Introduction to DCC - retired

DCCWiki, a community DCC encyclopedia.
Jump to: navigation, search
This logo identifies a product that conforms to the NMRA DCC Standards.

If you are new to model railroading and are wondering “Exactly what is this DCC thing I keep hearing about?”, the DCCWiki is the place to find out.

In a nutshell, Digital Command Control lets you operate a model locomotive as realistically as it looks. With the level of detail available today, why would you not want your locomotive to behave to a level that matches its appearance?

This site is a Wiki, meaning anyone can contribute information. It is not only for experts, but also for beginners. You don’t need to know anything about DCC. This site was created as a learning tool, as well as a tool to allow others to share their knowledge.

This page will attempt to answer some of the basic questions you may have. Additional information is also available by following the links you find in the article. Maybe in time you will contribute as well.

The Beginner’s Guide to Digital Command Control

First Question, “What Exactly is DCC?”

DCC is an acronym, identifying the Digital Command Control Standard created by the National Model Railroad Association.

Embossed logo.jpg

The NMRA decided that what was needed in the command control arena was a standard. So committees were created and discussions held, eventually resulting in a proposed standard for command control. The NMRA Working Group decided that Digital was the way to go, and built their standard around digital technology. Previous command control systems were built around analog technology, with some systems built around digital technologies.

Prior to the introduction of the NMRA Digital Command Control Standard, the market was fragmented into a number of completely incompatible command control systems. See DCC History for more background.

Before you start thinking that a bunch of old guys sat around a table smoking cigars, sipping Scotch and making decisions, that wasn’t the case. The NMRA is made up of people just like you and me. The working group members offered their time and knowledge to create the DCC Standard, along with many other standards, so that everyone from you to the manufacturer benefits, and the hobby benefits as well from these efforts. Not only did they ask modellers what they wanted, but they also took input from manufacturers in the model railroading industry, from small companies to large.

By defining a command control standard, which the NMRA named Digital Command Control, you are assured that any DCC branded product should work with any other DCC branded product. Those products will have the DCC Logo seen at the top of the page. The football shown below may also be present, but at this time many manufacturers of Digital Command Control products do not submit their devices for conformance testing by the NMRA.


What the NMRA did was make a digital command control system possible which could be manufactured by many companies while offering interoperability between them, and between modeller's DCC layouts and equipment. Much like you would expect your HO Scale equipment to work properly on any HO scale layout's track. Or that your N Scale boxcar wouldn't look out of proportion on your N Scale layout. This means a better experience for everyone, and a competitive marketplace for DCC systems and accessories, and even DCC equipped locomotives.

What is Command Control?

Lionel Trains, note third rail and transformer on left

Many modellers are familiar with Analog operations. In the 1930s the predominate form of control for your trains was Alternating Current with three rails, much like Lionel is today. Even HO was three rail AC operation in the beginning. It was just easier to implement at the time. At first some electrically operated trains used high voltage (for example, 120VAC) on the rails, but soon it was recognized this was not a good idea for toys sold for children. Transformers were added to the equation, allowing the creation of a low voltage (technically, below 48VAC, but typically in the 12 to 16VAC range) which was applied to the rails.

A transformer is an AC device, used to step voltages or currents up or down. They create a lower voltage for domestic use, such as the 120/240V circuits used in North America, from a higher supply voltage. Many devices don’t need that much voltage, so small transformers step the line voltage down to much lower voltage.
A transformer would be used with three rail trains, such as those from Lionel. For HO today, a power pack, containing a transformer and additional circuitry, supplies Direct Current to the track for analog operations.

Some pioneering modellers used outside third rail with HO, and in time, Flat Direct Current began to appear. The third rail was easier to wire than two rail, as issues like reverse loops didn’t cause problems. With the introduction of the more prototypical two rail operations, wiring got a little more complex.

Flat DC was called that because it used a battery, such as two 6V automotive batteries (as automobiles used a 6V electrical system until the 1950s), to supply the current needed to operate trains. By controlling the voltage on the track, speed and direction were determined. The legacy of battery power lives in the NMRA Recommended Practices, as full speed should be achieved when 12VDC is available on the rails. Many published reviews also reference speed at 12VDC for ease of comparison. (NMRA RPs allow up to 16VDC on the rails for analog operations.)

