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A Look at Solid State Interlocking

a)      Check setting of route. If any point is not in the desired position see if it is free to be moved. If it is free then move it to the desired position

b)      Set flags that the points required for the movement is not available for other movements

c)      Check that the track circuits over which the train has to move are free that is no other train is waiting on any of them

d)      Check that no signal in exactly opposite direction is being given at the same time

e)      Clear the signal

f)        Wait for the movement of the train monitoring the track circuits

g)      When the train crosses the signal given to it put back the signal to red.

h)      Hold the points which have been set over the route till the train completes the journey over the point held.

i)        Release the route holding of points ahead known as overlap once it is established that the train has come to a stop.

This is not a very complicated logic and any body can do it easily. But then why go into specialized software for Railway signaling? Again the reason is the consideration of safety as the software cannot determine that everything is safe when it is not. Normal software will never guarantee this good enough for Railway Signaling where human life is involved.

If we think in terms of conventional computer equipment we expect a operating system to sit between the failsafe hardware and the application software. The operating system will be able to take care of allocation of hardware elements to those called by the application software allocating to it necessary resources as required. However the operating system for failsafe application has to be failsafe also. Therefore it is necessary that if a bit is called to be output the command given to the hardware is crosschecked. He operating system can do so by employing similar techniques as used for failsafe hardware that of feed back and command to withdraw if the input command do not match the output. We now need to understand the difference the software in the failsafe hardware at the last stage and at this stage. In the last stage the output and input drives actual signaling devices and so has  to satisfy the physical equipment  while the Operating system only drives this hardware outputs and checks the inputs without any outside world interference. This means that the world driven by the operating system is virtual in nature and thus can be more flexible. To make this clear Windows 98 prints a page from a document but you may change the printer without change of the operating system as the operating system only feeds the information to the printer driver. The same will apply here. How it can simplify the situation is this that a reference to Home Signal is enough to ensure conditions of operations a home signal to be  carried safely by the hardware.