In 1980-82 the UK Department of Education and Science had begun the Microelectronics Education Programme to introduce microprocessing concepts and educational materials. In 1982 through to 1986, the Department of Industry (DoI) allocated funding to assist UK local education authorities to supply their schools with a range of computers; the ZX Spectrum was very useful for the control projects.
ZX Spectrum 16K/48K (1982)
Released by Sinclair in 1982 and available with either 16 kB (£125, later £99) or 48 kB (£175, later £129) of RAM and 16 kB ROM, the original ZX Spectrum is remembered for its rubber keyboard and diminutive size. Owners of the 16 kB model could purchase an internal 32 kB RAM upgrade daughterboard, which consists of 8 dynamic RAMs and few TTL chips. Users could mail their 16K Spectrums to Sinclair to be upgraded to 48K versions. To reduce the price, 32 kB extension was actually comprised of 8 faulty 64 kilobit chips with only one half of their capacity working and/or available.
Also available were third-party external 32 kB RAMpacks that mounted in the rear expansion slot. As with the ZX81, "RAMpack wobble" caused by poor connection with the expansion was the bane of many users, causing instant crashes and sometimes ULA or CPU burnout.
ZX Spectrum+ (1984)
This 48 kB Spectrum (development code-name TB) introduced a new QL-style enclosure with a much needed injection-moulded keyboard and a reset button, retailing for £180. An upgrade package for older machines was also available. Most hard core users (programmers and gamers) disliked the new keyboard.
ZX Spectrum 128 (1986)
Sinclair developed the 128 (code-named Derby) in conjunction with their Spanish distributor Investrónica. Investrónica had helped adapt the ZX Spectrum+ to the Spanish market after Spanish courts decreed all computers with 64 kB RAM or less must support the Spanish alphabet (including ñ) and show messages in Spanish.
New features included 128 kB RAM, three-channel audio via the AY-3-8912 chip, MIDI compatibility, an RS-232 serial port, an RGB monitor port, 32 kB of ROM including an improved BASIC editor and an external keypad.
ZX Spectrum +2 (1986)
The +2 was Amstrad's first Spectrum, coming shortly after their purchase of the Spectrum range and "Sinclair" brand. The machine featured an all-new grey enclosure featuring a spring-loaded keyboard, dual joystick ports, and a built-in cassette recorder dubbed the "Datacorder" (like the Amstrad CPC 464), but was (in all user-visible respects) otherwise identical to the ZX Spectrum 128. Production costs had been reduced and the retail price dropped to £139-£149.
The new keyboard did not include the BASIC keyword markings that were found on earlier Spectrums, except for the keywords LOAD, CODE and RUN which were useful for loading software. However, the layout remained identical to that of the 128.
ZX Spectrum +3 (1987)
The Spectrum +3 looked similar to the +2 but featured a built-in 3-inch floppy disk drive (like the Amstrad CPC 6128) instead of the tape drive. It initially retailed for £249 and then later £199 and was the only Spectrum capable of running CP/M without additional hardware.
The +3 saw the addition of two more 16K ROMs, now physically implemented as two 32K chips. One was home to the second part of the reorganised 128K ROM and the other hosted the +3's disk operating system. To facilitate the new ROMs and CP/M, the bank-switching was further improved, allowing the ROM to be paged out for another 16 KB of RAM as well as offering three 16 KB pages for the display RAM.
The ZX Spectrum +3 was the final official model of the Spectrum to be manufactured, remaining in production until December 1990. Although still accounting for one third of all home computer sales at the time, production of the model was ceased by Amstrad in an attempt to transfer customers to their CPC range.
ZX Spectrum +2A /+2B (1987)
The +2A was produced to homogenise Amstrad's range. Although the case reads "ZX Spectrum +2", the +2A/B is easily distinguishable from the original +2 as the case was restored to the standard Spectrum black.
The +2A was derived from Amstrad's +3 4.1 ROM model, hosting a new motherboard which vastly reduced the chip count, integrating many of them into a new ASIC. The +2A replaced the +3's disk drive and associated hardware with a tape drive, as in the original +2. Originally, Amstrad planned to introduce an additional disk interface, but this never appeared. If an external disk drive was added, the "+2A" on the system OS menu would change to a +3. As with the ZX Spectrum +3 some older 48K, and a few older 128K, games were incompatible with the machine.
The +2B signified a manufacturing move from Hong Kong to Taiwan.
This article is derived from http://en.wikipedia.org/wiki/Sinclair_Spectrum