Solid State Interlocking

Interlocking hardware SSI utilises a two-out-of-three redundancy 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 Multi Processor Modules (MPMs), two Panel Processors and a Diagnostics Processor (DMPM). In the event of a single MPM failure, an SSI system is capable of continuing to operate on the two remaining MPMs; furthermore, the interlocking functionality does not actually require the DMPM to be operational, as this is used for the technician's terminal only. 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 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. 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 and future ==

Having been developed in the UK, SSI has been widely installed across Great Britain, and has some penetration of other Western European markets. The first operational deployment of SSI was at Dingwall in 1984, where it was used in conjunction with Radio Electronic Token Block (RETB) signalling. In service, SSI has demonstrated higher than anticipated reliability: while the average mean time between failure (MTBF) of the equipment was calculated to be around 2.5 years, long term real world performance has shown an average MTBF of ten years. A further advantage is that they are designed to be programmable with an operator’s specific signalling principles, offering greater flexibility as a result. ==History==

Throughout much of the 20th century, early signal boxes that were typically manually-operated and limited to line-of-sign operations were being progressively replaced by larger power signal boxes, which made extensive use of electromechanically-operated relays that required sizable equipment rooms and frequent servicing. During 1980, three reports produced by a SSI working group were published; these detailed the planned pilot scheme, the safety impact, and the new technology's effects. It was determined that introducing SSI offered savings in three key areas – interlocking equipment, line circuits, and panel circuits; furthermore, that the correct safety techniques would provide an adequate safety level in principle, which had been developed from experiences gained with other microprocessor systems that were then being introduced to the market. It was decided to base SSI around a second-sourced microprocessor that was a widely used industrial standard at that time, and was amply supported with software development aids. A key target for the development of SSI was to achieve a cost saving of 15 percent in comparison to traditional relay interlocks. Another key objective was to avoid any changes to the view of the signalling system to both operators and drivers alike. The BR team were responsible for much of the work behind the design and development of both the hardware and software, including the scheme design language, data transmission system, trackside interfaces, safety validation, and overall project management; GEC and Westinghouse's principal task was to produce the fully-engineering production equipment. The majority of the interlocking software was written by a single individual. During the 1990s, various upgrades and improvements to SSI were mooted. However, these ambitions were heavily disrupted by the privatisation of British Rail and the dismantling of BR's research division. == References ==