Update:

Actual code has been added below for those who want to look at the details

Logic for single block
Logic for trailing-point main block
Logic for trailing-point diverging block
Logic for facing-point main block

JMRI: Simple Signal Logic

Background information of the logic used to control simple signals

This page describes the logic used by the JMRI Simple Signal panel to control signals.

On Single Block

This signal protects one end of a straight through block, with no signaled turnouts.

    void doSingleBlock() {
        int appearance = SignalHead.GREEN;
        int oldAppearance = ((SignalHead)outputs[0]).getAppearance();
        // check for yellow, flashing yellow overriding green
        if (watchedSignal1!=null && protectWithFlashing && watchedSignal1.getAppearance()==SignalHead.YELLOW)
            appearance = SignalHead.FLASHYELLOW;
        if (watchedSignal1!=null && watchedSignal1.getAppearance()==SignalHead.RED)
            appearance = SignalHead.YELLOW;

        // check for red overriding yellow or green
        if (watchSensor1!=null && watchSensor1.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor2!=null && watchSensor2.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor3!=null && watchSensor3.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor4!=null && watchSensor4.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;

        // show result if changed
        if (appearance != oldAppearance)
            ((SignalHead)outputs[0]).setAppearance(appearance);
    }

On Main Leg of Trailing-Point Turnout

This signal is along the main route through a turnout, which is defined as the direction taken by trains when the turnout is closed. It's protecting the frog of the turnout so that trains will stop before running through a turnout set against them.

    void doTrailingMain() {
        int appearance = SignalHead.GREEN;
        int oldAppearance = ((SignalHead)outputs[0]).getAppearance();
        // check for yellow, flashing yellow overriding green
        if (watchedSignal1!=null && protectWithFlashing && watchedSignal1.getAppearance()==SignalHead.YELLOW)
            appearance = SignalHead.FLASHYELLOW;
        if (watchedSignal1!=null && watchedSignal1.getAppearance()==SignalHead.RED)
            appearance = SignalHead.YELLOW;

        // check for red overriding yellow or green
        if (watchSensor1!=null && watchSensor1.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor2!=null && watchSensor2.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor3!=null && watchSensor3.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor4!=null && watchSensor4.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchTurnout!=null && watchTurnout.getKnownState() != Turnout.CLOSED)
            appearance = SignalHead.RED;

        // show result if changed
        if (appearance != oldAppearance)
            ((SignalHead)outputs[0]).setAppearance(appearance);
    }

On Diverging Leg of Trailing-Point Turnout

This signal is along the diverging route through a turnout, which is defined as the direction taken by trains when the turnout is set to "thrown". It's protecting the frog of the turnout so that trains will stop before running through a turnout set against them.

    void doTrailingDiverging() {
        int appearance = SignalHead.GREEN;
        int oldAppearance = ((SignalHead)outputs[0]).getAppearance();
        // check for yellow, flashing yellow overriding green
        if (watchedSignal1!=null && protectWithFlashing && watchedSignal1.getAppearance()==SignalHead.YELLOW)
            appearance = SignalHead.FLASHYELLOW;
        if (watchedSignal1!=null && watchedSignal1.getAppearance()==SignalHead.RED)
            appearance = SignalHead.YELLOW;

        // check for red overriding yellow or green
        if (watchSensor1!=null && watchSensor1.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor2!=null && watchSensor2.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor3!=null && watchSensor3.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchSensor4!=null && watchSensor4.getKnownState() != Sensor.INACTIVE) appearance = SignalHead.RED;
        if (watchTurnout!=null && watchTurnout.getKnownState() != Turnout.THROWN)
            appearance = SignalHead.RED;

        // show result if changed
        if (appearance != oldAppearance)
            ((SignalHead)outputs[0]).setAppearance(appearance);
    }

On Facing-Point Turnout

This signal is protecting the points-end of a turnout. Depending on whether the turnout is thrown or closed, the train will take two different routes, and the signal will protect different next blocks.