TheRK05 is adisk drive manufactured byDigital Equipment Corporation whose removabledisk pack can hold about 2.5 megabytes of data. Introduced 1972, it is similar toIBM's 1964-introduced2310, and uses a disk pack similar to IBM's2315 disk pack, although the latter only held 1 megabyte.[1] AnRK04 drive, which has half the capacity of an RK05, was also offered.[2]: p.1-1, p.1-4
Systems on which it can be used include DEC'sPDP-8,[3]PDP-11,[4]: p.43 andPDP-15.[4]: p.33
The RK05 was a moving headmagnetic disk drive manufactured byDEC.[5][a] It stored approximately 2.5MB on a 14", single-platterIBM-2315-style front-loading removable disk cartridge. The cartridge permitted users to have relatively unlimited off-line storage and to have very fast access to such data.
ThePDP systems to which it could be attached had numerous operating systems for each computer architecture, so by changing disk pack another operating system could also be booted. Although the 14-inch cartridge could not fit in a shirt pocket,unlikeDECtape, the RK05 provided personal, portable, and expandable storage.
Although a minimal PDP-8/A came with only one drive, most computers were configured with at least one additional storage device; some systems had four[b] drives.[4]: p.53
Occupying 10.5 inches (6U) of space in a standard19-inch rack, the drive was competitive at the time. The cartridge contained a single, 14"aluminum platter coated withiron oxide in an epoxy binder. The twoferrite andceramicread/write heads were pressed towards the disk by spring arms, floating on anair bearing maintained by the rotation of the disk. They were positioned by avoice coil actuator using alinear 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 pressurized air to the cartridge, excluding most contaminants that would otherwise causehead crashes.
When used on 16-bit systems such as thePDP-11, the drive stored roughly 1.2 megawords. When used on 12-bit systems such as thePDP-8, the drive stored 1.6 megawords (so roughly the same bit capacity, albeit formatted differently). Multiple drives weredaisy chained from their controller usingUnibus cabling; a terminator was installed in the farthest drive.
The 16-bit (Unibus) controller was known as theRK11; 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 theRK8E; it supported up to four RK05 drives. 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, theOS/8 operating system split each drive into two logical volumes, for example,RKA0 andRKB0, representing the outermost and innermost cylinders of the drive.
Prior to the introduction of DEC's own drives, DEC resold two drives fromDiablo Data Systems (later acquired byXerox),[6] the 1.22 megabyte RK02, of which very few were shipped, and the 2.45 megabyte RK03[5] (Diablo Model 31). These were interface compatible with the RK05; RK03 and RK05 disks could be interchanged as well.[2] The RK04 was DEC's counterpart to the RK02,[2]: p.1-4, p.2-6 for low-density storage of 600 Kwords, and the RK05 was DEC's counterpart of the high-density RK03, storing 1.2 Mwords.
RK05-compatible non-DEC RK05s were produced and marketed.[8]
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