Digital Command Control

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A Digitrax DCC system
A Digitrax DCC system

Digital Command Control (DCC) is a scheme for controlling locomotives on a model railroad layout that allows one or more locomotives to be independently controlled on the same electrical section of track.

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[edit] How DCC works

Each locomotive is equipped with a mobile DCC decoder that receives signals from the track and provides power to the motor and any lights the model may have, and in certain cases, other on-board functional devices such as uncouplers, smoke generators and sound generators While DCC is only one of several alternative systems for digital model train control, it is often misinterpreted to be a generic term for such systems.

In DCC, a Command station and booster combination places both the power and the messages on the track using a scheme where the actual power modulation encodes the digital bits of the messages. A stationary decoder can also be attached to the rails at a fixed spot, to allow control of switches and lights.

One large advantage of using DCC over traditional DC systems is the simplification of the wiring required to operate more than one locomotive on any layout at the same time. DCC makes it easier to run more than one train at a time under independent control.

A short, mid packet, example of a DCC signal and its encoded bit stream.
A short, mid packet, example of a DCC signal and its encoded bit stream.

DCC uses bipolar DC, which is a form of alternating current, to provide power and transmit data, while traditional systems use DC for power. Because the power is bipolar DC, the DCC signal does not follow a sine wave. Instead, the control system quickly switches the direction of the DC current, resulting in a square wave. The length of time that the current is flowing in each direction provides a method for encoding data. To represent one, the time is short (nominally 58µs for a half cycle). A zero is represented by a longer period (nominally 100µs for a half cycle).

A DCC decoder installed in a N scale steam locomotive.
A DCC decoder installed in a N scale steam locomotive.

[edit] DCC can normally power a single analog model locomotive

In a segment of DCC-powered track, it is possible to power a single analog model locomotive by itself or in addition to DCC-aware engines through a method known as zero stretching. In this manner, zero bits on the track can be extended to create a net effect where current appears to the engine to be flowing in one direction or another. However, because the power is actually closer to AC, many DC motors heat up much more quickly than they ordinarily would on DC power, and some motor types (particularly coreless electric motors) can be damaged by a DCC signal.

[edit] DCC protocols

The DCC protocol is the subject of two standards published by the NMRA: S-9.1 specifies the electrical standard, and S-9.2 specifies the communications standard. Several recommended practices documents are also made available. The Standards and Recommended Practices are available here.

[edit] DCC compared with other systems

There are several competing digital train control systems, most notably the European standard, Selectrix, and the proprietary Märklin-Motorola system, which is used exclusively in the model train products of the Märklin company. DCC, however, is the most widespread system. One of the reasons for this is that DCC is a widely adopted standard both in North America and in Europe. DCC was originally developed by a German company Lenz that decided to allow its system to become an open standard. As a result, DCC was adopted as a standard by the National Model Railroad Association and it is also a NEM standard.

[edit] DCC manufacturers

[edit] See also

[edit] External links

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