KB986: Digital Command Control: The Wave of the Future-Early Version

This article was last updated on Oct. 11, 2010, 7:37 p.m. | Print Article | Leave Feedback

By Zana & A.J. Ireland

This clinic is titled Digital Command Control: The Wave of the Future. Recently, I was asked the question: "DCC has arrived and is the technology of today, everybody's using it so, why don't you change the title of your clinic?" My answer is that digital command control is still evolving rapidly. Today's digital command control is moving beyond the basic NMRA DCC Standards and RPs to encompass many new technologies that will compliment and enhance the basic things we can do with the track format defined by the NMRA. It's an exciting time for model railroaders all over the world and DCC is still the wave of the future in train control.

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.

You become the engineer at the throttle, controlling speed and direction of one or more locomotives, each with its own performance characteristics. As a good engineer, you must obey the signals and watch for other trains because, there are no electrical blocking restrictions with DCC to prevent those pesky "cornfield meets."

Conventional DC Operation

Multiple Train Control Using Conventional Blocking

With conventional blocking, train operation depends on track wiring that can be extremely complex. Each block is powered and wired separately to allow more than one train to move around the layout. Trains move one block at a time by using insulated blocks and toggle switches to control power routing. This requires a lot of time and expense to wire and debug before you are up and running. Once you have it wired, you have to learn to "play the piano" and remember the rules to keep the trains moving.
Why Digital Command Control is Different

Multiple Train Control Using Digital Command 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.

Digital Command Control will revolutionize the way you run your railroad and it doesn't have to cost an arm and a leg. Whether you have an existing railroad or are starting a new one, DCC can work for you and let you run your trains the way you've always wanted!

The NMRA DCC Standard


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.

Because of the DCC standard we have already seen the cost of Digital Command Control systems drop dramatically. In the early days, a "starter" system ran about $1000 and decoders were $95 each. Today a system that does much more than those early systems costs about $325 and decoders can be purchased for less than $30.00.

Today's NMRA DCC Standard provides a framework for interoperability without precluding manufacturer innovation. Some innovations we have seen that are not required or covered by the standard include: automatic reversing boosters and devices, 128 speed step control, analog locomotive operation, various cab bus systems, a network for layout operation, cost effective decoder harnesses, block detection systems, sound decoders, system upgradeability, new "painless" ways of installing decoders and much more to come. The standard is just the starting point!

Recommended Practices (RPs) are adopted from time to time to give manufacturers additional guidelines for interoperability. Several RP's have already been adopted to cover the NMRA recommended locomotive plugs, the extended packet format that allows for decoders to receive and process more information, the programming RP and the "fail-safe" RP. The NMRA DCC working group is continuing to work on additional RP's and refinements to the standard. Once new RP's are adopted manufacturers will begin to incorporate the ones that make sense in the marketplace. Hopefully, these new RP's and changes to the standard can be incorporated in a way that will be backwardly compatible with existing equipment.

What does the "DCC symbol" mean? How is it different from an NMRA "Conformance Seal"?

Manufacturers that build interoperable DCC equipment compatible with the NMRA's DCC Standard use the DCC logo to let customers know that they support the NMRA's standards effort by producing compatible equipment. Various groups who support the DCC effort, including the DCC working group and the DCC SIG also use the logo. This symbol is not a conformance seal.

The NMRA conformance & inspection program covers all aspects of model railroading interchange, not just DCC. Many people who have heard a lot about the NMRA DCC standard are surprised to learn that the NMRA actually has standards covering couplers, track gauge, wheels and much more. The NMRA conformance and inspection program was relatively inactive until 3 or 4 years ago. Now, the NMRA is working to revive this program. To that end, the NMRA has established a conformance testing program for DCC equipment and for other model railroad products as well. The NMRA is now issuing conformance seals based on the tests they are performing. Let's briefly review the conformance seals that have been issued for equipment manufactured by DCC companies and "non-DCC" companies. (Since locomotives must conform to more non-DCC than DCC standards & RP's we have not counted the ones that follow the NMRA plug RP as DCC products.) In 1996 (the first year of the C&I revitalization), 9 seals were issued (8 for products made by DCC manufacturers and 1 for other products). In 1997, 13 conformance seals were issued (2 for DCC and 11 for others). Through June of 1998, 51 conformance seals have been issued (none for DCC specific products although some previous seals were updated). As you can see, the C&I program has grown beyond just DCC.

