1.1 This International Standard specifies procedures and technical requirements for the development of programmable electronic systems for use in railway control and protection applications. It is aimed at use in any area where there are safety implications. These may range from the very critical, such as safety signalling to the non-critical, such as management information systems. These systems may be implemented using dedicated microprocessors, programmable logic controllers, multiprocessor distributed systems, larger scale central processor systems or other architectures. 1.2 This standard is applicable exclusively to software and the interaction between software and the system of which it is part. 1.3 Software safety integrity levels above zero are for use in systems in which the consequences of failure could include loss of life. Economic or environmental considerations, however, may also justify the use of higher software safety integrity levels. 1.4 This standard applies to all software used in development and implementation of railway control and protection systems including – application programming; – operating systems; – support tools; – firmware. Application programming comprises high-level programming, low-level programming and special-purpose programming (for example, Programmable Logic Controller ladder logic). 1.5 The use of standard, commercially available software and tools is also addressed in this standard. 1.6 This standard also addresses the requirements for systems configured by application data. 1.7 This standard is not intended to address commercial issues. These should be addressed as an essential part of any contractual agreement. All the clauses of this standard will need careful consideration in any commercial situation. 1.8 This standard is not intended to be retrospective. It therefore applies primarily to new developments and only applies in its entirety to existing systems if these are subjected to major modifications. For minor changes, only clause 16 applies