Digital Command Control (DCC) is an exciting field in model railroading that uses digital computer technology to operate model railroad trains.
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What It Does
DCC puts digital control information into the electrical power. I'll call this the DCC signal, although its both power and control information. The DCC signal is put on the model railroad's tracks.
Then DCC "mobile decoders" in locomotives on the tracks use the DCC signal to power and control their locomotive's speed, direction, and lights. "Stationary decoders" may also be connected to operate the track's turnouts and other accessories.
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How It WorksThe user operates a throttle or "cab" equipped with a numeric keypad and other control buttons. Information from the throttle is sent to a DCC "command station" which converts it into DCC control information. This DCC control information is then fed to a "booster" which uses it to create the DCC signal and feed it to the tracks.
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Why It's ModularThe throttle, command station, and booster may be individual electronic components or bundled in multi-function units. DCC's modular design makes it extremely flexible. Large layouts with lots of locomotives will require more power than than one single booster can deliver; and every "engineer" wants his or her own throttle. DCC accommodates these requirements. Its modular design lets model railroaders tailor their DCC system to their own specific needs.
Basic DCC starter systems are... all-in-one units. If you're buying an all-in-one, make sure that it is expandable or can be integrated with the manufacturer's other, modular components.
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CompatibilityThe DCC standards specify voltage levels, pulse durations, and digital information formats. This means that a decoder from any manufacturer will respond to commands and programming signals from DCC systems made by any manufacturer.
Connection of DCC control system components has been left up to the individual manufacturers. Unfortunately this means that throttles, command stations, and boosters from different manufacturers aren't usually compatible.
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SoundSome manufacturers offer "sound decoders". In addition to implementing all the standard DCC locomotive control functions, these decoders are connected to a small audio speaker through which they play locomotive sounds. Diesel locomotives play a diesel motor hum which changes pitch relative to engine speed. Steam locomotives emit the characteristic chuff sounds at speed appropriate rates. Brakes squeal and valves hiss. High end sound decoders feature programmable sound tables. Using a... computer equipped with proprietary sound decoder programming software, and interfaced to the DCC controller, users of high-end sound decoders may customize the sounds played by their locomotives.
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Control ExecutionEach locomotive on the track layout is equipped with a uniquely addressed DCC mobile decoder. Using a throttle, the operator can select any locomotive on the track by its address and alter its speed, direction, lighting, and other features. DCC allows multiple throttles to control different locomotives simultaneously. Unselected locomotives continue running at their last settings. The brand and model of command station determines how many locomotives may be run simultaneously on your layout.
The... DCC decoder has a power circuit that converts the DCC power signal into DC power for the motor and lights. The decoder also has digital logic which uses the power signal information to control the locomotive.
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Decoder ProgrammingDecoder programming has evolved a bit since the introduction of DCC. Originally locomotives could only be programmed on a "programming track". DCC controllers have special terminals for connecting a programming track. The locomotive cannot run on the programming track, it can only be programmed there.
Decoders are configured by setting "configuration variables", or CVs. Today some decoders allow CV values to be programmed while the train is running on the main track. The one... exception is that the CV values specifying the locomotive's unique address cannot be changed while running; addresses can only be changed on the programming track.Continue to 9 of 12 below.
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Decoder AddressingMost decoders are shipped with their locomotive address set to 3. Each locomotive on a layout must have a unique address, so new DCC equipped locomotives and those that have just had decoders installed need their addresses reprogrammed. Many modelers program a locomotive's address to the locomotive's road number, the number painted on the locomotive itself.
Programming addresses can only be done when the locomotive is on a programming track. This is because the programming signal would... change the addresses of all locomotives on your track if you didn't isolate the locomotive you want to program on the programming track.
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Reading Decoder Configuration Variable (CV) ValuesEarly decoders could only be written to. When a CV was programmed with a new value the decoder could only respond by flashing the lights and pulsing the motor of the locomotive to indicate that it had received the update. Most decoders still flash lights and pulse motors, but newer decoders also have the capability to transmit CV values back to the DCC command station through the tracks, though not all command stations and throttles can receive and display them.
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Current DCC standards have been adopted by both the National Model Railroad Association (NMRA) and the European Union of Model Railroad and Railroad Friends (MOROP).
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An excellent introduction to the workings of DCC electronics can be found on the website of the Armadillo & Western Railroad Co. However, this page hasn't been revised for a while and specific models of DCC decoders it mentions have been replaced by newer, smaller size models.