Storage Module Device

Lore & Background

The SMD family began with the CDC 9760 40 MB drive in December 1973. The CDC 9762 80 MB variant was announced in June 1974, followed by the CDC 9764 150 MB and CDC 9766 300 MB variants in 1975 (all capacities unformatted). A non-removable media variant family called MMD, with 12, 24 and 48 MB capacities, was announced in 1976. The SMD interface was supported by many 8-inch and 14-inch removable and non-removable disk drives, and was rarely used with microcomputers except sometimes in large file servers.

The CDC 9762 80 MB variant used 5 × 14" platters, with the top and bottom platters being guard platters not used for storage, often made from data platters with too many errors. The remaining 3 platters provided 5 data surfaces and one servo surface (the upper surface of the center platter). The CDC 9766 300 MB variant used 12 × 14" platters, again with guard platters, giving 19 data surfaces and one servo surface. Both had 823 cylinders, and sector size and sectors per track depended on initialization; for example, on GEC 4000 series minicomputers, a configuration of 34 sectors of 512 data bytes each per track was used.

SMD disk packs required head alignment to assure interchangeability of media between drives. Control Data shipped its 100,000th SMD drive in July 1981. By 1983 at least 25 manufacturers had supplied SMD drives, including Ampex, Century Data Systems, CDC, Fujitsu, Hitachi, Micropolis, Pertec, Priam, NEC and Toshiba.

Reader's Guide

The Storage Module Device interface played a pivotal role in the era of mainframes and minicomputers, establishing itself as the dominant hard disk interface for large-capacity and large-diameter drives until the Intelligent Peripheral Interface emerged. Its design, based on two flat interface cables for control and data, enabled a standardized connection that was adopted by numerous manufacturers, fostering interoperability across systems. The SMD interface's data transfer rate of 9.6 Mbit/s was sufficient for the applications of its time, and its support for both removable and non-removable media made it versatile for various storage needs. The family's evolution from the 40 MB CDC 9760 to the 300 MB CDC 9766 reflected the rapid growth in storage capacity demands. The introduction of the MMD variant in 1976 addressed the need for non-removable storage. SMD's eventual replacement by SCSI marked a shift toward smaller, faster interfaces, but its legacy as a foundational standard for large-disk storage remains significant. The requirement for head alignment to ensure media interchangeability highlighted the precision engineering needed for removable disk packs.