According to the NMRA, an NMRA Conformance Seal is not an endorsement or guarantee by the NMRA. It is merely a statement that a particular product passes a particular test to determine whether, in the opinion of NMRA volunteers, it conforms to a particular NMRA Standard. It is important to remember that the NMRA conformance tests are administered and defined by dedicated NMRA volunteers who are working very hard to turn the C&I program into a useful tool for NMRA members.

The real issues are interoperability and getting the features
you want at a reasonable price.


Whether a particular DCC product carries the DCC compatible logo or an NMRA conformance seal is not really the most important thing to consider when choosing a DCC system. You really need to look at the "big picture." In some cases, a product that carries the NMRA seal may not have the features you really want to run your railroad. For example, there are DCC products that have the NMRA seal but do not offer 128 speed step control and others that do not provide for easy operation of non-DCC equipped locos. In the end what is really important is interoperability and getting the features you want at a reasonable price.

When we talk about interoperability, we mean that components from one DCC manufacturer will work with DCC components made by another DCC manufacturer. The purpose of the Standard and RPs are to insure this interoperability. Because of the complexity of the technology involved with DCC standard, this is not always as easy as it sounds. In some cases, the Standard & RPs do allow conflicts to occur. Nobody's perfect but the DCC community is working together to resolve issues as they come up. The bottom line is that in over 99% of all cases, DCC products that claim to be DCC compatible do in fact interoperate very well. In the few cases where problems have occurred, DCC manufacturers have moved quickly to provide solutions. The DCC Industry (Digitrax, Lenz, North Coast Engineering, Ramtraxx, Real Rail Effects, SoundTraxx, Wangrow, Easy DCC, and several others) have been producing interoperable systems for many years now without the NMRA's conformance process. The formalization of the conformance process by the NMRA will in no way obsolete or diminish the interoperability of DCC compatible equipment that is currently on the market.

Several DCC manufacturers build DCC compatible equipment but choose not to submit products for conformance testing. Does this really impact the value of these products to you as a consumer? Are they "defective" in some way? The answer in both cases is absolutely NOT! There are a several valid reasons that a particular product might not have an NMRA conformance seal: 1. The NMRA DCC standard/RPs may not apply to that product. For example, there is no Standard or RP that covers DCC throttles, boosters or command bus or network strategy that the DCC systems use. 2. The NMRA DCC standard is narrowly drawn in some areas and loosely drawn in others and in some cases a manufacturer may feel that it is more important to offer a particular feature than to slavishly follow a particular NMRA directive. Instances like this typically do not affect interoperability or compatibility but might preclude a given product from having a conformance seal. The Roco Digital Crane is a good example of this. 3. It is the opinion of some DCC manufacturers that the NMRA's conformance testing has not been adequately documented and verified and therefore, they choose not to submit products for testing. These manufacturers have all made their concerns known to the highest level of the NMRA and are working with the organization to address these issues in the hope that the C&I program can become useful to both manufacturers and consumers alike.

You will definitely continue to see products that are DCC compatible as well as products with the NMRA conformance seal. When you choose a DCC system, be sure it is either NMRA DCC compatible or conforming and, most importantly, that it has the features you are looking for!

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 and Xpress Net are "polled" buses. 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

* 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, Transponding and Detection Devices, Sound and other specialty decoders.
Command Stations-Generate the DCC Packets

Basic Command Stations: Control speed and direction of a limited number of trains. Some allow programming, others do not. These stations usually cost between $200 and $400.

