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.
DCC system was originally developed by German company Lenz GmbH in the 1980s for two German model railway manufacturers, Märklin and Arnold which later pulled out of the co-operation due to patent issues. Lenz GmbH has continued to develop the system. When the NMRA Command Control committee requested submissions from manufacturers for their proposed command control standard in the 1990s, Märklin and Keller Engineering submitted their systems for evaluation. The committee had settled on digital early on in the process, and was impressed by the Märklin/Lenz system. The NMRA would eventually licence the protocol from Lenz and extend it, into what is now the NMRA DCC Standard.The system was later renamed to DCC and was adopted as a NMRA standard.
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 the acronym "DCC" unambiguously means the NMRA standard protocol. Sometimes the term Digital Command Control is used to describe any digital model railway control system, while strictly speaking it should refer to NMRA DCC only.
How DCC works
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 58µs for a half cycle), while a zero is represented by a longer period (nominally at least 100µs for a half cycle).
Each locomotive is equipped with a mobile DCC decoder that takes the signals from the track and, after rectification, routes power to the motor as requested. Power can also be routed to lights, smoke generators, and sound generators. A stationary decoder can be attached to the rails to allow control of turnouts, uncouplers, 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.
Protocols
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 available.
The proposed standard was published in the October 1993 issue of Model Railroader magazine, prior to its adoption.
The DCC protocol defines signal levels and timings on the track. DCC does not specify the protocol used between the DCC command station and other components such as additional throttles. A variety of proprietary standards exist, and in general, command stations from one vendor are not compatible with throttles from another vendor.
Advantages
The great advantage of using DCC over traditional DC systems is the simpler wiring needed to operate more than one locomotive at a time. Before, to operate more than one locomotive independently, the track had to be wired into separate "blocks" with switches selecting which controller powered which block of track. If an operator failed to switch control of a block before his locomotive entered, a short circuit or loss of control was possible. With DCC, many layouts can be wired as a single large block, and each operator can control his locomotive without worrying about crossing a block boundary.
DCC controllers can include an "inertia" simulation, where the locomotive will gradually increase or decrease speeds in a realistic manner without continuous inputs from the operator. Mobile decoders are available which will adjust the power to try to maintain a constant speed, again without burdening the operator. Most DCC controllers allow an operator to set the speed of one locomotive and then quickly select another locomotive to control its speed.
Recent developments include on-board sound modules for locomotives as small as N scale.
Competing Systems
In Europe, Selectrix is an open NEM standard, but the Märklin-Motorola system is proprietary and used only in Märklin products. From the U.S., the Rail-Lynx system provides power with a fixed voltage to the rails while commands are sent digitally using infrared light. Other systems include the Digital Command System and Trainmaster Command Control.
Several major manufacturers (including Märklin and latterly Hornby), have entered the DCC market alongside makers which specialize in it (including Lenz, Digitrax, NCE Power Pro and CVP Products' EasyDCC, and Train Control Systems).
See also
External links
- The NMRA's trademarked DCC logo
- DCC History at the DCCWiki
- The DCCWiki
- NMRA Standards and Recommended Practices page
- Wiring for DCC
- Hornby DCC Product Information Page
- Model Rectifier Corporation - Manufacturer of DCC related products
- Digitrax - Manufacturer of DCC related products, Mobile Decoders, Starter Sets, etc
- Train Control Systems - Manufacturer of DCC related products
- NCE Power Pro System - Manufacturer of DCC related products
- CVP's EasyDCC System - Manufacturer of DCC related products
- Digital Command Control or DCC Explained