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DCC - Digital Command Control

Features

 

Summary

DCC allows the simultaneous operation of almost any number of model locomotives on complex track layouts without the use of conventional block switching, ie. each locomotive does not have to be controlled within a single section of track allocated specifically to it. The speed and direction of each locomotive can be set independently regardless of where it is on the track (a feature, naturally, which has to be used with care!). Control can also be exercised over multiple locomotives treated as a single unit (MUs or consists) and over accessories such as lighting, signalling, and points (turnouts).

The basis of DCC is to supply power to the track in a form which also contains coded information to control the model locomotives and accessories. A fixed voltage (nominally 14 volts for HO scale) is applied to the track. The voltage polarity is continuously reversed electronically between positive and negative, such that the time periods between polarity reversals represent the required control data. A period of 58 microseconds positive followed by 58 microseconds negative represents a binary '1' bit, while consecutive positive and negative periods of 116 microseconds represent a binary '0' bit. Defined sequences of bits are used to represent commands to change speed, for example, and by how much, and to identify which locomotive should respond to each command.

Since the polarity of the applied voltage is positive and negative for equal lengths of time, the net voltage on the track, as seen by a normal (non-DCC) model locomotive, is zero (especially as the polarity reversals are occurring at around 6000 times a second), and the locomotive will remain stationary.

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WARNING :  While most model locomotive motors will not react to the DCC track voltage, other than by making a buzzing' noise, in some cases the motor will become very hot and can fail. It is recommended, therefore, that non-DCC locomotives are NOT placed on a track where DCC is in operation.

 

Unlike a normal model locomotive, a locomotive intended for DCC contains a decoder unit which performs two functions. The decoder picks up and rectifies the alternating voltage applied to the track and makes it available as a direct voltage which can be applied to the locomotive motor. The decoder also interprets the stream of bits encoded in the track polarity reversals as a set, or packet, of command bytes (a byte is a group of 8 bits) and acts upon them to apply an appropriate proportion of the rectified track power to the motor, making the locomotive move forward or backwards at the required speed. Each decoder is given a unique identity (or address) and will only act on command packets which have that address value encoded into them.

As well as controlling locomotive speed, decoders normally provide several auxiliary outputs which can be used to drive locomotive lights or sound units, or other effects when activated by receipt of an appropriate command packet. Because power is always present on the track when using DCC, operation of these auxiliary functions is totally independent of the locomotive speed. Lights, for example, operate at constant brightnesss even when the locomotive is stationary.

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Further Information

See the Links page for links to suppliers and other websites with information on DCC.

The definitive guide to DCC ‘Digital Command Control’ was written by Stan Ames, Rutger Friberg & Ed Loizeaux in 1998 (revised 1999) and is published by Allt om Hobby, a Swedish company, in cooperation with the NMRA. More details can be found on Rutger Friberg’s website. Other useful publications are available from Digitrax, who also provide a lot of data in PDF format while you can freely download.

 

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Page Last Updated  30 May 2006