Full Featured Command Stations: Control speed and direction of up to 127 trains. Can access between 99 and 9,999 locomotive addresses. Control accessory decoders. Control limited throttles. Allow programming of decoders. These stations offer a wide variety of options and features. They cost between $350 and $800.

Computer Control Command Stations: Control the layout from your PC or MAC. Software prices range from "Freeware" to over $100. Some packages require command stations to generate the DCC packets others use boosters and the computer directly generates the packets.

Multi-Format Command Stations: Can generate command control signals for DCC along with command control signals for other command control systems at the same time on the same track. For example, DCC decoders and Marklin "Motorola" format decoders can run on the same set of track with a multi-format command station.

Power Supplies/Transformers

All DCC systems require an external power supply. Follow the manufacturer's recommendations to get the best performance from your system. Generally, you can use any model railroad transformer to get started. Bear in mind that most of these railroad transformers were designed for block operation, running one train in a block. As your DCC system grows you will probably need to upgrade your transformer so that you can run more trains in a section. Some DCC manufacturers offer cost effective ready to run transformers. Build it your self transformer kits are also available if you want to save a little cash!
Boosters-Boost the DCC Signal onto the track

The various manufacturers call these devices boosters, power boosters or power stations. These devices take the DCC signal generated by the DCC Command Station and electrical power generated by the transformer and combines them to provide the power with the encoded digital packet signal to drive the rails. This device is called by several different names by the various DCC manufacturers.

Standard Boosters simply boost the DCC signal and Auto reversers allow for complete automation of reverse loops. Boosters come with current ratings from 2.5 amps to 8 amps (the maximum legal limit). Boosters range in price from the NMRA F9 "build it yourself" to around $300.

Throttles (Cabs)- Input Your Commands to the Command Station

Full Featured Throttles (Cabs): Can access addresses for locomotives on the layout. Can set up consists of locomotives. Some Full Featured Throttles can assign trains to limited throttles and control locomotive functions and control accessory decoders on the layout.

Limited Throttles (Cabs): Throttles that are used as input devices with Full Featured and Computer controlled Command Stations.

Wireless Throttles (Cabs): Radio and IR Throttles that are used as input devices to radio and/or infrared receivers. These receivers relay the input information to the command station.

Most DCC throttles are different from any conventional throttle you have ever used. This is because DCC gives you many more options than you had with conventional throttles. All of these throttles have the traditional throttle & direction control, like throttles you might have used in the past. In addition, these units might also access locomotive functions (turn lights on and off, activate sounds, etc). Some of these throttles even let you run more than one train at a time. Some customers want simpler DCC throttles or throttles that are more like throttles on an older system they ran before DCC. Simple DCC throttles are available but they don't give you access to all the possibilities of DCC. If you are worried about complex throttles, think back to the first time you read about block control and how complicated it all seemed then. If you are worried about how to explain these new-fangled doo dads to your operators, consider the "joys" of explaining how to run your present blocked system to them. Converting to DCC does involve a learning curve but the rewards of prototypical operation are worth it!

Mobile Decoders - Decode the DCC Packets Received from the Command Station Via the Rails and Tell the Locomotive What To Do.

These are the "chips" that go in the locomotives. Sometimes they are called receivers but they are really more that just receivers. Decoders actually decode the DCC signal and control the engine's speed and direction. There are many different decoder choices available. Decoders let you program locomotive characteristics like acceleration, deceleration and, starting and mid-point voltages. Some may have built in light and function controls as well. Some can simulate lighting effects like Mars lights, ditch lights, Gyra lites, rotating beacons & other special effects. There are other decoders that include sound and motion control in a single unit. Mobile decoders cost between $20 and $200 depending on the manufacturer and the features you choose. You can even build decoders yourself from a kit.

Standard DCC decoders typically have an address range from 1 to 127 and Extended Packet Format (EPF) decoders have addresses from 1 to 9,999.

Some DCC decoders can be used to run Hi-rail locomotives like Lionel and American Flyer and three rail AC Marklin Locomotives. Check with the manufacturers on this one!

