Like Sinclair's own Interface 1, the DISCiPLE was a wedge-shaped unit fitting under the Spectrum. It was designed as a super-interface, providing all the facilities a Spectrum owner could need. In addition to floppy-disk and parallel-printer interfaces and a "magic button" (see Non-Maskable interrupt), it also offered twin joystick ports, Sinclair network ports and an inhibit button for disabling the device.
At the rear of the unit was a pass-through port for connecting further devices, although the complexity of the DISCiPLE meant that many would not work, or only if the DISCiPLE was "turned off" using the inhibit button.
The DISCiPLE was a considerable success but its sophistication meant that it was expensive and the plastic casing, located beneath the computer itself, was sometimes prone to overheating. These factors led to the development of the +D.
The popularity of the DISCiPLE, considering its relatively high price — approaching that of the Spectrum itself, and considerably more if a pair of disk drives and a dot-matrix printer were included — led to the formation of a user group and magazine, INDUG, which later became Format Publications.
In October 1993 "The Complete DISCiPLE Disassembly" was published in book form, viewing the "GDOS system 3d" 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 DISCiPLE, see the entry on MGT.