The control system contains three types of state machines: CrossingSignal, TrainPosition and Flash. CrossingSignal performs the direct control of the Red light and enable/disables "Flash" sending it commands: Cmd_Flash_Enable and Cmd_Flash_Disable. TrainPosition transforms the sequences of sensor changes into states which represent train positions. Flash carries out only auxilary function: it generates the flash cycle.

FLASH generates a cycling Do signal that is used to drive a flashing (yellow) crossing light. FLUSH oscilates between two states. The oscilation are enabled/disabled by the command Enable or the digital input Enable. The oscilation period is determined by the Timer.

It is the first and only instantiation of the Flash state machine.

Di

The state machine switches on/off the (red) lamp and enables/disables the (yellow) flash lamp. It does it by knowing the states of the state machines TrainPosition.

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On entering the state the (red) lamp is switched off but the (yellow) flash lamp is enabled. There are two imaginable situations: - if any TrainPosition state machine goes into the state Coming the CrossingSignal goes to the state On, - if any TrainPosition state machine goes to the state Unexpected the CrossingSignal goes to the state Undefined. Note that the transition of the TrainPosition state machine to the state Disabled is ignored by the CrossingSignal (it is an intern blocking mechanism between two TrainPosition state machines that control train movements on a single rail).

On entering the state the (red) lamp is switched on and the (yellow) flash lamp is disabled. There are two imaginable situations: - if any TrainPosition state machine goes to the state Leaving or Going, CrossingSignal goes to the state Going - if any state machine TrainPosition goes to the state Unexpected or Missing, CrossingSignal goes to the state Undefined

All state machines TrainPosition work and are in the state Start but the there is no distinct information about the trains positions. Therefore on entering the state the state machine switches on the (red) lamp and enables the (yellow) flash lamp. This is the responsibility of an operator to reset the system. Resetting the system the operator acknowledges that there are no trains in the supervised area. The reset signal goes directly to all state machines TrainPosition that go to the state NoTrain. At that moment the CrossingSignal system takes control over of the crossing signals going to the state Off.

On entering the state the (red) lamp is switched off and the (yellow) flash lamp is enabled. There are two imaginable situations: - if all TrainPosition state machines are in the state NoTrain, CrossingSignal goes to the state Off - if any TrainPosition state machine goes to the state Unexpected or Missing, CrossingSignal goes to the state Undefined

TrainPosition state machine is a part of a systems that controls crossing signals on a train/road crossing. TrainPosition does not perform any direct control function; it just translates the sensor changes into states that represent the train position. The train movements is detected by Sensors L, M, R (check boxes Left, Middle, Right on the picture). By the specification the Sensors L and R are numbered as 1 and 2 (for one direction 1->L and 2->R, for the other direction it is vice-versa). Sensor 1 is the sensor which signals that a train has entered the controlled zone. Sensor 2 is the sensor which signals that a train is leaving the controlled zone. In the system configuration (see project) the proper sensors are then used for each direction. We assume that a train may come from both directions but only one train can enter the sensor zone. The crossing signals may be in the following states: - both lights gray if the control system is not working, - yellow light flashing if there is no train between sensors 1 and 2 or a train just passed the sensor M and is still between sensors M and 1 moving towards 2, i.e. leaving the sensors zone, - red light on if there is a train between sensor 1 and M moving towards M, i.e. aproaching the road, - red light on and yellow light flashing if the situation is not well defined: after a system start, when received an unexpected sensor signal and when the expected sensor signal has not come after a certain time. These situations are considered as unsafe ones and can be resolved only by a manual control: if the situation is cleared the operator may Reset the system. The time needed by the train to pass the distance beetween two sensors is measured by a timer. By entering most states the timer is started. When the timer elapses the situation is considered as an unsafe one and TrainPosition enters the Missing state (train disappeared?).

It is the first instantiation of the TrainPosition state machine: reflecting the position of trains coming on the rail 0 from the left side.

It is the second instantiation of the TrainPosition state machine: reflecting the position of trains coming on the rail 0 from the right side.

It is the third instantiation of the TrainPosition state machine: reflecting the position of trains coming on the rail 1 from the left side.

It is the fourth instantiation of the TrainPosition state machine: reflecting the position of trains coming on the rail 1 from the right side.

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The system goes always into the Start state.

Without the state DelayStart the state machine TrainPosition enters immediately the state Start and the state machine CrossingSignal does not get the event. In effect the outputs Do_Light are not initialized (see the state machine CrossingSignal).

The functioning of the state machine is started by the Reset button which causes TrainPosition to go into the state NoTrain.

TrainPosition waits in this state until a "partner" state machine which controls the other direction returns into the state NoTrain. The Delay timer delays the return to the state NoTrain which ensures that the Sensor_1 is there already inactive.

The timer is halted. TrainPosition waits for a sensor signal. Receiving an active signal from Sensor_1 (a train enters the sensor zone) TrainPosition goes to a state Coming. An active signal from Sensor_M means that something is wrong with the system and TrainPosition goes to the Unexpected state.

The train is over the Sensor 1 entering the controlled zone. The expected changes are: the Sensor 1 becomes inactive (short train) or the Sensor M becomes active (long train) which leads to transitions correspondingly to states Approaching or ApprPresent. The erroneous changes like an active signal from the Sensor 2 forces TrainPosition to go to the Unexpected state.

The train is between Sensors 1 and M moving towards M. If the Sensor M becomes active (the train crosses the road) TrainPosition goes to the Present state. The unexpected active signal from the Sensor 1 (the train has changed its movement direction or another train has entered the controlled zone?) or 2 (the train has missed the Sensor M or another train entered the controlled zone?) forces TrainPosition to go to the Unexpected state.

There is a long train over both Sensors 1 and M moving towards M. If the Sensor 1 becomes inactive TrainPosition goes to the Present state. If the Sensor 2 becomes active TrainPosition goes to the state AllPresent. Disappearence of the Signal M is considered as an error.

The train is over the Sensor M. If the expected sensor changes occur: Sensor M becomes inactive or Sensor 2 becomes active TrainPosition goes correspondigly to the states: Leaving or LeavPresent. If Sensor 1 becomes active it is an error.

There is a very long train over all three Sensors 1, M and 2. The only expected change is Sensor 1 becoming inactive. Other sensor changes are errors.

The train is over the Sensor M and 2 moving towards 2. The only expected change is Sensor M becoming inactive. Other sensor changes are errors.

The train left the sensor M and is between M and 2. The only expected change is Sensor 2 becoming active. Other sensor changes are errors.

The train is over the Sensor 2 leaving the controlled zone. The only expected change is Sensor 2 becoming inactive. Other sensor changes are errors.

By entering the state the alarm UnexpectedSensor is generated and the Timer is stopped. TrainPosition can leave this state if the Reset button is activated and all sensors are inactive. In such a case it goes to the NoTrain state. The active Reset button while at least one sensor is still active generate an alarm info about this situation.

By entering the state the alarm MissingSensor is generated and the Timer is stopped. TrainPosition can leave this state if the Reset button is activated and all sensors are inactive. In such a case it goes to the NoTrain state. The active Reset button while at least one sensor is still active generate an alarm info about this situation.