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. Analog locomotives tend to "sing" when sitting still on DCC layouts. This noise decreases as the analog locomotive accelerates and runs. The noise is caused by the DCC track signal. This noise can be significantly reduced by using conductive brush lubricants such as Aero-Car Technology's "Conducta" and by assuring that there is no vibration inside the locomotive that will add to the noise generated. It is best to park your analog locomotive on an un-powered section of track when it is not running to cut down on heat build up inside the engine. If you want more information on Aero-Car Technology's products contact your local hobby dealer.

Accessory (Stationary) Decoders - Control Turnouts and Other Layout Accessories

Control stationary accessory devices such as switches and building lights. Stationary decoders cost between $50 and $85. Some control more than one accessory and some allow you to use either slow motion (Tortoise type) switch machines or solenoid (Atlas Snap type) switches. If you want to build these yourself, printed circuit boards and instructions are available from the DCC Working Group.

Other Optional Devices

The possibilities with DCC are nearly endless. New products are being developed at a rapid pace so if there is something you wish your layout could do, there will probably be a way to do it with DCC before long.

Today there are several computer based decoder programmers, automatic reversing devices, power management devices and block detection devices. In the future we will see signaling systems, sound systems and more.

With the adoption of the DCC standard there is a variety of different equipment available. You have lots of choices of features and price ranges. Since the market is changing so rapidly, it's best to contact the equipment manufacturer, importer or dealer to get the latest information on any system you are considering.

Special interoperability note: DCC decoders and boosters are generally interoperable but command stations are not. For example, you can use Digitrax decoders with Lenz command stations or Lenz decoders with Digitrax command stations. You can use Digitrax and Lenz decoders together with either command station. Note that some systems use components produced by a common manufacturer and have a common command bus structure that does allow some throttle interoperability.

Track Wiring Considerations


Now let's turn our attention to track wiring. You will need to consider your layout power bus wiring, your command bus or network wiring and in some cases separate feedback bus wiring. With DCC the signal and the power go hand in hand so your locomotive must have good conductivity to insure reliable train control. DCC is more tolerant of dirty track than some other command control systems because of the fact that DCC commands are sent over and over to the decoders. Periodic track cleaning will still be needed.

Track Wiring

Early proponents of DCC touted the fact that you can hook up your railroad with just two wires. While this is technically correct, there are some issues that need clarification here. If you are wiring a new HO layout it is a good idea to use at least 12 gauge wire with feeders to each rail every 10 feet or so as a power bus. If you have an existing layout, the general rule is that if you can run regular DC engine around the layout, the wiring should be able to run DCC without problems.

Unless you need to section your layout for added power, the only gaps you need are for hard shorts such as reverse loops and un-insulated frogs. If you are already wired for block control, you probably don't need to rewire to use DCC. Just open all your blocks so that the entire track has power and you are ready to go. If you are using common rail wiring and you wish to section your layout, you will need double gaps to separate the sections.

Remember, no matter how you control your trains, you should always use safe wiring practices.

Reasons To Section Your DCC Layout.


Even though blocking is not required for train operation with DCC, sectioning the layout has two advantages:

1. To provide additional power to operate more locomotives than one power supply can handle. For example a 4 amp booster and power supply will operate between 6 and 10 average N-scale locomotives, between 4 and 6 HO locomotives and 2 to 4 G scale locomotives. You can run more equipment by sectioning the layout and adding additional boosters and power supplies. For large-scale operations you can use higher current boosters to deliver more power to individual sections if needed to run more trains. Just a note about boosters and current ratings: most DCC boosters will require an external fan in order to output the stated maximum current for extended periods of time. This is not an issue for most modelers but if you experience booster shutdown, you should consider adding fans to increase heat-sinking capability.

2. To prevent total layout shutdown when shorts occur in any given section. If a short occurs in one section, only that section shuts down, the rest of the layout keeps operating. The reason for this is that all of the boosters are linked to the command station and will continue to receive the DCC signal and output it to their own section of track.
Wiring the Command Bus or Network & Feedback Bus (If required)

Follow your system manufacturer's instructions for wiring your DCC Command Bus or Network and Feedback Bus.

