Each track section will have a signalling aspect at it beginning. This unit will indicate either a red (stop), green (go), yellow (the next aspect is at red, for headway). The signalling aspects are separate module of the signalling system. The inputs from C167(3) will determine what aspects are shown. Since the signalling aspects are a separate functional unit of the system it should independently restore to a safe state if the unit that it communicates with fails.

A controller has to be build into the signalling aspects to take in the input from the C167(3) and signal the appropriate aspect. If the C167(3) was to fail, the controller should identify this and the signalling aspect should restore to a fail safe state (red aspect).

The C167(3) transmit a pair of bits (X0, X1) to each signalling aspect unit. These bits will tell the signalling aspect unit what aspect it should be indicating.

The truth table above shows the relationship between the inputs and the outputs. The first line of the table, where both inputs are zeros is the failure input (i.e. C167(3) has failed or the transmission lines have been disconnected). So for fail safe operation this input will issue a red aspect.

There are several ways of implementing the above truth table. A separate microcontroller can be used to read the inputs and issue the outputs. The alternative is to combinational digital logic. The boolean functions of the output are:

The red aspect function is the exclusive-NOR of the two inputs. The yellow and green functions can be re-arranged using De-Morgan's theorem so they can be implemented using NOR gates rather than AND and NOT gates. NOR gates are more readily available on integrated circuits than AND gates. The NOT function is implemented using NOR gates by tying the input together.

Figure 1: Controller logic

Figure 1 above shows the controller logic implementation using NOR and Ex-NOR gates. The signalling aspect unit has its own power supply and is not reliant on other units to supply it with power.