+D

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The +D (or Plus D) was a floppy disk and printer interface for the Sinclair ZX Spectrum home computer, developed as a successor to Miles Gordon Technology's earlier product, the DISCiPLE. It was designed to be smaller, cheaper, simpler and thus more reliable.

It discarded a number of the less important features of the earlier product — the network and joystick ports, the inhibit button and the pass-through connector — and replaced its ancestor's plastic wedge-shaped design which fit under the Spectrum with a simple flat metal slab which protruded from the rear of the computer.

It provided only floppy-diskette and Centronics parallel interfaces, plus a Non-Maskable interrupt button. The +D's casing was simple folded steel, which was not only stronger than before but acted as a heatsink, improving reliability. Apart from the missing ports, though, it was software-compatible with the larger device.

The +D's DOS was named G+DOS, and was compatible with the DISCiPLE's DOS, GDOS. SAM DOS for MGT's SAM Coupé was backwards-compatible with GDOS and G+DOS.

The +D and DISCiPLE so extended the capabilities of the Spectrum that a user group was formed to discuss the system's newly-expanded potential. The group, INDUG, published a newsletter called Format, and later went on to become Format Publications.

"The Complete +D Disassembly" was published viewing the "G+DOS system 2a" version, which is now usable online at the writers page http://www.biehold.nl/rudy/index.htm for both DISCiPLE and +D usergroups like Format (UK) or DISCiPLE-Nieuwsbrief (NL) produced enhancements like e.g. not 3×3 but 5×3-bit grayscale printing and HP-printer settings. In later years a complete new system called UNI-Dos was developed and put in a ROM for the DISCiPLE and +D both, making up 2 new ROMs.

For more information on the design, specification and use of the +D, see the entry on MGT.

The +D design was later licensed by Datel and it continued on sale for some years after MGT's demise. Its design and chips have been released into the public domain^{[citation needed]} and it still remains available commercially or even as a DIY project.