Plugs
There are two types of modern British Telecom plugs – 431A and 631A.
431A is 4-way and 631A 6-way. They fit a standard "Type 600" telephone socket. There are also plugs with only two contacts commonly seen on modem leads. These are a recent introduction and do not seem to be easily available as separate parts.
The Sinclair QL and 128k ZX Spectrum home computers used type 630W connectors for their serial ports (on the Spectrum 128 and +2, MIDI signals could also be sent from the port). These closely resemble standard 631A connectors but the keying slots on the back of the plug have a different size and position.
The 430A and 630A plugs had the latch on the opposite side of the plug, and were used as headset plugs on some switchboards and as handset connectors on some telephones, e.g. Ambassador.
The 631A and 630A plugs are also used for connecting sensors to interfaces for computer based measurements in educational environments, the former for connecting analogue sensors and the latter for digital sensors. Companies using these plugs include Vernier, TI and Casio, for interfaces connecting to their graphical calculators, and in the Netherlands CMA. They all use the same pinning.
Read more about this topic: British Telephone Sockets