+D

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The +D (or Plus D) was a floppy disk and printer interface for the 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 disk 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 Complete +D Disassembly" by Rudy Biesma documents the "G+DOS system 2a" version.^[1] An enhanced version called Beta DOS fixed bugs and added features.

In later years a complete new system called UNI-DOS was developed by SD Software for the DISCiPLE and +D interfaces.^[2]

The popularity of the DISCiPLE led to the formation of a user group and magazine, INDUG, which later became Format Publications. Usergroups like INDUG/Format in the UK or DISCiPLE-Nieuwsbrief in the Netherlands produced enhancements such as extended printer support.

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.

• How to build a +D for yourself Archived 2010-12-06 at the Wayback Machine
• rwapsoftware.co.uk still offer the +D Disk Interface for sale