Safeguarding a Railway Control System with Path Expressions

摘要

The report shows how path expressions can be used to specify the safetyrequirements of a discrete control system in the form of a safety layer between the controlling software and the controlled hardware. The task of the safety layer is to check if the actions that the control layer wants to perform are allowed. The authors examine two different philosophies for the safety layer: A fully transparent layer that remains hidden in the background until the control layer tries to do something illegal and then notifies the control layer, and a layer that cooperates with the control layer in such a way that the control layer will never even try to do anything wrong.