation of Maynard, Massachusetts. It stored approximately 2.5 MB on a 14", single-platter IBM-2315-style front-loading removable disk cartridge.

Occupying 10.5 inches of space in a standard 19 inch computer rack, the drive was competitive at the time but would now be viewed as laughably small, having only twice the storage capacity of a typical floppy disk. The cartridge contained a single, 14" aluminum platter coated with iron oxide and polished smooth. The two ferrite and ceramic read/write heads were pressed towards the disk by spring arms, floating on an air bearing maintained by the rotation of the disk. They were positioned by a voice coil actuator using a linear optical encoder for feedback. The track density was 100 tracks-per-inch. The bit density along the track was about 2200 bits-per-inch. Discrete electronics computed the velocity profile for seeks commanded by the controller. An absolute filter (HEPA filter) provided pressurised air to the cartridge, excluding most contaminants that would otherwise cause head crashes.

When used on 16-bit systems such as the PDP-11, the drive stored roughly 1.2 megawords. When used on 12-bit systems such as the PDP-8, the drive stored 1.6 megawords (so roughly the same bit capacity, albeit formatted differently). Multiple drives were daisy chained from their controller using Unibus cabling; a terminator was installed in the farthest drive.

The 16-bit (Unibus) controller was known as the RK11; it allowed the connection of up to eight RK05 drives. Seeks could be overlapped among the drives but only one drive at a time could transfer data.

The most-common 12-bit (Omnibus) controller was known as the RK8E; it supported up to four RK05 drves. The RK05 disk had more than 4096 sectors and so could not be addressed completely by a single PDP-8 12-bit word. To accommodate this, the OS/8 operating system split each drive into two logical volumes, for example, RKA0 and RKB0, representing the outermost and innermost cylinders of the drive.

[edit] Variants

• An RK04 was also produced; it had half the capacity of the RK05 but sold poorly.
• The RK05-E was the final version of the standard-density RK05 drive. It contained many reliability enhancements compared to the earlier versions.
• An RK05-F was later produced. By "fixing" the otherwise removable cartridge in place, it was able to avoid the cartridge-to-cartridge compatibility problems that limited the capacity of the ordinary RK05. As a result, it could operate at twice the normal track density, doubling the capacity of the drive to about 5MB.
• Prior to the introduction of DEC's own drives, DEC resold two drives from Xerox's Diablo Data Systems division. Known as the RK02 and RK03, these were electrically compatible with the RK04 and RK05, respectively; their disks could be interchanged as well.