page_title: Digital Command Control
section_title: How DCC works
hierarchical_section_title: Digital Command Control / How DCC works
caption_attribution_description: Esempio di pacchetto DCC
context_page_description: Digital Command Control 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. 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.
context_section_description: The system consists of power supplies, command stations, boosters and decoders.
  A DCC command station creates the digital packet. Many command stations are integrated with an amplifier (booster) which, in combination with its power supply, modulates the voltage on the track to encode digital messages while providing electric power. For large systems additional boosters may be employed to provide extra power.
  The voltage to the track is a pure digital signal. The DCC signal does not follow a sine wave as it is not an  AC waveform. The command station/booster quickly switches the voltage on the rails on and off, resulting in a modulated pulse wave. One rail is always the inverse of the other, with each data pulse repeated. The length of time the voltage is applied provides the method for encoding data. To represent a binary one, the time is short (nominally 58 µs), while a zero is represented by a longer period (nominally at least 100 µs). As there is no polarity, direction of travel is independent of the rail's phase.
  Each locomotive is equipped with a multifunction DCC decoder that takes the signals from the track and, after rectification, routes power to the electric motor as requested. Each decoder is given a unique running number (address) for the layout, 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. Stationary decoders 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 large harmonic 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.
caption: A short, mid packet, example of a DCC signal and its encoded bit stream
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