As technology advanced, conversion of your house current (120VAC) to low voltage direct current became possible at a reasonable price. Still, you controlled the track, and for operations, more complex wiring was needed to allow independent control of multiple trains using multiple throttles.

All this wiring was needed to create blocks, or sections of track with independent power for control of a train. This needed a lot of wires and toggle switches to make it all work.

Command Control

Command Control attempted to address the issue of complex wiring by allowing control of two or more trains on the same track, without a lot of complicated wiring. Most command control systems were Analog in nature, and relied on a constant track voltage for operation.

Carrier Control

Carrier Control is just another word for Command Control. It may be a little more accurate, as the constant track voltage supplied power, with a low voltage, high frequency alternating current signal riding on top. A receiver could strip this signal off the track voltage and be controlled by it, which in turn controlled power supplied to the motor.

In any case, any motive power on the layout had to be equipped with a receiver. Without it, a runaway would occur as soon as track power was switched on.

Constant Lighting

As command control didn’t appeal to many modellers, it did not gain widespread acceptance. But the idea of using a high frequency carrier was applied to constant lighting. Track power would be varied to control speed and direction, and this also changed the intensity of headlights, and particularly, lighting in passenger coaches. Again, a low voltage high frequency signal would be applied to the track. Circuitry in the coaches would use that signal to power lamps, which would stay lit at a constant brightness. The track voltage would be blocked, so it would not affect the lighting. Additional components in the locomotive would block the AC signal from the motor, which would respond to the direct current supplied by the throttle.

NMRA Digital Command Control


The NMRA’s Digital Command Control (DCC) built on the experience of the past with command control systems, none of which were successful. If a giant like General Electric couldn’t be successful selling their ASTRAC command control system in the 1960s, would anyone else be successful? Even Hornby was very successful selling their Zero 1 Command Control system, yet financial difficulties meant the product's development stalled and the Zero 1 was soon discontinued.

The NMRA identified a number of reasons for command control’s failure. Some modellers expressed the opinion that the worst enemy command control had was command control itself. The main reason for the failure of command control was a complete lack of compatibility. Whichever system you chose meant you were tied to it. Today that would be called Vendor Lock-in. Many of these costly command control systems were made by small companies, which could disappear at any time.

The NMRA addressed that issue by creating a Standard. They defined Digital Command Control at the railhead. The manufacturers were free to do whatever they wanted before and after the railhead, as long as what was on the rails met the DCC specifications in the Standard.

This lowered the barriers to entry into the command control field, allowing many new entrants to supply DCC compatible equipment, and didn’t tie the modeller to one vendor. This created a viable command control market for many suppliers as modellers (their customers) no longer feared being tied to an expensive control system that might be obsolete tomorrow, and out of production next year.

This eliminated the limitations imposed on the command control market by the costly and totally incompatible command control systems that were available at the time. This situation did not foster a large market that would appeal to most modellers. Your command control equipped locomotives would not function on your friend’s layouts, limiting the appeal of command control greatly.

Additional information on the evolution of Command Control can be found one the DCC History page.

How Does Digital Command Control Work?


Why did the NMRA settle on using digital instead of analog for their command control standard?

Simple. Digital technology offered the possibility over time of ever more powerful electronic devices, while the costs declined. Newer technology with more power would become available at a lower cost than last year’s offerings. We’ve all experienced that with consumer electronics in general.

The power of the microprocessor was only going to increase, while its size and cost would decrease. What cost $100 in the late 1970s would sell for $6 ten years later.

Many DCC components, such as decoders, are built around a device called a microcontroller. One of the earliest microcontrollers was the TMS-1000, which would form the basis of the Hornby Zero 1 system. Today you can buy an eight bit microcontroller for about 60 cents each in quantity. Zero 1 used a single microcontroller in its command station, while a DCC decoder installed in your locomotive is built around a microcontroller. Despite this, in adjusted dollars, decoders can be purchased for a lot less than a Zero 1 engine module (their version of a decoder) cost 30 years ago.

The difference between a microprocessor and a microcontroller?

