Periferico created by Sinclair Research for the ZX Spectrum, launched in 1983. It was originally created to build a LAN that could be used in schools, but was revised before its release to add a mass storage system known as ZX Microdrive. Was connected to the expansion bus, staying under the Spectrum and leaving it in a more ergonomic position inclined. To avoid bad contacts had two screws replacing two of the original casing of the Spectrum (the covers of the Spectrum Traian these holes in order to ensure the attachment). To ensure the expansion was an expansion bus connector, so you could still use the other interfaces.
The ZX Interface 1, adds an RS-232 (DE-9 in the rear) to be used with printers capable of operating up to 19 kilobytes per second. The device made available to users of 2-port network can connect up to 64 ZX Spectrum. This system was named ZX NET.
ZX Interface 1 created by Sinclair Research
This was achieved by connecting a ZX Interface 1 to ZX Spectrum that each wanted to connect, using a 3m cable that came with the interface is then connected. The data could be sent or received at 100 bits per second to one or more stations, or you could make all the Broadcasting stations acting as a server and other clients. Another feature that has the ZX Interface 1, is to control up to 8 ZX Microdrive units, which could be made available to the ZX NET, accessing a unit at a time by any of the connected clients. You could also connect a ZX Printer, and make it available to the ZX NET.
Interface 1, was inside an 8Kb memory ROM containing the software to handle the ZX Microdrive, RS-232 port and network connectivity. This ROM adds commands to the ZX Spectrum as OPEN, CLOSE , MOVE, ERASE, FORMAT and CAT, but these commands are printed on the keypad of the ZX Spectrum, they do not fulfill their function if there is a ZX Interface 1 connected to the machine. To avoid incompatibility, that was appealed to swap ROM rather than part of the Spectrum ROM access when needed andalusia code. However some problems with games that used the portion of RAM reserved for the control of the Microdrive.
A variation of the ZX NET was the one I use with the Sinclair QL QLAN receive the name and was designed to be interoperable with the ZX NET, but this difference in speed was problematic.

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