RK05

Digital Equipment Corporation
Industry	Computer hardware
Founded	1957
Headquarters	United States

Digital Equipment Corporation's RK05 is a disk drive featuring a removable disk pack with a capacity near 2.5 megabytes. Launched in 1972, its design is reminiscent of IBM's 1964 2310 and utilizes a disk pack similar to IBM's 2315, although the latter offered only 1 megabyte of storage. A lower-capacity model, the RK04, providing half the storage size, was also available.

This drive was compatible with DEC's PDP-8, PDP-11, and PDP-15 systems.

Overview

The RK05 was a moving head magnetic disk unit produced by DEC, capable of storing roughly 2.5 MB on a single 14-inch removable platter cartridge modeled after the IBM-2315 style. This removable cartridge allowed users to extend storage capacity offline while maintaining rapid data retrieval.

Multiple operating systems were supported on PDP platforms, enabling boot operations from various disk packs. Although physically larger than DECtape, the RK05 offered portable and scalable storage solutions.

Basic PDP-8/A setups typically included one drive, but most configurations incorporated additional units; certain systems, such as the DEC DATAsystem-535, supported up to four or even eight drives.

Technical details

The drive occupied 6U (10.5 inches) in a standard 19-inch equipment rack. Its cartridge housed a single 14-inch aluminum platter coated with iron oxide embedded in epoxy. The read/write heads—composed of ferrite and ceramic—floated on air bearings sustained by the disk’s rotation and were positioned by a voice coil actuator guided by a linear optical encoder for feedback. Track density reached 100 tracks per inch, with bit density along tracks approximately 2200 bits per inch. Seek velocity was precisely managed by discrete electronic components. A HEPA filter supplied pressurized air to prevent contamination and avoid head crashes.

On 16-bit architectures like the PDP-11, the drive held about 1.2 megawords; on 12-bit machines like the PDP-8, it stored roughly 1.6 megawords—both reflecting identical bit capacity but differing formatting schemes. Multiple drives connected via Unibus cabling in daisy-chain fashion; the final unit featured a terminator.

The RK11 controller for 16-bit Unibus systems accommodated up to eight RK05 drives, allowing overlapping seeks but limiting data transfer to one drive at any given time.

The standard 12-bit Omnibus controller, RK8E, supported as many as four drives. Due to the RK05’s sector count exceeding the PDP-8’s 12-bit addressing limit of 4096 sectors, OS/8 partitioned each drive into two logical volumes (for example, RKA0 and RKB0), corresponding to outer and inner cylinder groups respectively.

Predecessors

Prior to developing its own disk drives, DEC marketed two models from Diablo Data Systems (later acquired by Xerox): the RK02 with 1.22 MB capacity and limited distribution, and the RK03 (Diablo Model 31) offering 2.45 MB storage. Both were interface-compatible with the RK05; disks from the RK03 and RK05 were interchangeable. The RK04 served as DEC’s equivalent to the RK02 for low-density storage of 600 Kwords, while the RK05 matched the high-density RK03 at approximately 1.2 Mwords capacity.

Variants

The RK05E represented an improved reliability iteration of the original standard-density RK05.
The RK05J was the last version featuring a standard-density removable pack.
The RK05F was a fixed-pack variant without removable cartridges, which resolved compatibility concerns and doubled track density to nearly 5 MB capacity.

Compatibles

Drives compatible with the RK05 format were also manufactured and sold by companies other than DEC.