Digital Command Control

Digital Command Control (DCC ) is a standard for digital train, signal, and switch control of model trains. The standard is based on the development of the German company Lenz Electronics ( for Marklin ). The oldest digital decoder DCC are thus essentially the delivered in the 1980s locomotive decoder by Arnold ( N scale ) and Marklin (Digital DC for H0- Hamo and lane 1).

Later, Märklin had separated from the bilge and then digital DC said system; Lenz continued to develop the system and tried to recognition as a standard by the NMRA.

Technical Basics

Since around the end of the 1990s, the system DCC is in 2- rail 2-wire area the most widely used digital system, only in 3-rail 2-wire area (Märklin H0 ) is the Märklin-Motorola digital system prevalent although in 2004 a new system was put on the market: Märklin Systems ( mfx ). Another, also long standing digital system is SelecTRIX (mainly in N scale ).

DCC both the necessary voltage for operation of the trains as well as control information from a digital control unit are transmitted to the decoder via the track. Also switches and signals ( accessory ) can be controlled via the DCC signals. Standardization subject alone is the protocol ( the traffic on the tracks ), but not the communication between headquarters and other input devices, boosters ( = power amplifiers ), etc. In Europe, are of importance for the input side XpressNet (originally X bus of Lenz) plus and LocoNet (developed by Digitrax ). LocoNet integrated input (stationary ) feedback and booster control in a "bus", while it added two more signal buses at X-Bus/XpressNet.

The standard DCC was further developed in the course of time. Initially, there were still only 14 steps, 99 locomotives and 256 accessory addresses, the current standard requires 14, 27, 28 and 128 speed steps and 10,239 locomotive; most vehicles and some decoders are limited to 9999 locomotive addresses. Despite the development of the system, the current control and decoders are backward compatible, so that always the weakest component determines the overall possibilities.

The DCC standard has now been expanded to include a mechanism for feedback of information from the decoders to the central office via the track ( railcom ).

For easy installation of decoders also by non-specialists the interface connectors have been standardized. The six-pin plug according to NEM 651 is provided for confined spaces; the eight-pin plug according to NEM 652 offers an optional common return for lighting and other functions as well as a free pin for an additional function.


Packet format

The data rate of the track is approximately 8000 bits per second. Each DCC packet begins with a 10-14 bit-long preamble is used for synchronization of the decoder. In a standard packet, a follow address byte, a command byte and a check byte which are in each case preceded by a start bit. The whole package is completed by a stop bit. The check byte is calculated as the XOR operation on the address and command byte.

This Standard Package, however, is no longer the rule, since it is only suitable for controlling locomotives with these constraints. For the switches and other function decoder and modern locomotive decoder extended package format has been defined, which may contain between three and six data bytes.

Motor control

A decoder converts the motor in the appended track non- sinusoidal alternating current with the help of a bridge rectifier to direct current. The motor is supplied with this DC clocked transistors. Unlike in analog mode, not the voltage, but the duty cycle determines the engine speed ( pulse width modulation).

Reading of decoders

Decoder save configuration-relevant data in eight -bit wide registers ( Configuration Variable, CV). There are up to 1024 CVs. The DCC standard requires as a compulsory CVs before the CV1 ( base address ), CV7 (Decoder version ), CV8 ( manufacturer ID) and CV29 ( decoder base configuration). The meaning of some CVs is also standardized, others are manufacturer specific verifiable.

DCC was the simplicity sake originally designed as a unidirectional protocol. The only way to recover from transmission of information to the control center was the so-called pulse acknowledgment (ACK ). In this case, the decoder for 6 ms increases its power consumption to 60 mA by turning on his motor or other consumers. This increased current consumption is recognized by the Central and interpreted as confirmation of the last programming command. Thereby, it is also possible to read the value of a certain CV. In older decoders, this query is only byte-wise possible, so that in extreme cases up to 256 possible values ​​of a CV must be polled until eventually an ACK comes from the decoder. Newer decoders can be bitwise queried so that the maximum number of necessary queries per CV reduced to eight.


From the company Lenz and a " working group railcom " with other manufacturers of DCC components an extension of the DCC protocol was developed that enables the decoders to send data over the track during operation. Railcom was standardized by the NMRA RP 9.3.1 than in 2007. To make room for the railcom transmissions, breaks in the light emanating from the Central DCC signal are cut. In a pause of up to 8 bytes can be transferred. The evaluation of the railcom data streams can either locally (that is usually at the level of separate track circuits ) or ( usually at the level of the DCC office) be global. Classic application of a local detector is the vehicle detection in the context of track occupancy detection. A global detector can not assign a local transmitting decoder, but they may include applications such as the Register of a locomotive at the head office.

Mixed operation

Since it is in principle possible ( strictly speaking alternately in milliseconds ) to send Märklin-Motorola digital system and DCC signals at the same time on a track, both systems use particular many 3-rail 2-wire (Märklin H0) systems simultaneously. Almost all on the market digital control units ( centers) are to produce signals capable of DCC and Märklin-Motorola. Also Märklin own current headquarters, the Central Station 2, can next Märklin -Motorola and DCC mfx also generate signals and thus control locomotives and turnouts with DCC decoders.