Digital Command Control

Digital Command Control (DCC) systems are used to operate locomotives on a model railroad (railway). Equipped with DCC, locomotives on the same electrical section of track can be independently controlled. 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. Several major manufacturers offer DCC systems.

How DCC works

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

A DCC command station, in combination with its booster, modulates the voltage on the track to encode digital messages while providing electric power.

The voltage to the track is a bipolar DC signal. This results in a form of alternating current, but the DCC signal does not follow a sine wave. Instead, the command station quickly switches the direction of the DC voltage, resulting in a modulated pulse wave. The length of time the voltage is applied in each direction provides the method for encoding data. To represent a binary one, the time is short (nominally 58µs for a half cycle), while a zero is represented by a longer period (nominally at least 100µs for a half cycle).

Each locomotive is equipped with a mobile DCC decoder that takes the signals from the track and, after rectification, routes power to the motor as requested. Power can also be routed to lights, smoke generators, and sound generators. A stationary decoder can be attached to the rails to allow control of turnouts, uncouplers, operating accessories (such as station announcements) and lights.

In a segment of DCC-powered track, it is possible to power a single analog model locomotive, by itself or in addition to the DCC equipped engines. The technique is known as zero stretching. Either the high or the low pulse of the zero bits can be extended to make the average voltage (and thus the current) either forward or reverse. However, because the raw power contains a heavy AC component, DC motors heat up much more quickly than they would on DC power, and some motor types (particularly coreless electric motors) can be damaged by a DCC signal.

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 available.

Advantages

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A DCC decoder installed in an N scale steam locomotive.

The great advantage of using DCC over traditional DC systems is the simpler wiring needed to operate more than one locomotive at a time. Before, to operate more than one locomotive independently, the track had to be wired into separate "blocks" with switches selecting which controller powered which block of track. If an operator failed to switch control of a block before his locomotive entered, a short circuit or loss of control was possible. With DCC, many layouts can be wired as a single large block, and each operator can control his locomotive without worrying about crossing a block boundary.

DCC controllers can include an "inertia" simulation, where the locomotive will gradually increase or decrease speeds in a realistic manner without continuous inputs from the operator. Mobile decoders are available which will adjust the power to try to maintain a constant speed, again without burdening the operator. Most DCC controllers allow an operator to set the speed of one locomotive and then quickly select another locomotive to control its speed.

Recent developments include on-board sound modules for locomotives as small as N scale.

Competing Systems

In Europe, Selectrix is an NEM standard, but the Märklin-Motorola system is proprietary and used only in Märklin products. From the U.S., the Rail-Lynx system provides power with a fixed voltage to the rails while commands are sent digitally using infrared light. Other systems include the Digital Command System and Trainmaster Command Control.

DCC, however, is the most widespread. DCC was originally developed by Lenz of Germany, and they allowed the system to become an open standard. DCC has been adopted by both the NMRA and the NEM as a standard. Several major manufacturers (including Märklin and latterly Hornby), have entered the DCC market alongside makers which specialize in it (including Lenz, Digitrax, and Train Control Systems).

External links

• NMRA Standards and Recommended Practices page
• Wiring for DCC
• Hornby DCC Product Information Page