Digital Command Control
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Digital Command Control (DCC) is a scheme for controlling locomotives on a model railroad layout that allows more than one locomotive to occupy the same electrical section of track. Each of the locomotives has a decoder between the track and the motor, an electronic circuit that reads messages sent over the track and controls the amount of electricity delivered to the motor. A command station/booster 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. Devices that are fixed in one spot, such as switches and lights, can also be controlled by DCC.
The key advantage of using DCC over traditional analog systems is the elimination of electrically isolated blocks of track to allow the operation of multiple locomotives on the same layout. Several other digital control systems have been developed, but DCC is most common. It was originally developed by Lenz and was adopted by the National Model Railroad Association (NMRA), a very influential group in model railroading.
DCC essentially uses alternating current or bipolar DC to provide power and transmit data, while traditional systems use direct current for power. However, the DCC current 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. A zero is represented by a longer period.
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 scheme, 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 an analog segment, and some motor types can be seriously damaged with only a brief encounter with DCC track.
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.
DCC manufacturers
- Atlas
- Bachmann
- Digitrax
- ESU
- Model Rectifier Corp.
- Lenz
- NCE Corporation (http://www.ncecorporation.com)
External link
- NMRA Standards and Recommanded Practices page (http://www.nmra.org/standards/)