Digital Command Control

Digital Command Control (DCC) is a standard for a system to operate model railways digitally. When equipped with Digital Command Control, locomotives on the same electrical section of track can be independently controlled.

The DCC protocol is defined by the Digital Command Control Working group of the National Model Railroad Association (NMRA). The NMRA has trademarked the term DCC, so while the term Digital Command Control is sometimes used to describe any digital model railway control system, strictly speaking it refers to NMRA DCC.

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 58 µs for a half cycle), while a zero is represented by a longer period (nominally at least 100 µs for a half cycle).

Each locomotive is equipped with a mobile that takes the signals from the track and, after rectification, routes power to the motor as requested. Each decoder is given a unique running number (address), and will not act on commands intended for a different decoder, thus providing independent control of locomotives anywhere on the layout, without special wiring requirements. Power can also be routed to lights, smoke generators, and sound generators. These extra functions can be operated remotely from the DCC controller. can also receive commands from the controller in a similar way to allow control of turnouts, uncouplers, other 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), depending on the choice of commercially available base systems. 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.

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