Digital Command Control (or DCC)

Digital Command Control makes reliable, realistic train operation and simplified layout wiring a reality. With DCC you control multiple trains independently on the same section of track without blocking. In the real world, engineers control the speed and direction of real trains. Engines operate under their own power independent of the track. Each engine has its own motion characteristics such as how fast it speeds up (acceleration) and how long it takes to slow down (momentum). A locomotive's performance is influenced by whether it is operating alone or as part of a multiple unit lash-up. The weight of the train also affects its performance. DCC gives you reliable control over all these variables.

The way it was

Before the digital world, we lived in the analog world. The speed of a model train was controlled by how much DC voltage was applied to the rails. The higher the voltage, the faster the locomotive moved. To change the direction of the train, you simply flicked a switch which changed the polarity on the rails.

The problem was that you could only run one train at a time. If you wanted to run the train around on a loop to go back from where you came from, you needed some fancy switches and wiring. This leads us to...

...Block Control

In order to control two trains, with different directions and speeds, you had to rely on "block control". This involved lots of wiring and double-pole double-throw (dpdt) switches, which divided a layout into sections or blocks. By dividing the layout into separate blocks, you could control whatever train was in that block. If you wanted to take your train from one block to another, you had to check and make sure that the dpdt switch was thrown in the right direction. The throttle didn't control the train, it controlled the track. When the train passed from one block to another, you had to change the track you controlled in order to maintain control of the train... and if the two trains somehow ended up in the same block, there was no way to do anything about it.

However, if you wanted to run more than two trains, you had to install a whole bunch of special rotary switches - a complex and expensive proposition. This required an array of power supplies and throttles (one for each train), switches, and wires. The more complex and detailed your operating scheme, the more complex the wiring. All helper districts and yards had to be carefully preplanned. Adding a single operator and train to your schedule might require rewiring most of the layout! There had to be a better way!

DCC Control

With DCC, train operation depends on the decoder installed in the locomotive. The track is powered by a command station and/or booster connected to a transformer. Each locomotive operates independently over the track. Several locomotives can be moving at different speeds and in either direction at any time on the same electrical section of track. Blocking is not required for train control. It's easy to move engines around in the yards and park them close to one another without worrying about where the insulated sections are. It's easier to operate trains in the wide open spaces, too! DCC lets you run your trains instead of running your track.

The NMRA Digital Command Control Standard defines the basic communications structure at the track level for digital control signals via the rails. The standards specify a communication protocol between transmitter and decoder without specifying transmitter and decoder hardware. The data needed to operate each decoder is transmitted in packet format on the rails in the form of a balanced square wave. This baseline packet format allows for interoperability among equipment made by different companies that support the standard.
Interoperability is the most important advantage of the standard. Interoperability means that if you have a DCC compatible decoder, you can run it with any DCC compatible command station. This is very important since the major part of your investment in any DCC system is in the decoders. We have all heard the horror stories: "I have a fortune invested in this equipment and now I can't even get spare parts any more much less expand my system!!!" Any system that is available from more than one source is not as likely to disappear and leave its users stranded. Also, having equipment available from multiple suppliers creates competition in price and features to the benefit of the end user.
The standard does not cover the actual command stations or control equipment used to operate the decoders or the features they offer. You can buy a full-featured DCC command station or a basic DCC command station. You can spend more money or less money. There is a place in the market for both low end and high-end equipment. You decide what makes sense for you and your railroad.

What can DCC Do For Me?

DCC has advantages for everyone from the beginner to the advanced modeler and for every layout from the smallest to the largest. For beginning and intermediate modelers (most modelers classify themselves at this skill level) the advantages of reasonably priced simple command stations and simple layout wiring are very important. Start with a relatively low cost command station and add components as your interest grows. If you decide you want more advanced features and functions from your command station or if you want to add a computer, it's an easy transition from basic to full-featured command stations. The equipment you already own moves on with you as you add more features to your system. Your largest investment in time and money is in the decoders you install in the locos. These are upwardly compatible as you expand and add to your system. By simply adding components you can grow into a more advanced system at your own pace and as your budget allows.
Most home layouts are small or medium sized. They typically have a limited amount track available for block control. DCC has a real advantage in these situations. Since blocking is not required you can operate more locos in a smaller area.
For the large home or club layout DCC offers truly prototypical operation and minimum wiring hassle.
Modular layouts running with DCC can operate more than 2 or 3 trains at a time. Let's face it, the outside loop running clockwise and the inside loop running counterclockwise all day isn't very exciting. The ease of wiring makes modular hook up simple and lets you get operating sessions up and running more quickly.

What Are The Components of a DCC System?

All DCC systems are made up of various components that are connected by a command bus. Generally, DCC decoders and boosters are interoperable and DCC command stations are not interoperable. This is because each DCC manufacturer uses its own command bus structure. The way communications are handled by any given system are very important to overall system performance and to system expandability. When you are making your decision about which system to choose we recommend that you look carefully at what each manufacturer's of bus structure has to offer. Some factors to consider are ease of hook-up, ability to run multiple devices without slower response times, future expansion capabilities and overall system architecture.
Digitrax's LocoNet is a collision sense multiple access bus with carrier detect. Lenz's X-bus is a "polled" bus. Wangrow/NorthCoast bus is similar to X-bus. As other manufacturers enter the market they are adopting their own communications structures.
Components of DCC systems
Simple DCC system diagram.
To create a DCC system you will need each of the following:

One Command Station (to generate the command signal)

One or more Power Supplies (for power to run the locomotives)
One or more Boosters (to combine the signal with the power and put them on the track)
One or more Throttles (to send your commands to the system)
One or more Mobile Decoders (to decode the signal and control the locomotives)


Most DCC Manufacturers provide everything you need (except for the transformer) in starter sets.
Optional equipment:
Automatic Reversing Devices
Accessory Decoders for turnout and other accessory control
Programming Devices
Signaling and Detection Devices


What if I want to run a regular analog engine?

With most DCC systems you can run one analog locomotive (without a decoder) along with the digital ones. This lets you convert your fleet gradually. You may also have some locomotives are too small or too valuable as collector's items to be converted but you still want to run them on your DCC layout. If one of your friends brings his unconverted locomotives over to run on your layout, your DCC system can probably handle it. And it goes the other way too, if you want to run your DCC equipped locomotive on a regular DC layout, many DCC decoders automatically convert to DC operation if there is no DCC signal present. Check with your manufacturer about the availability of this feature.