Failsafe and Fault Tolerant Systems

Embedded System:

An Embedded system is a combination of hardware and software to do a specific job, unlike a general purpose computing system like a PC even though having good amount of hardware and software is not an embedded system because it does not do a specific function.

Real Time System: A Real time System is an Embedded System, Which operates on current data and not on saved data.

Hard Real Time System or Mission critical System: A real-time computer system must react to inputs from controlled object and from the operator. The instant at which a result must be produced is called a deadline. If by missing a firm deadline a catastrophe could happen, then the deadline is called hard. A real-time computer system that must meet at least one hard deadline is called a hard real-time computer system or a safety-critical real-time computer system.

Railway Interlocking System: A railway interlocking system controls the traffic in a railway station, and between adjacent stations. The control includes train routes, shunting moves and the movements of all other railway vehicles in accordance with railway rules, regulations and technological processes required for the operation of the railway station.

Interlocking Logic: A term used for the logical relationships between physical entities in the railway yard such as points, signals, track circuits, and so on. In SSI, this is programmed in the Software; in relay-based interlocking this is hardwired into the relay circuitry, and in ground-frame interlocking it is manifest in the mechanical linkages between physical components.

Ground-frame interlocking: An Interlocking System When built using mechanical linkages between Levers (Physical Entities) is called Ground-frame interlocking System.

Route Relay Interlocking System (RRI): An Interlocking System When built completely using Electro mechanical relays is called as Route Relay Interlocking System.

Solid State Interlocking System (SSI): An Interlocking System When built using Electronics replacing traditional Mechanical Levers and Electro mechanical relays is called as Solid state Interlocking System.

Reliability: The reliability can be defined as the ability of an item to perform a required function under stated conditions for a stated period of time.

Redundancy: The existence of more than one means of accomplishing a given function. Each means of accomplishing the function need not be necessarily identical.

Hardware (Software Diversity): Two or more different Versions of Hardware (Software) working in a system to achieve a same result.

Failure: The termination of the ability of an item to perform a required function.

Maintainability: The ability of an item, under stated conditions of use, to be retained in, or restore to, a state in which it can perform its required function, when maintenance is performed under stated conditions and using prescribed procedure and resources.

Availability: The ability of an item (Under combined aspects of its reliability, maintainability, and maintenance support) to perform its required function over a stated period of time.