BBC Master and Archimedes first appeared as features of Electron expansion units, including ROM cartridge slots and the Advanced Disc Filing System, a hierarchical improvement to the BBC's original Disc Filing System. Having been envisaged as the basis of a portable computer with "a very strong emphasis on communications" during its development, supporting both modem and Econet interfaces, the BT Merlin M2105 product subsequently combined the Electron with communications functionality, and the Acorn Communicator developed such concepts further, introducing networking support. The availability of the Electron at discounted prices from 1985 onwards led to increased demand for third-party software and expansions for the machine. While it may not have been as popular as the Spectrum, Commodore 64 or Amstrad CPC, it did sell in sufficient numbers to ensure that new software titles from established producers were being produced right up until the early 1990s, with mainstream publications dedicated to the machine having effectively supported it for five years beyond the point at which Acorn's own support had ceased. Hardware expansions Since the Electron provided only a selection of video output ports, a cassette port and the expansion connector, a range of additional expansions were produced to offer ports and connections to various peripherals. The first expansions were largely joystick and printer interfaces or sideways ROM boards. For instance, First Byte Computers developed an interface and software which allowed a "switched" joystick to be used with the majority of software titles. This interface became very popular and was sold by W.H. Smiths, Boots, Comet and hundreds of independent computer dealers, selling as many as 23,000 units over a two-year period, helped by a bundling agreement with Dixons. Acorn's own expansion strategy was led by the Plus 1 which offered a combination of ports and cartridge connectors, followed by the Plus 3 disc drive unit, but by early 1986 the more general range of expansions had broadened to include floppy drive and RS423 interfaces, Teletext adapters, and other fundamental enhancements to the base machine. Multi-function expansions Since the Electron's expansion connector was the basis of practically all external hardware expansions for the machine, unless an expansion propagated this connector to others, as was done by the Acorn Plus 3, the capabilities of any given expansion would limit the capabilities of the expanded machine. Thus, expansions offering a single function, such as joystick ports or a printer port, would need to be disconnected