Digital Command Control

The digital command control (DCC), in French “digital control system” is a standard used in the model railway to individually control of locomotives or track accessories by modulating the supply voltage of the way.

This system, defined by a National Model Railroad Association ( NMRA ) standard, was subsequently adopted by the European Union of Railway Model Trainers and Friends of the Railways ( MOROP ).

The locomotives and accessories (lights, sound effects powered units) and network accessories (switches, routes) each have a unique address. The coded signal sent on the channel gives commands to the equipment while providing the power.


The DCC system was initially developed by Lenz Elektronik GmbH (Germany) for the Märklin and Arnold rail model suppliers . The first decoders appeared at the beginning of 1989 for Arnold (scale N) and mid 1990 for Märklin (scales Z, H0 and 1).

In 1992 the NMRA began to evaluate this system as a possible candidate for a future standard of control. The system has been selected and named DCC. The NMRA has a license from Lenz and the official ‘9’ series were issued at the end of 1993 1 .

MOROP then adopted the system under the NEM 670 2 standard .


A control unit modulating the voltage of the track is interposed between a power supply box and the power supply cables of the track.

Mobile (locomotives) and fixed (lights, switches, lighting) equipment are equipped with decoders enabling them to interpret the control signals. These decoders include a number of parameters, called ‘CV’ or ‘configuration variables’. Only certain configuration variables are mandatory, but not all: some are recommended, others optional, or even available to each manufacturer who assigns them according to his needs. It is the same for their content, which varies according to the manufacturer 3 .

Control units

The control panels allow either manual control using a command interface or an automated command. It is thus possible to define routes and traffic scenarios by adjusting the position of the switches and the control of the locomotives (equivalent to the train paths ). The plants can be controlled by computer. There is control software available on Windows or Linux.

The manual controls are either directly on the control panel, or with a wired remote control or radio, or via tablet or smartphone.


Locomotive decoders incorporate a dimmer for controlling electric motors. They also control lights, lights, sounds, smoke generator and sometimes a stall system.

Accessory decoders generally provide all-or-nothing signals and are often equipped with relays.

The setting of the decoders is done most generally using the control unit by passing into a so-called ‘programming’ mode which transmits the programming signals on the channel or on the output wires. A specific track segment is typically used to program each equipment individually without disturbing others.


‘Boosters’ are current amplifiers, which retain the voltage form of the signal present at their input, but allow the equipment to use a higher intensity, providing greater power for a network with many locomotives or accessories running. simultaneously. The output voltage can be adjusted to a different value than the input one, only the shape of the pulses being kept in order to keep the transmitted codes. There may be several ‘boosters’, to separate the power supply from the accessories and the track and also to create a number of ‘cantons’ to detect trains. The same signal will be sent to the different boosters.

Control panels

The command interface sometimes called ‘regulator’ (cab) can take several forms:

  • A control panel directly on the central
  • A computer with a special program (connected by wire or wifi to the central)
  • A box connected by a wire to the central, sometimes called ‘mouse’
  • A wireless box connected by radio to the central
  • A wireless phone (analog link) controlling the central
  • A ‘smartphone’ connected by wifi to the central

The majority of plants agree to have multiple orders operating simultaneously. This allows for example each operator to fly his own locomotive. Panels on larger units or computers, for example, allow the display of an image of the network, facilitating a more global view and control of accessories.

The interface buses of the control panels

Some manufacturers have defined an electrical interface and a protocol to connect the controls (or radio receivers) to the control panel. These standards are not an integral part of the DCC standard and there is a variety of offers from the manufacturers. In English, these buses are called ‘cab buses’. In parentheses providers using the bus 4

  • LocoNet (Digitrax, Uhlenbrock, Fleischmann) – also used for feedback –
  • XpressNet / X-bus (Lenz, Arnold, Hornby, Roco, CT-electronik, ZTC)
  • NCE Network (NCE)

Principles of coding

The control signals are grouped in packets sent successively, each of the packets being intended for a single address. There may be several decoders with the same address, which will then receive the same orders.

