Safety of a block instrument is ensured by a foolproof logic sequence and relay interlocking. Safety for the Block Interface Equipment means it must correctly acquire the signal and reproduce it at the other end without any error. In case of any failure, it is detected and the system is lead to a fail -safe state.

However, there are some very remote possibilities when t he system may not be able to detect a failure which may lead to an unsafe situation. This is defined as Mean Time between Wrong side Failure (MTBWSF) and according to RDSO specification SPN 147/2005, the figure is 10 9 hrs. This can be very well ensured by using 2 out of 3 hardware redundancy logic at the I/O Interface and processor level. With 2/3 redundancy system, as soon as an anomaly is detected, the system is forced to a fail -safe state leading to a RED signal. Mishaps are thus avoided, but the mov ement of the train is also restricted. To increase the availability of the system, thus triple hardware redundancy has been proposed at the I/O interface and Processor level so that even if a failure is detected in one set of hardware fail safe operation can still continue with 2 sets of hardware, thereby increasing availability. Better availability is also ensured by using dual redundant power supply unit for the system.

• Designed for fail safe operation.

• Triple redundant hardware at all levels with 2/3 decision logic ensures high degree of safety as well as availability.

• System meets CENELEC & ORE 155 guidelines for safety and reliability.

• System meets RDSO specification SPN-144 and SPN-147/2005 for all types of Block Instruments in operation in Indian Railways.

• Simple, modular and rugged design.

• High performance 16 bit Intel 80C196KC Microcontroller used for better software management.

• CMOS/FET active components to ensure low power consumption and better reliability.

• Hi-rel components, relays and connectors used to ensure high MTBF.

• All input-output connections are optically and/or galvanically isolated to eliminate ground noise and earth faults.

• Rugged communication protocol for transmission of error-free data.

• Dual power supply in hot-standby mode increases system availability.

• 16-digit dot-matrix alpha-numeric display for user friendly man -machine interface.

• Environment specification meets IS-9000.

The system is housed in a 19 inch subrack, 6U (10.5 inches) high, which is fixed inside a closed rack of approx 5 ft high. The front panel of the closed rack is a hinged door fitted with transparent plastic sheet so as to enable the operator to see the LEDs and displays of the system. The door is also provided with a lock to protect the system from unauthorized access. The rear panel of the closed rack is normally fixed with screws. The various cards which have been used in the system are as follows:

The above modules are housed in a passive backplane which has the provision of accommodating 12 modules. The basic system consists of two power supply units connected back to back, three system units and one communication driver unit. The above modules consume six fixed slots. The subsequent six slots are for accommodating two nos. of each of Input, Feedback and Output modules. The last slot is not used.