Generally following two types of redundancy techniques are used for achieving fail-safety
in the design of signaling systems:
Hardware Redundancy – In this case, more than one hardware modules of identical design with common software are used to carry out the safety functions and their outputs are continuously compared. The hardware units operate in tightly syncronised mode with comparison of outputs in every clock cycle. Due to the tight syncronisation, it is not possible to use diverse hardware or software. In this method, although random failures are taken care of, it is difficult to ensure detection of systematic failures due to use of identical hardware and software.
Software Redundancy – This approach uses a single hardware unit with diverse software. The two software modules are developed independently and generally utilize inverted data structures to take care of common mode failures. However, rigorous self check procedures are required to be adopted to compensate for use of a single Hardware unit.
Hybrid Model - The hardware units have been loosely synchronized where the units operate in alternate cycle and the outputs are compared after full operation of the two modules. Therefore, it is no more required to use identical hardware and software. Although the systems installed in the field utilize identical hardware and software, the architecture permits use of diverse hardware and software. Moreover, operation of the two units in alternate cycles permits use of common system bus and interface circuitry.
There are two methods of programming the SSI for a particular station, namely – Geographical Programming & Free-wire Programming. Most of the SSI systems adopt geographical programming where Control Table of the station is fed to the SSI system. This gives great relief to the user as he is not required to make the circuit diagram, data for any station can be programmed very easily. However, this method does not provide flexibility in terms of change of interlocking practice or interlocking rules. Adoption of change in interlocking practice of a particular railway requires changes to be made in the executive software and entails revalidation of the software. On the other hand, Free-wire programming, which requires circuit diagram to be prepared for each station and programmed into the EPROM as Boolean expressions, provides total flexibility as any given circuit can be programmed without touching the executive software. The price, however, is to be paid in terms of preparation of circuit diagram for each station.