Digitrax LocoNet requires a 6 conductor phone wire network phone jack type outlets. These outlets can be daisy chained around the layout. This system is topologically similar to an Ethernet type computer network. LocoNet does not require a separate feedback bus.

Lenz's X-bus and X-press Net require a 5-conductor command bus with DIN jacks. This system requires a separate feedback bus.

The Wangrow/Ramtraxx/North Coast cab bus is similar to the Lenz X-bus.

Throttle Connectors


There are several different connectors in use by different manufacturers for plugging throttles in to the command bus or network. You may prefer a different plug in connector for your throttles than the one your manufacturer ships with their throttles. Generally, you can rewire any throttle to use any plug arrangement that you prefer as long as you use the correct pin out. So, if the system you like uses DIN5s and you would rather have stereo jacks or RJ12s ask the manufacturer for throttle re-wiring instructions.
The Dreaded Reverse Loop

You can operate reverse loops manually or automatically using DCC. You must double gap (completely isolate) both ends of the reversing section. If you choose manual operation you will power the reverse section separately and use a switch or relay to handle the polarity change as the locomotive enters and leaves the reversing section. If you use an auto reversing strategy you will power the reverse section separately and use an auto reversing booster or other auto reversing device to handle the polarity change. Note that when the polarity change occurs DCC equipped locomotives will continue at the speed and in the direction commanded but any analog engines running will reverse direction because they "see" the polarity change and respond to it. If you choose the auto reversing booster strategy, you will need at least two boosters. One will be the system reference booster and the second will be the auto reverser. The good news is that you can run more than one reversing section on a single auto-reversing booster. Also, note that some auto reversing devices require that you make changes to locomotive wiring where the pickups are not "side by side" on the locomotive. This is an issue in many steam locomotives where one power pickup is on the locomotive and the other is on the tender.

How Many Trains Can I Run?

The actual number of trains you can run is determined by several factors. Seriously, how much room do you really have to run trains? For most people the answer is "Not Enough!" To figure out how many trains you can run with DCC you'll need to know the address range supported by your system and your decoders, how much power you will need to run a given number of locomotives and how many throttles your system will support.

Address Range: DCC systems can access anywhere from 6 to over 9,000 addresses. This is the number of addresses you can assign to your decoders, not necessarily the number of locomotives you can run at a time. Some decoders can only use "2-digit addressing" others can use both "2 digit" and "4 digit addressing". The advantage to 2 digit addressing is that it is much simpler to use. The advantage of 4 digit addressing is that you can assign the number painted on the side of the locomotive as its address. Most DCC systems can run both types of decoders on the same layout.

Power Requirements: The number of trains you can actually run will ultimately be determined by the amount of power you supply to your layout. Each DCC booster is rated for between 3 & 8 amps. This means that you can run as many locomotives as your booster can power. To run more locomotives, you'll need to add more boosters.

How Many Throttles Can Your System Support?: Another factor that determines how many trains you can run is the number of throttles your system will support. DCC systems support from 4 to over 200 throttles. Check with your manufacturer if you are planning to have a lot of operators.

How Can I Customize Each Locomotive's Performance?

Each decoder installed in your locomotives can be programmed to have its own unique personality. When you program DCC decoders, the command station sends programming information to decoders and the decoders store that information for future use. You do not have to open up the locomotive to program decoders. Just press a few keys and you are ready to go. Each decoder can have a different personality and it "remembers" its programming until you change it. We use configuration variables or "CVs" to set up various operating characteristics in our decoders.

DCC decoders have a wide variety of features. Not all features are important to everyone so, you will find decoders available in a wide variety of feature combinations and price ranges. The following is an outline of most of the features available in today's decoders. Check with your manufacturer to be sure whether the decoder you are buying has the features that are important to you. Remember that DCC decoders are interoperable and you don't have to put the same decoder in every locomotive.