A microprocessor is a part of a computer, that needs external integrated circuits (RAM, ROM, I/O) to become a functional computer. A microcontroller has many of these attributes already on the die (the little piece of silicon integrated circuits are made of.) They are usually application specific, as the programming needed is already onboard, either as part of the manufacturing process, or burnt in later. Once it is programmed, that is what it is going to do.

By incorporating many of the parts needed to make a computer into one small integrated circuit (or chip), it can be made very cheaply, and incorporated into another product very easily. A DCC decoder can be made with a few external parts and a simple microcontroller, which can retail for $20 to $30. Using a microprocessor would mean additional parts, a larger package, and additional costs.

Small, low cost microcontrollers made small decoders possible, small enough for N scale. Powerful but low cost Digital Signal Processors made digital on-board sound a reality. These things made decoders, command stations, full featured handheld throttles, and signalling systems possible.

The other component is software. The software (or code) determines how the microcontroller will behave. The software code can be written once, and used on multiple products later, allowing the manufacturer to amortize those costs over years. A basic DCC command station or decoder can be implemented in hardware and software easily, and additional features added later within the software. A more sophisticated sound decoder can use the same basic software, with the additional code for controlling the sound. In the case of OEM decoders, the buyer can decide what features he wants, and the supplier can deactivate or customize decoder features accordingly.

The power of the microprocessor made Digital Command Control possible. But it did not come with the requirement to own a computer or know a programming language. Simple programming was possible using your throttle. Decoder and system features could be customized to your needs.

DCC System Components

A complete Digital Command Control System consists of a number of components, generally: one or more throttles connected to a single command station, which is connected to one or more boosters, which drive one or more decoders.




The throttle (also known as the cab) is the controller, often handheld, used by the operator to control a single locomotive, or a group of locomotives in a single train. Throttles connect to the throttle network, which is usually brand specific, so generally throttles from different manufacturers cannot be interchanged.

Throttle Network

The throttle network is the connection that ties the entire system together. The throttle network determines a lot regarding the performance and expandability of the system. Your throttles must also be compatible with the network. Every supplier either has his own network, or uses an existing network from another manufacturer. This can be a limitation for external accessories. Technical issues can also place limits on the capacity of the network, not usually an issue, but with future expansion it could be.

Command Station

This is the glue that holds everything together. The command station is connected via the throttle network to your throttle and other devices, such as boosters and detection systems. It takes commands from the throttle and formats them into a DCC packet, which is passed onto the booster, or via the throttle network to other devices.

The technical term for this device is an Encoder. An encoder is a device, circuit, transducer, software program or algorithm that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security or compression.

As a rule, your layout only needs one command station, but it can have multiple boosters. Since the command station is the "boss", commanding all those beneath it, there is no need for a second one. If you see yourself running a lot of trains, chose a system with a command station that has the capacity you need, as some are very limited in terms of the number of trains is can control.

Programming Track

Does your choice of system include a programming track output?

These outputs are designed to allow easier programming of decoders, while limiting the power available to prevent damage if a mistake was made during installation. The programming track can be part of the layout, or a special track on your workbench. For sound decoders you will probably need a programming track booster.


Booster DCS200.jpg

This is a digital signal amplifier. It takes the digital packet from the command station and boosts (amplifies) it to the track voltage required. It is between the track and the command station. In a large installation there may be several boosters deployed.

Since boosters only need the DCC Packet from the command station, it is possible to add boosters from another manufacturer, subject to some limitations. It is best to verify compatibility before investing in a booster.

Integrated Command Station/Booster

RammTraxx would introduce the Integrated Command Station and Booster, where both devices are combined into a single package. This eliminated the need to have two devices, two power supplies and a connection between them. While this arrangement is now quite common, standalone boosters are still needed. But you only need one command station, which can be used to supply signals to many boosters. You cannot have two command stations on the layout, as they will cause conflicts and other issues.

Power Supply

Dcs50 zephyr.jpg

Power is provided to the components, particularly to the booster by the power supply. This is usually a separate item. Depending on your system, it may or may not be included. As a rule, a DCC layout has a much larger demand for current than an analog Direct Current layout , as usually only one booster is employed to provide power to the entire layout. If you have more than one booster, you may need a large power supply or multiple supplies.


Related topic: Configuration Variable.

A decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode.