A DCC control station, in combination with its ‘booster’ (signal amplifier), modulates the voltage of the channel to send digital messages while providing the power supply.

The voltage of the channel is a bipolar continuous signal. This gives a form of alternating current, but the signal is not sinusoidal. Voltage inversions are instantaneous giving a pulsed signal. The duration of the pulse in each direction provides the coding of the signal. To define a bit of ‘1’, the duration is short (58μs for a half-cycle) while a bit ‘0’ is represented by a long period (at least 100μs for a half-cycle).

Each locomotive is equipped with an onboard decoder that retrieves the signal from the track and after recovery, provides the power to the engine following the setpoint. The power can also be used for fires, smoke generator and sound. A fixed decoder can be attached to the rails for control of switches, decouplers and other track accessories such as voice announcements and lighting.

With some power plants, in a DCC-powered track segment it is possible to use a single analog-controlled locomotive in addition to the locomotives controlled by DCC. The technique is called zero stretching. The high or low pulse is changed to give a mean value of voltage in one direction or the other. However, the transients of the DCC signal heat engines and some locomotives equipped with ironless rotor motors can be destroyed with this principle of operation.

Advantages and disadvantages

The control is fully centralized and each equipment has its own decoder, the wiring is significantly simplified compared to the relay control systems.

For an automated circulation, the sequences are defined by programming without any modification of wiring, which opens great possibilities of modifications and reorganization.

However, all locomotives must be ‘digitized’ by equipping them with a decoder. If on modern machines, the connection of a decoder is planned at the time of construction, the old equipment must be modified and re-wired. The cost of retrofitting is therefore important if we add the price of the plant to that of locomotive decoders and accessory decoders.

If the wiring is simple, it is replaced by an important programming step of the decoders.

Initially, the DCC was only sending orders without being able to collect feedback information (‘feedbacks’). To fill this gap, several incompatible systems have been created. The DCC standard has since been supplemented by the ‘Railcom’ standard, but the other already well-established standards have created a fragmentation of offers that is detrimental to good development. The addition of a back-signaling system allows complete automation but increases the price of the installation.


The DCC system was originally intended only for sending orders without the possibility of feedback, whether from the track or from the locomotives. This has led to the development of ‘bus’ retro-signaling in particular to report to a central occupied track sections. There are therefore several standards available.


In 2006 Lenz, in coordination with Kühn, Zimo and Tams, started to develop an extension to the DCC protocol allowing feedback from the decoders to the control panel. This return signal makes it possible in particular to indicate which train is currently running on such section (canton), but also makes it possible to give the real speed of a locomotive. The name of this feedback system is Railcom and was standardized in 2007 by NMRA RP 9.3.1.

Other standard back-signaling

In parentheses manufacturers offering equipment following these standards organized in ‘ bus ‘.

  • Loconet (Digitrax, Fleishmann, Uhlenbrock, Roco, Zimo)
  • S88 (ESU, Fleishmann, Tams, Uhlenbrock, Viessmann)
  • RS-feedback (Lenz)

Schemes of principle

DCC for garden train

In a garden train, clogging of rails and junctions poses specific problems for the use of DCC by obstructing good signal and power transmission.

It is expected in principle when a channel feed that the signal DCC is transmitted by a main cable of large section called ‘feeder’, connected at very regular intervals (2 to 6 m) to the track to maintain a good quality of current.

There are alternative possibilities.

  • The radio transmission of the DCC signal, the trains then being powered by battery, or even by the way but with energy storage.
  • The transmission of the DCC signal in the possible way in the absence of contact between the track and the wheels (the channel then serving as an antenna). In this case, an energy storage system of a few tenths of a second is mounted on the decoder (Lenz ‘gold’).


  1. ↑  [ archive ] standard 9
  2. ↑  [ archive ] Standard 670
  3. ↑  [ archive ]
  4. ↑  [ archive ]

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