Long distance data transmission of safety information for the solid state interlocking

British Rail´s Solid State Interlocking (SSI) uses a nonstandard baseband transmission technique for communicating with its trackside equipment. Screened twisted pair cable is used as the transmission medium for a data highway which can be accessed simply by a large number of trackside interface units distributed along the railway. The authors describes the SSI long distance terminal (LDT), as the standards converter is now called. The methods used to perform standards conversion and maintain safe operation under fault conditions are described and the application of the equipment to both point-to-point and drop-and-insert transmission systems is discussed. Test results are presented as evidence of the high level of availability expected