In Digital Command Control, the decoder is mounted in a locomotive (mobile decoder) or under the benchwork (stationary decoder). It is connected to the command station, by the power bus via the rails, or as in a stationary decoder, which can also be connected to the throttle network. A steady stream of digital information is presented to it, and the decoder only responds to certain addresses assigned to it. When a DCC Packet addressed to it is received, it decodes the packet and acts on the instructions contain within.

Mobile Decoders

Usually when someone refers to a ‘’DCC Decoder’’, they usually mean a mobile decoder.

Mobile decoder means just that: it is mobile because it is installed in a locomotive, its tender, or another car such as a boxcar if it can’t fit in the locomotive. The mobile decoder is what makes things work. It takes the DCC signal, reads the address, and if the packet is addressed to it, acts accordingly.

DCC decoders can be customized using defined Configuration Variables (CV), the most basic being the address you set. That address is used to take control of that decoder using your throttle. The decoder will also respond to specified addresses as well, such as the Emergency Stop address.

Stationary Decoders

Stationary decoders are used for controlling fixed items. Such as turnouts, crossing gates, or signals. A very typical application is the remote control of turnouts. They can be powered from the track, or from their own power supply. Commands can be received via the track or the throttle network. They can be simple, or allow a degree of automation themselves, or be controlled by a computer running the appropriate software.

The standard defines a specific addressing scheme for stationary decoders so they will only react to commands sent to them while ignoring commands meant for mobile decoders.

Getting Started in DCC


As DCC is a standard defined by the NMRA, there are a number of options available to you. There are basic DCC systems and Starter Sets available from a number of sources, and more sophisticated DCC systems are also available from a variety of vendors. Do some research first, so you don’t paint yourself into a corner by investing in a system with limited features or a limited upgrade path. This is an investment, and properly made, will bring you years of enjoyment.

"While decoders are DCC compatible amongst themselves, the DCC system is not." You will be forced to stay within the manufacturer's lineup for accessories and upgrades. It will save money in the long run to spend a little more now, than having to replace your entire DCC system in the future because you have outgrown it.

There are a number of parameters to consider: How big a system to you plan to have in the future, computer interfaces, radio control? Even if you don’t think it is important today, it may be important sooner than you think. One thing to consider is the commercial software that is available: Does it support the system you plan to get? Locking yourself into a vendor-specific software package can be another limitation. A good resource is the list of systems supported by JMRI, a free, open-source package that you will hear about.

Computer interfaces can also offer the ability to use your smart phone as a throttle, over WiFi. The computer takes the commands from the phone or iPod, converts them to the needed format, and passes them onto the command station. Computers can also offer an additional throttle, route control, ability to operate signals and switches, and more.

Before you say you don’t need a computer interface, look at DecoderPro.

A Basic DCC System

A Bachamn Dynamis DCC System
A more advanced NCE DCC System

A basic DCC Starter System could be built around a microcontoller. By combining the throttle, command station, and booster into one package, you have a simple computer that responds to your inputs on the throttle by creating the required DCC signal, which is then amplified and applied to the track. A decoder equipped locomotive will then respond to your throttle inputs.

More complex starter sets are also available, with handheld throttles, connected by wires, or IR, or even radio signals. They usually offer more capacity in terms of trains that can be controlled, more power to the track, and additional features and configuration possibilities.

Some command stations offer features making MU lash ups easier to do, another consideration. Let your interests guide you. Some modellers are more interested in prototypical signalling systems than actual operations, so they get a DCC system to enable that.

Do It Yourself DCC

Yes, that is possible. You can build your own boosters, and even decoders, as the system is open. Depending on your skill, anything is possible.


Controlling locomotives has always been the focus of any command control system. Some DCC systems may support the ability to operate one locomotive which is not equipped with a decoder. This feature is known as Zero Stretching. Not all systems support this feature.

Can I run DCC and Analog

Yes and No.

You could wire the layout’s power so you can switch between analog and DCC control, but once you discover DCC’s power and flexibility, you’ll probably not bother.

Can you use analog and DCC at the same time?

No. Expensive mistakes will happen.

Can I use a DCC Decoder Equipped Locomotive on an Analog layout

Yes, with one provision: Analog mode must be enabled in the decoder.

