Digital command control (DCC) is, without a doubt, the greatest improvement in model train technology since the birth of the hobby. The ability to control our model locomotives like the prototype has long been the desire of every modeler. When combined with sound modules, digital controllers can be used to create realistic locomotive operation with sound effects which are controlled by the operator or programmed to operate automatically. Digital decoders for steam, diesel, or electric locos can be installed and in some cases are available as drop-in units for a specific manufacturer’s locomotive. Turnouts, lights, and other special effects can be operated by fixed digital decoders. The ability to run multiple trains, at different speeds, in opposite directions on the same section of track is only one of the advantages of DCC. Special effects such as momentum allow locomotives to act like the real thing when the throttle is opened and the engine responds with a programmable increase in acceleration. The same is true when the throttle is closed and the loco slows gently to a stop. Braking power can be set to determine the amount of time for the engine to reach a full stop when applied. The effects are programmed through variables called CV’s (configuration variables) and are defined by numbers such as CV-1 etc...
Model trains are usually controlled by adjusting the D.C. track voltage and switching polarity to change the engines direction. Throughout this discussion we will refer to this as conventional control. In order to run more than one train at a time requires wiring track sections as blocks and using separate controllers for each locomotive. Switches are used to select which controller is connected to a given block and two trains cannot occupy the same block at the same time. This arrangement is called cab control and for many years before the advent of command control was the only way to run multiple trains simultaneously.
Digital command control applies a constant voltage to the rails and uses a decoder (receiver) installed in the locomotive to decode digital signals from the command station to determine how much voltage to send to the motor as well as which direction to travel. Unlike conventional control, each digital command control decoder with its unique address will decide what direction and how fast the train moves and control is independent of the track voltage and polarity.
A typical Digital command control system is composed of the following devices:
A Command Station which takes input from the operator and sends coded messages to the decoders telling them what action to take.
A Booster combines signals from the command station with output from the track power supply and sends both through the track rails to the decoders. Some layouts may use several boosters and divide the layout into separate power districts.
A Power Supply usually consists of a step-down transformer and supplies electrical power to the system.
Decoders located in or near all remote devices controlled by the DCC system. For example, decoders in locomotives decipher signals from the command station to control and adjust the power to the loco’s motor to match the desired speed.
A Throttle is the device used to input commands by the operator to the command station. This unit usually has a keypad for programming and a throttle control to adjust speed.
Digital Command Control is a computer system but one does not have to be a computer guru to understand how the system works or how to install it on your layout. I will try to provide some information on basic computer circuits and coding for those hobbyists who want to know more about the technology but thankfully this is not necessary to install and program the equipment.
Years ago, when it became apparent that DCC was going to be a viable system, the need for standardization prompted the NMRA to develop specifications for the components and timing of the DCC signal. These specifications cover the make-up of the signal only and do not address the electrical components. What this means is that any decoder, regardless of make will work with any other manufacturers system. What it does not mean is that command stations, boosters, and other equipment are interchangeable between manufacturers. Most DCC firms use their own proprietary methods of communication between their units and there is no standard covering this. The message here is---with the exception of decoders, do not mix and match DCC components.
The advantages of DCC as mentioned above are obvious. The main disadvantage may be the price of a full-blown system although this should be weighed against the cost of a block wired cab control system with all of its wiring components and controllers. If you already have a conventional cab control layout –then the decision must be made as to the advantage of re-wiring for DCC. I have heard comments from other modelers who are not in the mood to change all of those switches, indicator lights, and control panels that they worked so hard to install years ago. I have also heard from those who made the switch and swear by the increased control and ease of wiring of command control. This decision, of course, is up to the individual. I made the choice about eight years ago and at this point my layout is all DCC and 80% of my locomotives are equipped with sound. In the beginning, I was hesitant to tear apart a prize steam locomotive and install a decoder in the tender-and naturally my first effort did not work. The problem, however, was not in the installation but in the way I programmed the decoder. In my anxiety to get things running, I skimmed over the operational instructions that came with the decoder (which by the way was a different manufacturer than the command system). I feel foolish giving the following advice---read the instruction pamphlet before programming the device! Most locomotives today are “DCC ready” which generally means that one unplugs a dummy connector and simply plugs in the decoder. The dummy connector allows the engine to run on a layout wired for conventional D.C. voltage operation.
There are a large number of DCC firms who produce both complete systems and individual components such as decoders and other products. For the beginner, there are limited feature systems which are fairly inexpensive and simple to install. I hope that the subjects covered in this overview and the accompanying pages will give the reader enough information to decide what these features are and which ones will meet their needs.
Digital Command Control Theory of Operation
Digital Command Control Technical Information