On This Day - 14th July

IBM engineer Robert H. Dennard files a patent for a new type of semiconductor memory in which each memory cell can store a bit using just one transistor and one capacitor.

The stored electrical charge represents the data and must periodically be refreshed, giving the technology its name: Dynamic Random-Access Memory (DRAM).

The design dramatically reduced the number of components required for each memory cell compared with existing semiconductor memory approaches, allowing much greater memory densities to be achieved.

DRAM would become one of the most important technologies in the development of affordable computers, eventually providing the main memory used by generations of personal computers, including the ZX Spectrum.

In July 1961, Clive Sinclair founded Sinclair Radionics, the company that would establish him as one of Britain's best-known electronics entrepreneurs. Initially operating from a small premises in London, the company began by developing and selling electronic products, including radio kits, amplifiers and other low-cost electronic equipment. Sinclair's approach was to make technology affordable to a much wider audience by keeping products small, simple and inexpensive.

Sinclair Radionics went on to produce a wide variety of innovative products, including the Sinclair Executive pocket calculator, one of the world's first genuinely pocket-sized electronic calculators. The company also developed the Black Watch digital watch and the company's early television products. Although Sinclair Radionics eventually ran into financial difficulties, its work laid much of the groundwork for the Sinclair computer business.

The company's experience in designing low-cost electronic products would prove particularly important when Sinclair entered the emerging home-computer market. Following the financial difficulties of Sinclair Radionics, Clive Sinclair established Sinclair Research, which went on to produce the ZX80, ZX81 and ZX Spectrum and transform the British home-computer market.

The **ZX Microdrive** was Sinclair Research's compact storage system for the ZX Spectrum and ZX81. Announced in 1983, it was intended to provide a much faster and more convenient alternative to cassette tape, while remaining considerably cheaper than a conventional floppy-disk system. Each Microdrive cartridge was a small removable loop of magnetic tape, allowing programs and data to be stored and retrieved without the long loading times associated with ordinary cassette recordings.

A Microdrive cartridge could store approximately **85KB of formatted data**, although the actual usable capacity depended on the way the storage was formatted and the system software being used. Data transfer was considerably faster than cassette, with the Microdrive capable of loading and saving files in seconds rather than minutes. Cartridges could also hold multiple files, making the system much more useful for serious programming and data storage. The Spectrum's operating system provided commands for handling Microdrive files, while the BASIC language included support for loading and saving programs and data to the drives.

The Microdrive was not simply a storage device but part of Sinclair's wider expansion system for the Spectrum. The drives connected through the **ZX Interface 1**, which also provided networking and an RS-232-compatible serial interface. Up to eight Microdrives could be connected in a daisy-chain arrangement, giving a substantial amount of online storage for the period. The combination of Microdrives, networking and communications made the Interface 1 a significant expansion for users who wanted to move beyond the Spectrum's original cassette-based design.

Despite its advantages, the Microdrive had limitations. The cartridges used a continuous magnetic tape loop rather than the random-access media used by floppy disks, and the mechanism and cartridges were relatively delicate. Reliability could also be affected by worn or contaminated cartridges and drives. Nevertheless, the Microdrive was an ambitious and distinctly Sinclair solution to the problem of bringing fast, affordable mass storage to a low-cost home computer. It became an important part of the Spectrum ecosystem and helped demonstrate how far the machine could be expanded beyond its original specification.