DCC - Digital Command Control
for Model Train Sets


    Before DCC an electric model locomotive would be controlled by applying a variable direct current through the track; The train's direction would be changed by changing current polarity and speed by varying the voltage. This is the system most familiar to enthusiasts and has been used at every level from the train set oval to the most extensive exhibition layouts. It is tried and tested and it works. Through the application of electronics the level of control has been enhanced and analogue systems can provide acceleration and braking delay, load compensation, automatic operation and other sophistications that many people erroneously believe are only possible with DCC. The great majority of exhibition layouts in UK are still controlled this way and are a testament to what can be achieved. This is not achieved without cost however and needs considerable electronics knowledge. The great drawback of analogue DC is that the complications of control grow almost exponentially as a layout grows and all because of one inescapable fact; all locomotives on the same track will go in the same direction at the same speed varying only because of their physical characteristics. From this comes the need for numerous isolated sections, block control and complicated control panels. The National Model Railroad Association (NMRA) in the USA, in co-operation with the manufacturers, set the standard for a new way to control locomotives. This was called DCC.
    Although there are other competing standards the rest of this article will assume that of the NMRA. The NMRA Digital Command Control Standard defines a basic communications structure at the track level for digital control signals via the rails. The standards specify a communication protocol between transmitter and decoder without specifying transmitter and decoder hardware. The data needed to operate each decoder is transmitted in packet format on the rails in the form of a balanced square wave. This baseline packet format allows for interoperability among equipment made by different companies that support the standard. Interoperability is the most important advantage of the standard. Interoperability means that if you have a DCC compatible decoder, you can run it with any DCC compatible command station. This is very important since the major part of your investment in any DCC system is in the decoders. We have all heard the horror stories: “I have a fortune invested in this equipment and now I can’t even get spare parts let alone expand my system!!!” Any system that is available from more than one source is not as likely to disappear and leave its users stranded. Also, having equipment available from multiple suppliers creates competition in price and features to the benefit of the end user.

With Digital Command Control (DCC) you use a controller (also called cabs or throttles) to send information to a command station telling it what you want train X to do. The command station then takes this information, transforms it into a stream of digital 'packets' and sends it to the booster. The booster will add power to the packets, and broadcast the combined signal to the rails. In most modern systems the basic set combines the command and booster functions in a single unit; the NCE Power Pro and Gauge-master systems are typical examples. DCC systems send commands and decoders receive and act on them

The decoder-equipped locomotives on the railway constantly listen to the ’packet’ broadcast. Each information packet has an address component to it which should match the address of one of the decoders. Any decoder which is not the intended recipient of the packet simply ignores the data and its locomotive keeps on doing whatever it is doing - running forward, backward, lights on etc. The decoder, to which the data packet is addressed, will translate the packet into a command for the locomotive such as ‘slow down’, ‘stop’ or 'reverse direction’, and the locomotive will behave accordingly. The power on the tracks is alternating current (AC), and not DC or direct current. Full power is running through the tracks at all times while the decoder applies the appropriate amount of voltage and polarity to the motor based on the speed and direction in which you want the locomotive to travel.

Operation is far more exciting with each train running independently. You can use double heading or banking and match the speeds of locomotives from different manufacturers. You can program realistic acceleration and deceleration rates, or limit the top speed of a locomotive. DCC has advantages for everyone from the beginner to the advanced modeller and for every layout from the smallest to the largest. Once you have the basic system you can decide how much of the available functionality you want to use and can expand the system as your layout grows; the equipment you already own moves on with you as you add more features. Your largest investment in time and money is often in the decoders you install in the locos. These are upwardly compatible as you expand and add to your system. By simply adding components you can grow into a more advanced system at your own pace and as your budget allows. Most home layouts are small or medium sized. They typically have a limited amount of track available for analogue block control; DCC has a real advantage in these situations. Since blocking is not required you can operate more locos in a smaller area. For the large home or club layout DCC offers truly prototypical operation and minimum wiring complications. Layouts running with DCC can operate more than 2 or 3 trains at a time; the outside loop running clockwise and the inside loop running anti-clockwise all day is not very exciting. The ease of wiring makes connection simple and lets you get operating sessions up and running more quickly. The addition of sound or computer control are examples of the increasing range of features that you can take advantage of. Your railway can be as simple or complicated as you wish. Above all DCC enables you to control your trains just like the prototype.
Although DCC offers great advances in control it is not a cure-all for old problems and brings one new problem. If an item functions poorly using DC just adding DCC will not cure it. If a locomotive runs badly using DC the cause will still be there after you install your decoder. It is best to test a locomotive on DC before conversion if possible, even for new locomotives, and undertake any maintenance or repair that is needed. If your track is dirty or poorly maintained your railway will not run well. Wiring for DCC is different because all track is live all the time but there is just as much need for good connectivity. Points with dead frogs produce the same problems as before and live frogs are still better; the wiring is easier though. DCC cannot substitute for care and attention; in some ways it demands more. With DCC you really can run two trains at different speeds in opposite directions towards each other on the same piece of track with the obvious result. With even basic start sets able to run up to ten locos simultaneously you need to pay attention.
For the newcomer it is too easy to look at the technical specifications of DCC systems and be impressed by the numbers that are thrown at you without really understanding them. It is generally true that the larger the numbers the more powerful and capable the system is likely to be but it is not necessarily a guide to how easy it is to use.


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