Solid State Interlocking
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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.
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 consists of 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.
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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