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How Many Wires?

So DCC only requires two wires? Well, it depends upon exactly what you mean by "two wires".

Let's step back a little and consider a DC layout that can run more than one train at a time. It needs to be split into sections or blocks each of which can be switched to the output of two or more controllers. If you want to be able to simultaneously run three trains, for example, anywhere on the layout then you need three controllers and each block must have a three way switch to select the appropriate controller. With a common-return wiring scheme (one rail is connected throughout all sections), each block switch will be single pole, three way to switch the other rail in that block. If there is no common return then each block switch must be double pole to switch both rails in the block. The wiring for these switches can soon add up to a lot of wires.

Theory...

In theory, A DCC layout can be operated with just two wires between the booster and a single point on the track.

...Put Into Practice

In practice, the basic theory only applies to the simplest of layouts, and is extended to include a "power bus" and "droppers". This means that the two wires from the command station are extended around the layout, often running below the baseboard and co-incident with the tracks on the top. Short wires, or droppers, are used to connect to the track at regular intervals. This ensures that each length of track is supplied with a low resistance feed and does not rely on the current being fed from a adjacent piece of track through rail joiners. A bus in wiring terminology is a circuit (or collection of circuits) that connects to more than two points and so the two power wires and the droppers are collectively known as the power bus.

The Argument

The arguments over the number of wires start with the question "Do the droppers count as extra wires?". I do not propose a direct answer to that question, nor the question "Does It Really Matter?". Instead, I invite the reader to think in terms of the number of circuits that a layout requires. The DC example with three controllers has one possible circuit from each controller to the track that requires wiring up via the switches. The DCC layout has only one circuit. This is true regardless of the number of track segments fed by individual droppers. They are all part of the same circuit, the power bus.

Other Considerations

Larger DCC layouts may be split into "power districts", each with its own booster. This may be done because of the number of locos being driven, to enable fault isolation or to introduce auto-reverse sections on reverse loops. In this scenario, both the two wire and one circuit arguments break down since each additional booster adds a new circuit with two wires.

You Wait For A Bus And Three Come Along At Once!

Having introduced the power bus, let's look at the other reasons why a DCC layout may well require more than two wires or one circuit.

Accessory Bus

DCC accessory decoders are sometimes driven directly from the track power bus. This has two potential disadvantages:

• The current required to recharge a capacitor discharge unit (CDU) for driving solenoid point motors can be excessive and disrupt the track power bus unless the CDU is designed with a current limit in the recharge circuit.
• If a train is run up against points set the wrong way, a short circuit can occur which will cause the booster to shut down the track power. The lack of power means that the accessory decoder cannot be used to change the points to clear the short circuit.

The first problem can be avoided by providing a separate power supply to the CDU which takes only commands form the track bus. Both problems can be avoided by connecting accessory decoders to their own power district from which they take both power and commands. The power bus for this power district then becomes known as the accessory bus.

Cab Bus

A "cab bus" is used to connect throttles (handsets or cabs) to the command station.

Feedback Bus

A feedback bus is another bus used in some DCC systems to return information from the layout to the command station. The information could be, e.g., block occupancy, point setting, data from bidirectional communications.