ZX Microdrive

The ZX Microdrive is a magnetic tape data storage system launched in July 1983 by Sinclair Research for their ZX Spectrum home computer. The Microdrive technology was later also used in the Sinclair QL and ICL One Per Desk personal computers.

It is claimed the Microdrive was originally proposed by Andrew Grillet at an interview with Sinclair Research for what was to become the ZX81. Grillet proposed "a version of the Learjet Stereo 8 system, modified to allow two 64k core images per track for roll-out roll-in swapping using the KUTS protocol". The ZX81 had only 1k of RAM. Mr Grillet was offered better pay by Xerox Corporation, and never worked on the ZX81 or the Spectrum.Fact|date=April 2007 The primary engineer involved in the development of the Microdrive was Ben Cheese.

Typically of Sinclair Research, the ZX Microdrive was comparatively cheap (£49.95 at launch) and technologically innovative but also rather limited. Connection of a ZX Microdrive to a ZX Spectrum was done via the ZX Interface 1 unit, which cost £49.95 on its own, although both this and the Microdrive could be bought together for £79.95. Later, in March 1985, a Microdrive plus Interface 1 bundle called the "ZX Spectrum Expansion System" was sold for £99.95. The bundle included four Microdrive cartridges: the "Tasword Two" word processor plus "Masterfile" filing system, Quicksilva's "Games Designer" and "Ant Attack" game, an introductory cartridge, and a blank. A total of eight ZX Microdrive units could be connected to the Interface 1 by daisy chaining one drive to the next via an electrical connector block.

Microdrives used tiny (44 × 34 × 8 mm including protective cover) cartridges containing a 5 m (200 inch) endless loop of magnetic tape, 1.9 mm wide, driven at 76 cm/second (30 in/second); thus performing a complete circuit in approximately 8 seconds. The cartridges held a minimum of 85 kB when formatted on a ZX Microdrive (exact capacity depended on the number of "bad" sectors found and the precise speed of the Microdrive motor when formatting). The data retrieval rate was 15 kB/s (i.e., 120 kbit/s). It is possible to 'expand' the capacity of a fresh microdrive cartridge by formatting it several times. This causes the tape to stretch slightly, increasing the length of the tape loop so that more sectors can be marked out on it. This procedure was widely documented in the Sinclair community magazines of the 1980s.Fact|date=March 2007

Unfortunately, the system acquired a reputation for unreliability, and the cartridges were relatively expensive (initially sold for £4.95 each, later reduced to £1.99). Similar technology was used in other devices, such as the Rotronics Wafadrive, and was sometimes known as a "stringy floppy".

Microdrives were also used as the native storage medium of the Sinclair QL, which incorporated two internal drives. These were very similar to the ZX Microdrive, but used a different logical format and ran slightly slower, allowing each cartridge to hold at least 100 kB. The QL also included a Microdrive expansion bus allowing the attachment of up to six external QL Microdrives. These were never produced, probably due to lack of demand. It was however possible to connect ZX Microdrives to a QL by putting a twist in the cable.

In addition to the QL versions, dual internal Microdrives were included in the related ICL One Per Desk system (also badged as the Merlin Tonto and Telecom Australia Computerphone). These drives were re-engineered by ICL for greater reliability, and used a format incompatible with both ZX and QL Microdrives.

ee also

* Stringy floppy - similar system for the TRS-80 and others

External links

* [http://www.nvg.ntnu.no/sinclair/computers/peripherals/microdrive.htm ZX Microdrive information] – At Planet Sinclair
* [http://www.sincuser.f9.co.uk/037/news.htm "Sinclair User", April 1985, News section]