From the desk of
Sandeep Patalay
The Computer Based Interlocking Architecture
The Solid state Interlocking systems for Railways should
ensure the following:
- Fail safety
- Availability
- Reliability
- Maintainability
Generally following three types of
redundancy techniques are used for achieving fail-safety in the design of
signaling systems:
Hardware Redundancy –
In this case, more than one hardware modules of identical design with common
software are used to carry out the safety functions and their outputs are
continuously compared. The hardware units operate in tightly syncronised mode
with comparison of outputs in every clock cycle. Due to the tight
syncronisation, it is not possible to use diverse hardware or software. In this
method, although random failures are taken care of, it is difficult to ensure
detection of systematic failures due to use of identical hardware and software.
Software Redundancy –
This approach uses a single hardware unit with diverse software. The two
software modules are developed independently and generally utilize inverted
data structures to take care of common mode failures. However, rigorous self
check procedures are required to be adopted to compensate for use of a single
Hardware unit.
Hybrid Model - The
hardware units have been loosely syncronised where the units operate in
alternate cycle and the outputs are compared after full operation of the two
modules. Therefore, it is no more required to use identical hardware and
software. Although the systems installed in the field utilize identical
hardware and software, the architecture permits use of diverse hardware and
software. Moreover, operation of the two units in alternate cycles permits use
of common system bus and interface circuitry.
To ensure the
above said points hardware and software is designed accordingly. There are
various techniques to meet the above said requirements as discussed below:
Table
1:
Existing Failsafe Methods employed in Design of Computer Based Interlocking
Systems
Method Name
|
Method of Implementation
|
Type of Errors Detected
|
Practical Problems with the Method
|
Time Redundancy
|
The same software is executed on
the same hardware during two different time intervals
(Refer: Figure
5: Time Redundancy)
|
Errors Caused by transients. They
are avoided by reading at two different time Intervals
|
Single hardware Fault leads to Shut
down of the System. This method is not used since software faults are not
completely found in validation. And the Self diagnostics employed for
checking of hardware faults is not complete.
|
Hardware Redundancy
|
The same software is executed on
two
identical hardware channels
(Refer: Figure
6: Hardware Redundancy)
|
Hardware faults are detected since
outputs from both the channels are compared. And single hardware fault does
not lead to shut down of the system
|
Software faults are not detected
since the same software is running on two identical hardware channels.
Software Faults at design stage are still not detected.
|
Diverse Hardware
|
Identical Software is Executed on
Different hardware Versions
(Refer: Figure
7: Hardware Diversity)
|
Hardware Design faults at the
Initial stage are Detected
|
Software Faults at the design stage
are still not detected
|
Diverse software
|
The different software versions are
executed on the same hardware
during
two different time intervals
(Refer: Figure
8: Software Diversity)
|
Software Faults at design stage are
detected
|
Even though the software is
diverse, they are executed on the single hardware channel; single hardware
fault leads to Shut down of the system.
|
Diverse software on
redundant hardware
|
The different software versions are
executed on two identical hardware
channels
(Refer: Figure
9: Diverse software on redundant
hardware)
|
Software Faults at design stage are
detected and single hardware faults does not lead to system shut down
|
Hardware faults at the design stage
are not detected.
|
Diverse software on
diverse hardware
|
The different software versions are
executed on two different hardware
channels
(Refer: Figure
10: Diverse software on Diverse
Hardware)
|
Software Faults and Hardware Faults
are detected at the design stage.
|
This methods is rarely used, Since
design complexity involved is high
|
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