your portal to more fun in scale model railroading
your portal to more fun in scale model railroading
Welcome to the DCC pages!
This is the official site of the NMRA Standards & Conformance Department's Digital Command Control Working Group. The Working Group consist of manufacturers and NMRA volunteers, who develop, revise and extend the DCC Standards. The volunteers also test DCC equipment for compliance to the Standards. The participants in the DCC Working Group are experienced in electronics and computer technology. The Working Group is very active, meeting in person a few times each year and by video conference all other months.
DCC is an acronym for Digital Command Control. To explain what command control is, some historical background is needed.
Traditionally, the speed and direction of a model train has been controlled by varying the voltage and polarity on the rails. The higher the voltage, the faster the locomotive moves, the lower the voltage, the slower the locomotive moves. If the right rail is positive with respect to the left rail, the locomotive moves forward, if it is negative, the locomotive moves in reverse.
Being able to control the speed and direction of a train is great, but how do you control more than one? Model Railroaders have come up with many ingenious methods to control multiple trains. The basic method used has been block wiring. The model railroad layout is divided up into electrical blocks, each of which can control one locomotive. A cab (or throttle) is used to control each train. Arrays of selector switches connect each block. This method of control is also called cab control.
Probably the most ingenious method of cab control is the one called progressive cab control. As a train moves around the model railroad, the connection between the cab and the block is automatically switched by relays to the next block, and the present block is released for another train to use.
For a small layout, with one or two trains, block control was simple and straightforward to wire and install. For bigger model railroad layouts, the task can be immense.
The next evolutionary step is command control. Command control is a method of controlling individual locomotives (or trains) at the same time on the same rails. The first command control system was the Astrac system, developed by General Electric in 1964. As the electronics industry grew, so did the methods of controlling model trains.
Two of the most popular systems were Keller Engineering's Onboard system, and PSI's Dynatrol system, which both used audio tones to control each locomotive. Both of these systems worked well, but a user was still limited in the number of trains that could be controlled.
If imitation is the sincerest form of flattery, then CVP Products must be blushing. In 1978, Model Railroader magazine published a series of articles by Keith Gutierrez on how to build your own command control system. The system, called the CTC-16, could control up to 16 different trains, all on the same track. Many other companies used the same methodology to control 32 or 64 trains.
The problem with these systems was the lack of standardization. There was no common ground between the systems (with the exception of the CTC-16). In the late 1980's, the NMRA started investigating command control systems to see if there was enough common ground to create a Standard.
Rather than re-invent the wheel, the DCC Working Group studied all of the commercially available command control systems. Proposals were received from Keller and Marklin. The Working Group found that the best system to base the new standard on was a system invented by Lenz Elektronik, which was used at that time by Marklin for their 2-rail sets. This system offered the best signaling method electrically and offered the fewest limitations on expansion.
The NMRA Working Group expanded the design, allowing for control of ten thousand possible locomotives, turnouts, and multiple-unit lash-ups. The DCC Working Group continues to clarify and expand the existing Standards and Recommended Practices as the need arises.
Computers and computerization have reached into nearly every aspect of our lives. You now find microprocessors in your automobiles, your kitchen appliances, and in many areas of the workplace. Over the years there have been a number of control systems developed to help operate our model railroads. From the simplest DC 'Power Pack' with a rheostat control to walk-around hand-held throttles to more sophisticated radio-controlled trains and various forms of command control, we have continually tried to bring more control and realism to our model operation. With DCC, that goal is now within reach.
DCC is short for DIGITAL COMMAND CONTROL. It is not just another method of control, it is a standard for manufacturers to use to make their products compatible with other products on the market that meet this standard. In this respect, the playing field has been leveled and we can begin to reap the benefits of controlling things in ways that we have never been able to control before.
The DCC signal is a bipolar square waveform, which contains digital information. Long pulses are zero and short pulses are ones. A Command Station sends aproximately 8,000 bits per second. The frequency does vary between about 5kHz-9kHz depending on the information sent and the bit duration being near the nominal values and not the max or min values. This excludes zero stretching, which some manufacturers still support to allow operation of DC analogue locomotives on DCC systems. This coded signal controls a specific decoder placed in a specific locomotive causing it to use as much of the track voltage as it needs to move forward or backward, turn its headlight on or off or even dim it for meets with other trains on another track. Decoders can be set so that locomotives that could never have run together with a conventional power pack can double-head or even operate in pusher service without one of the locos working too hard or not hard enough.
Many of the newest locomotives have light boards that can be removed, and a decoder added in the same space. Others come equipped with a standardized NMRA compatible plug and socket to make decoder installation a snap. Today many locomotives are available factory equipped with an NMRA DCC compatible decoder installed, sometimes already configured for the loco. Decoders today offer many lighting effects, multiple lighting outputs, and prototypically correct sounds. Decoders are dropping in price while functions, sounds, and performance are improving.
Configuration of the systems and decoders depends upon the complexity of the system that you have. Each decoder has an address and will only read instructions addressed to that decoder. Digital because they are digital packets. Command because you use those packets to send commands to the decoder. Control because that is the result.
Block Wiring (with no switches) may still be required for proper power distribution, and/or detection & signal systems to function properly. However, in many cases block wiring and all of those block switches will no longer be necessary. No longer does the operator have to worry about turning a rotary switch to get power to their loco or to free up the block behind. Slow speed operation and switching moves may be improved and constant lighting is already there. You can control onboard sound systems and have flashing ditch lights and markers; all of the above or as many as you wish.
You may control more than one train or device with only one controller. Device? Yes, it doesn't have to be a locomotive, it could be a crane on the rails or in a nearby junkyard or at a construction site. How good is your imagination?
NMRA Bulletin references:
Jan 94 page 32 - Recommended Practices for Digital Command Control
Stan Ames
Feb 94 page 24 - Standards for Digital Command Control
Stan Ames and Dave Cooper
Apr 94 page 40 - Digital discussion
Edward Loizeaux, a member of the NMRA Command Control Working Group, takes a non-technical look at why the committee thinks digital standards should be approved.
Feb. 1996 pg 19 - The Reading Lines go DCC
Bill Gruber's Reading Lines layout has been featured in many model magazines. He shares how he converted this operations-intensive layout to Digital Command Control in this article.
Updated 4-Jun-2022