Digital Command Control (DCC) is a model railway control system which allows controls of individual locomotives, rather than control of sections of track. This is a powerful way of controlling multiple locomotives on both simple and complex layouts. To understand its benefits, it is necessary to understand the alternative approaches.

Conventional model railway control systems work by providing a power feed to a piece of track; any locomotives on that track respond by moving forwards or backwards. This is both cheap and simple to install, and is fine for simple layouts. However, if two locomotives are on the powered section of track, then both will move in the same direction.

In order to control several locomotives - to which most modellers will aspire - it is necessary to electrically isolate them from each other. In very simple layouts, this can be achieved using the power switching offered by points. For example, a locomotive on a siding will be isolated from the main line as long as the point is not switched to the siding. This allows the siding to be powered separately and might permit shunting on the siding while another train operates on the main line.

Most users will want more complicated - and realistic - operations. This might include several trains working on the same line. This is commonly achieved by dividing the line into isolated sections and powering those sections selectively. By choosing which controller is connected to which track sections, it is possible to control several locomotives independently. This is the basis of cab control. Cab Control is in widespread use throughout the model railway community and works well. However, it has a limitation: the track needs to be divided and wired into fixed sections, which ultimately impose limitations on the train movements which can be achieved.

In a Command Control system, a different approach is used. A DCC controller is able to communicate with each locomotive individually, allowing each to be given its own commands independent of the operation of other locomotives. This approach does not need the track to be divided into sections and locomotives can be controlled individually wherever they are located on the layout. A command control system relies on two fundamental units: a controller to broadcast commands to the track, and a decoder located within each locomotive to handle those commands.

The first Command Control system was the Hornby "Zero One" system. This was released in the late 1970's and was for a long time unique. It is now obsolete, although there is an active market in secondhand Zero One components. Many people have bad things to say about Command Control which are based on their experiences with Zero One. I never used that system; however I understand that it did work.

The DCC system was invented by Lenz, a German manufacturer. It uses a digital signal broadcast over the rails and provides a set of protocols to control both locomotives and accessories. The protocols have been designed to cope with power interruptions to the locomotive - for example when running over points. It has been adopted by a large number manufacturers and is by far the most popular command control system worldwide. The standard is now maintained by the American NMRA organisation.

DCC operated by applying a modulated DC signal onto the rails. This is generated by a control unit in response to operator commands. Each locomotive requires a decoder installed within it: this decodes and executes those commands intended for itself. The DCC system allows a large number of locomotives (100+) to operate at the same time on the same set of rails - the only real limits are budgetary!

DCC was originally designed to control locomotives. However it is perfectly possible - to use DCC to control points, signals, turntables and any number of other accessories around the layout.

Components of a DCC System

A DCC system consists of several components. Today some of these tend to be combined into a single unit but their function still exists. A typical system will consist of:

• One or more Throttles: the component with which the user controls speed and direction of locos.

• A Command Station: this unit takes commands from throttles and generates a set of data packets to be sent to the track.
• A Booster: this unit takes the DCC command packets and amplifies them to drive the rails. Boosters normally provide enough output current to drive several (5-10) locomotives.
• A Programmer: this unit is used to set up each mobile or accessory decoder, so that it knows its own unique Address (identification number) and other optional information.
• Several Mobile Decoders: one of these is installed into each locomotive to allow the command packets intended for it to be decoded and executed.
• Optional Accessory Decoders: those units decode DCC packets to control stationary accessories, for example point motors, signals and turntables.

A typical system might include one box which has the command station, programmer and booster function, into which one or more throttles can be plugged.

Mobile Decoders

These units are small modules designed to be installed into locomotives to provide individual control of them. Most DCC decoders allow relatively complex control of their attached locomotives. Typically the following functions are available:

• programmable acceleration and deceleration;

• programmable maximum speed;
• PWM speed control, allowing very slow minimum speeds;
• control of around 4 accessory outputs (e.g. lights).

