The DCC protocol


The DCC (Digital Command Control) protocol enables precise, flexible digital control of model trains, by sending digital signals across the track and using decoders to convert these signals into commands for locomotives, wagons and accessories.

Locomotives and their functions (lights, engine sound effects) as well as layout accessories (points, light signals) each have a unique address. The coded signal sent to the track gives orders to the equipment while supplying power.

This system is defined by an NMRA (National Model Railroad Association) standard.


Here's how it works:

  • The DCC control unit sends digital signals across the rails of the model railway.
  • Each locomotive or wagon is equipped with a DCC decoder which receives the digital signals.
  • The DCC decoder converts the digital signals into commands for the locomotive or wagon.
  • Commands can include speed, direction, lighting, sounds, etc...
  • The DCC control unit can send individual commands to each locomotive or wagon positioned on the same track, enabling precise, independent control of each receiver (decoder).

The DCC can also be used to program traffic scenarios, create routes, and simulate real-life rail traffic conditions.

The DCC frame

The principle of a DCC frame is to transmit digital information to a locomotive or accessory decoder. The DCC frame is sent in the form of frequency-modulated electrical signals on the track rails. Each frame contains a unique address identifying the decoder for which it is intended, as well as instructions for controlling the speed, direction and special functions of the locomotive or attachment. The frame is sent continuously at regular intervals to maintain control of the locomotive or attachment. The decoder interprets the frame and executes instructions to control the motor, lights and other functions of the locomotive or attachment.

Here are the basic principles of a DCC frame:

  1. A DCC frame is made up of binary bits (0 and 1) that are sent sequentially.
  2. The frame begins with a preamble to synchronize the various decoders (a sequence of several bits in the "1" state).
  3. Next, the frame contains an address identifying the locomotive or decoder that the DCC control unit wishes to control.
  4. After the address, the frame contains command bits which tell the locomotive, accessory or wagon what to do, such as move forward, move backward, stop, switch on the lights, etc...
  5. Finally, the frame ends with a check sequence to verify its consistency.

The number of bytes in a frame is not fixed, and may include more than 2 bytes of information. For example, to manage extended addresses, frames contain two address bytes.

In a nutshell, a DCC frame is a digital message sent by the DCC control unit to a DCC decoder to control a locomotive, wagon or accessory. It is made up of binary bits containing an address and commands to control the receiver.

General principle of a DCC decoder

A DCC decoder is an electronic device that controls the functions of a model train using a digital DCC (Digital Command Control) signal. The DCC signal is sent by a DCC control center, which sends digital data packets to each train decoder.

The DCC decoder is equipped with a microcontroller which interprets the DCC signal information and uses it to control train functions. The DCC decoder is also equipped with a power amplifier which controls the train speed (motor control).

The DCC decoder is programmable, which means modellers can configure train functions to suit their needs. They can also program several DCC decoders to control several trains on the same track.

In short, the DCC decoder controls the functions of a model train using a digital DCC signal, using a microcontroller and power amplifier to interpret the signal information and control the train's functions.