Full Life Cycle Case Study: development and verification of a suite of operating system level software services to EN 50128 SIL 4 

REP won a contract to produce a suite of operating system software services from which the customer could then build various applications for the rail industry.

The software executed on bespoke hardware cards developed by the customer, which would be mounted in industrial enclosures for deployment on trains and trackside sites.  The products control functions such as automated train operation and traffic movement.

The overall system was assessed by the client’s safety assessor as requiring the highest level of safety critical consideration due to the danger of loss of life should the system fail.  Therefore, the most rigorous level of the rail industry standard, EN 50128 Safety Integrity Level (SIL) 4, was applied.

The customer provided DOORS requirements and prototype code to use as a starting point. A software architecture, high-level design, and then low-level detailed design were all developed in UML using ARTiSAN.

Ada code was then written and statically analysed using SPARK (syntax checks) and Polyspace (data flow analysis) to reduce the chance of errors.

This Ada code could then be dynamically tested against its design using the unit test tool AdaTEST.  This dynamic testing proved not only that the code was functionally correct, but also that there were no scenarios where the algorithms could fail.
 
Finally, the code was tested as an integrated system on a hardware test rig to prove compliance with the high level design and the customer’s requirements.
 
Peer reviews were conducted at the end of every software lifecycle stage by an independent engineer to prove that the output was suitable to be passed to the next stage.

All activities were conducted using processes and standards from the internal Quality System, which was proven to comply with the requirements of EN 50128 through the use of a compliance matrix.

This project successfully concluded and subsequently led to further work on the successor to the system.