Most decoders offer very similar capabilities. They do vary substantially, however, in size and cost. Early decoders were quite large, and a challenge to fit. Today many locomotives are manufactured with decoder sockets, and "chipping" is simply a case of removing a blanking plug and plugging in the new decoder. Decoders can be fitted easily into "N" gauge tank loco, and into Z gauge locomotives: so it can be done!

Controllers

DCC controllers, on the other hand, are all very different from each other. A very wide range of products is available offering different facilities at different costs. It is essential to decide what you want from a DCC system to enable a system to be chosen. Some systems incorporate throttles into the main box; some use external hand-held throttles. Some allow any number of hand-held throttles; some may only allow one.

The principal differences between systems are generally those not controlled by the DCC standards. Some significant differences between different systems are:

• Number of locomotives controlled: some low cost systems will allow control of a small total number of locomotives (e.g. 10). More complex systems may allow control of a far larger number.

• Throttle Bus: some systems support a proprietary bussed connection between throttles, into which additional throttles may be plugged. This allows the number of simultaneous operators to increase as more throttles are purchased. Most such systems will allow the throttles to be plugged and unplugged during operation: this allows the operator to walk around the layout and plug the throttle into a socket local to some feature of the layout. The throttle bus specification may be made available as an open standard, allowing others to develop items which interface to it.

• Feedback Bus: the basic DCC standards allow signals to be sent from the command station to mobile and accessory decoders. They do not allow transfer of information in the reverse direction. To permit sensing of information within the layout (e.g. positions of points, positions of locomotives) some systems have a proprietary feedback bus. This allows sensors to be interfaced to the central unit and information from them to be made available to the operator. The feedback bus and throttle bus may be the same bus.
• Computer Interface: some systems have an interface to which a PC may be connected. This allows, for example, a PC program to automate operations on the layout - at least to some extent. Commercial programs are available to assist with this.

   

The choice of a controller system really hinges on the capabilities to which you may want to upgrade in the future. The low cost DCC controllers generally permit very limited expansion beyond the basic product. Others will cost more up-front, but will allow greater capability on day one and greater expansion potential. try to choose the system which, when expanded as far as you want to go, has the features you will want. Not all systems meet that test!

Why Use DCC?

There are many reasons why use of DCC may be of benefit. These are likely to be personal to you: your aspirations as a modeller and/or model trail operator will greatly influence your perception of DCC. Some potential reasons for choosing to use DCC are:

• Prototypical operation: DCC allows each individual locomotive to be controlled, not a section of track. It is quite possible to have three locomotives independently controlled with any two feet of track on the layout. (Whether you wish to do this is, of course, a different matter).

• Momentum simulation: most decoders will allow the user to simulate the momentum of a locomotive, with acceleration and deceleration programmed by the user. Some systems allow these values to be changed "on the fly" to simulate the load of a heavy train etc.
• Simplified wiring: There are different wiring rules for DCC, but in general there will be less wire beneath the layout. Certainly it is easier to plan the wiring required than with a block cab controlled layout.
• Upgradability: it is easy with many DCC systems to add new throttles to permit additional operators to control trains independently. My research suggests that this needs to be planned on day one with a cab controlled layout.
• Centralised control: It is possible to control all locomotives, points, signals, turntables etc. from a common system with a common set of interfaces. It is still possible to have control panels for points etc. if you want them in addition to command control. All of these resources can be interfaced to a PC for automation - if you desire.
• Ego: DCC is probably the highest level of technology generally applied for train control. It is modern, digital, upgradeable, sexy. Whether it is beneficial to you is really for you to decide.  
• Computer control: it is possible to interface a personal computer to a DCC system to control all of the functions of that system. There are software packages available allowing all of the operations of a model railway to be automated. This allows train operations according to a timetable, on-screen display of track status etc.

It is only fair to mention that the up-front costs of getting into DCC are significant and almost certainly higher than required for block cab control. This provides you with a control facility which can be used to achieve almost anything electrical on a model railway.

DCC in the UK

DCC is everywhere, with DCC systems available in most high street model shops. All the major UK manufacturers have DCC systems, and locos are available either DCC ready or with decoders already fitted. It is available for use, and ready for use, now. Decoders can be bought from approximately £10: the cost of entry is not the barrier it once was.