Storage Module Device
| Industry | Computer storage devices, Computer hardware |
|---|---|
| Founded | 1957 |
| Products | Storage Module Device (SMD) hard disk drives |
Storage Module Device (SMD) represents a series of hard disk drives initially introduced by Control Data Corporation in December 1973, beginning with the CDC 9760 model offering 40 MB of unformatted storage. This was succeeded by the CDC 9762, delivering 80 MB in June 1974, followed by the CDC 9764 and CDC 9766 models in 1975, which provided 150 MB and 300 MB capacities respectively (all unformatted). In 1976, a variant with fixed media called MMD was launched, featuring storage sizes of 12, 24, and 48 MB. The SMD interface, which originated from Digital Equipment Corporation’s RP0x interface, was officially standardized under ANSI X3.91M - 1982.
The interface for SMD drives employs two flat ribbon cables—designated "A" for control signals and "B" for data transmission—linking the disk drive to an interface controller that connects to the host system. This setup supports data transfer rates up to 9.6 Mbit per second. SMD technology gained widespread use in both removable and fixed disk drives measuring 8 inches and 14 inches, primarily within mainframe and minicomputer environments before being replaced by SCSI. It held a leading position as the preferred interface for high-capacity storage devices until the Intelligent Peripheral Interface appeared. Its adoption in microcomputers was limited, mostly confined to large-scale file server applications. By July 1981, Control Data had shipped its hundred-thousandth SMD unit. By the year 1983, more than twenty-five manufacturers—including Ampex, Fujitsu, Hitachi, Micropolis, Pertec, Priam, NEC, and Toshiba—were producing SMD-compatible drives.
CDC 976x disk geometry
The CDC 9762 model with an 80 MB capacity incorporates five platters measuring 14 inches each; the topmost and bottom platters serve as guard platters—they do not store data but match the size of the data platters and are frequently reused from defective disks. The three intermediate platters provide five surfaces used for data storage along with one servo surface located on the upper side of the central platter to assist with precise head positioning.
In contrast, the CDC 9766 model offering 300 MB consists of twelve platters of the same diameter, again featuring guard platters at both extremities. Among these, ten platters supply nineteen data surfaces plus one servo surface situated on a middle platter.
Both versions contain a total of 823 cylinders and position their servo surfaces centrally. The number of sectors per track and sector size differ depending on how the disk is initialized; for example, when configured for use with GEC 4000 series minicomputers, disks are formatted with thirty-four sectors per track, each sector holding 512 bytes.
To guarantee interchangeability of media across different drives, SMD disk packs required meticulous alignment of read/write heads.