GUIDE TO DIGITAL COMAND CONTROL

There is a wealth of information available about Digital Command Control (DCC) in hard copy (books), soft copy (the Internet) and oral (word of mouth);  there is also a wealth of misinformation and misunderstanding, particularly in the latter category.  

These short articles have been drawn from all three sources and hope, fairly briefly, to provide a little background, some reasons why and to dispel a few myths about Digital Command Control.

POTTED HISTORY

The story of DCC must start with analogue control, or direct DC, whereby an electric model locomotive is controlled by applying a variable direct current through the track;  direction is changed by changing current polarity and speed by varying the voltage.  This is the system most familiar to modellers 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 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 charecteristics.  From this comes the need for numerous isolated sections, block control and complicated control panels.

The search for a method of controlling locomotives individually began as soon as soon as the model electric locomotive arrived but the origins of DCC can be traced back to the 1940s with a two channel system by Lionel Trains using frequency control.  There was not great progress until the late 1970s when a number of sytems began to be developed of which Hornby's Zero 1 was an example along with others such as the Dynatrol system in the US and the Marklin/Motorola format in Europe.  All of these systems are forms of Digital Control but had the drawback that they were competeing manufacturers systems incompatable with each other.

The lack of compatability between manufacturers was a major factor prompting the National Model Railroad Association (NMRA) in the USA, in co-operation with the manufacturers, to negotiate a common standard available to all.  References to DCC have generally become accepted as referring to this standard but other, incompatable, systems can justify this description; the Marklin/Motorola System is an example.  On this website all use of the term DCC assumes the NMRA standard.

HOW IT WORKS

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 code and sends it to the booster. The booster will add power to the code, and broadcast the combined signal to the rails and hence to the decoders. In most modern systems the basic set combines the command and booster functions in a single unit; the NCE Power Pro and Gaugemaster systems are typical examples.

DCC systems send commands and decoders receive and act on them

Packet Broadcast

The digital instructions are sent out in 'packets' all of which comprise three elements:

  • An address component
  • An instruction
  • A quality control element which confirms that the message is correct. 

The decoder-equipped locomotives or accessories 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 was last told to do -  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 a form of 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.

The digital code is included in the power by varying (modulating) the AC waveform which runs at a much higher frequency than domestic electricity supplies.  All of the power is also the message which greatly enhances the effectiveness of transmission.

THE ADVANTAGES

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 counterclockwise 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.

WHAT DCC CANNOT DO

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.  Remember:

  • A short circuit is still a short circuit.  Although wiring is much simplified it must still be done properly and bear in mind the higher current ratings that are sometimes involved.
  • 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 several locos simultaneously you need to pay attention.

Automation

DCC systems are sophisticated manual controllers.  They use computer based technology but do not, by themselves, automate your railway.  Automatic train movements and shuttles can be achieved but require much additional hardware and software to do so.