Solid State Interlocking

Solid State Interlocking

Solid State Interlocking (SSI) is the brand name of the first generation processor-based interlocking developed in the 1980s by British Rail, GEC-General Signal and Westinghouse Signals Ltd in the UK.

Interlocking hardware

SSI utilises a 2-out-of-3 architecture, whereby all safety-critical functions are performed in three separate processing lanes and the results voted upon. An SSI interlocking cubicle comprises three Interlocking Processors or Main Processor Modules (MPMs), two Panel Processors and a Diagnostics Processor.


Geographic interlocking data, relating to the area of railway under control, is installed using EPROMs contained in plug in memory modules. The interlocking program contained in each of the MPMs interprets this data to allow safe passage of trains through its area of control.

External hardware

Trackside equipment such as signals and points are connected to nearby 'trackside functional modules' (TFMs). Each module has a number of outputs and inputs. Each output drives an individual function, such as a signal lamp or an AWS inductor. Certain outputs are capable of driving a flashing lamp directly. The inputs are used to send information back to the interlocking, such as indications determined by track circuit relays or points detection circuits, for example.

There are two kinds of TFM; the signal module (identified by a red label) and the points module (black label). A maximum of 63 TFMs may be addressed by one SSI interlocking; in practice the number will be limited by timing issues and the need to allow for future expansion.

Data links

Communication between interlockings and TFMs is by electronic data packages termed 'telegrams'. Telegrams are transmitted via 'data links', comprising twisted pair copper cable. The data links are duplicated for availability.

A 'data link module' (DLM) is the interface between the data link and the TFMs. A DLM has a blue label.

For transmission over longer distances, fibre-optic cable and pulse-code modulation may be used. Another type of module, the 'long distance terminal' (LDT) is available for this purpose. An LDT has a gold coloured label.

Market penetration

SSI is widely installed within Great Britain, and has some penetration of other Western European markets. It was first used at Dingwall in 1984 in connection with RETB signalling. The first conventional SSI scheme was at Leamington Spa in 1985.



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