High voltage spikes from certain controllers will damage a Digital Command Control decoder. The decoder warranty is void when this happens. As always, read the instructions or contact the manufacturer to verify if your locomotive's decoder is compatible with an analog power source.

Most modellers will configure the decoder to ‘’’NMRA Digital Only’’’ for a very good reason: If the track signals get distorted, the decoder may think it is on analog power, and switch over to analog mode. Since DCC puts full voltage on the track at all times, the locomotive immediately accelerates to full speed, becoming a runaway. To avoid that, analog mode must be disabled.

Note: Some manufacturers will recommend you do not run their product on a DCC layout without first installing a DCC Decoder. Read the instructions first to determine if such a restriction exists.

Decoder Installation

Locomotives can come as DCC Equipped, DCC Ready, or just analog.

You can chose to buy a locomotive with a decoder, or even sound, installed at the factory. You can buy one without a decoder and install your choice of decoder too. See the articles on Decoders and Decoder Installation.

Cost of Conversion

MX645 1800uF16V-640.gif

Many people bring out the argument that "Going to DCC is expensive." It is, with the cost of the equipment, but in the grand scheme of things, when building a layout it will cost no more than analog, with less issues along the way. Your biggest upfront expense is the system you chose.

The next argument is with respect to existing layouts and equipment rosters: “It would cost too much to install decoders in all my locomotives.”

Only if you want to install a decoder in every piece of motive power you own. Even if you have 300 locomotives, realistically, a percentage of them may not be easily converted to DCC. The question is, how many do you regularly use? Ten, twenty locomotives? They also don’t have to converted at once, you can convert your favourites first, then add some more, and decide if the balance need to be, or are worth conversion. A locomotive that runs poorly on DC will run poorly on DCC as well.

In many cases, new modern locomotives are much easier to convert, as well as being smoother running and better detailed. Many are now available with sound decoders already installed, with some manufacturers installing custom sounds and other features.


HO Layout 1.jpg

Wiring is one of the most important aspects, as well as a source of controversy. Good wiring is a must. As with trackwork, invest time and attention in this aspect and don’t try to cut corners. See the articles on wiring for tips and best practices. Poor or substandard wiring can cause expensive problems or maintenance issues that are hard to track down.

Wiring can be divided into two segments: The Track or Power Bus, and the Feeders. The Track Bus is always heavy gauge wire (14AWG or better). The feeders are lighter gauge, typically 18 to 22AWG. They are short lengths soldered to the rails, and then connected to the Track Bus.

In all cases, your wiring must pass the Quarter Test.

Reverse Loops

Reverse loops are as much a problem with DCC as they are in analog wiring. This issue can be solved easily using Auto Reversers, which automatically detect a polarity issue and correct it for you.

Power Districts

Power Districts are the DCC equivalent of a ‘’block’’. There are a number of devices out there that can be used to create a power district, including boosters. The more common application in an electronic circuit breaker. While most small layouts will operate as on large district, they are useful to isolate a short to one area, leaving the rest of the layout under power and operational.


Trackwork is important for smooth, reliable operations, and wiring is involved. The problems occur in reverse loops and turnouts. If you laid your track carefully, you will have few problems. Some modifications may be needed for turnouts, or if you are in the planning stage, use turnouts that are considered DCC Friendly or compatible. Older turnouts may introduce shorts.

One issue is rail joiners: do not rely on them to carry power.

Which DCC system is the Best?

Quick answer: None. No one system is the best. The best system for you depends on your wants and needs.

When selecting a system, do your homework. Try them out at a club, or fellow modeller’s layouts. Ask them questions about their system. Download and read the manuals for the systems you are interested in. Ultimately, what is important to you.

Do you plan to run many trains, or sound? DCC can fuel an addiction: Once you can run two trains at once, you will want more running. Once you have your first sound locomotive, you will want more too. Keep that in mind.

Addressing is another issue, does it support two or four digit addresses? Control of turnouts? Is it expandable in the future? Is the throttle comfortable to hold and use? Are accessories that can add features easy to get?

Remember, this is a large investment, so planning ahead can save you money and frustration in the future.

This is the beginning of the DCC Tutorial.

Additional Reading