Locomotive Address

The locomotive address is a two digit (CV01) or four digit number (CV17 & 18) assigned to a certain decoder. This is the number you will use to access the locomotive in your system. Some systems use color designations instead of numbers but in reality, these colors correspond to numbers.

Locomotive Speed Controls

Because DCC is a digital system, discrete speed steps define locomotive speeds. The DCC standard calls for 14 forward and reverse steps for speed control. Some decoders offer advanced 28-step operation to give you even more speed control. And if that's not enough, how about 128 step operation. With 128 step operation you have extremely fine speed control. You can really make those locomotives crawl! The ability to take advantage of more speed steps depends on the throttle you are using. The number of speed steps a particular decoder can use is determined by the manufacturer, some systems use CV29 to set up which mode the decoder will operate in.

Speed Stabilization or Back EMF Speed Control


This is cruise control for your locomotives. Some decoders have this feature that lets you set a speed for your locomotive and have it run at that speed "up hill and down dale." It is also called load compensation. This is particularly useful for low speed operation when 128-speed step control is not available. Decoders that offer scaleable speed stabilization let you select how much of this effect your system will implement with any given locomotive. This type of speed stabilization let's you avoid the problem of the "pushy pusher" that was inherent with non-scaleable versions of back emf decoders. In this scenario, because the stabilization is constant, speed stabilized rear end helpers would often create the "concertina" effect with trains moving up grades.

Acceleration & Deceleration Rates


Acceleration is the rate at which the decoder increases speed from one speed step to the next in response to a new increase speed command. The acceleration rate (CV03) can be set to simulate train weight. Deceleration is the rate at which the decoder decreases speed from one speed step to the next in response to a new decrease speed command. The deceleration rate (CV04) can be used to simulate inertia. Just like the prototype, you can set your locomotives to get off to a slow start because of a heavy load and to take a long time to come to a stop because of the inertia of the train once it is moving.
The Throttle Response Curve

It's easy to confuse the throttle response curve with acceleration & deceleration. The throttle response curve is the relationship of the motor voltage (throttle setting) to the speed step command sent by the command station. Acceleration and deceleration are the rate of change from one speed step to the next up or down.

The Graph below shows the various curves that can be created using V-start and V-Mid adjustments or by programming the user loadable speed table for each discrete speed step.

Adjusting the Loco's Throttle Response Curve


The default motor voltage/speed curve is a straight line from stop to maximum speed. However, since locomotives don't really accelerate this way, DCC decoders let you alter this speed curve to simulate prototypical train motion. Let's look at the different ways to control locomotive speed and motion that are available with DCC. The chart above shows the default speed curve and how you can modify it by using V-start, V-mid & V-max.g V-start, V-mid & V-max.

Discrete Speed Steps

Because the signal is digital, the throttle response curve has 14, 28 or 128 discrete speed steps.
Start Voltage

You can set the start voltage by using CV02. The higher the start voltage, the higher the locomotive's initial speed when started. This adjustment is used to trim the locomotive to compensate for its motor efficiency. If you have a locomotive that takes a lot of voltage to get started, this adjustment can be helpful.

Mid Point Voltage


The mid-point voltage adjustment allows the motor speed curve to be altered at step 15, the midpoint of the motor voltage curve by using CV06.

Max Voltage

The maximum voltage adjustment lets you set the maximum voltage to be applied at the top speed step. Use the maximum voltage CV05 to limit the top speed of your locomotives.

Start voltage, mid point voltage and maximum voltage can be used to quickly and effectively set your locomotive's throttle response curve to simulate the prototype.
Loadable Speed Tables

If you wish to be more precise in setting your throttle response curve, loadable speed tables let you define each individual speed step for a locomotive. Once you have defined the speed curve you like, you can use the forward and reverse multiplier to move the curve up or down in speed.

Setting up a loadable speed table involves setting many CV's since you will set a value for each of 28 speed steps. Many DCC users find that using a computer based programmer makes this process much easier. When you use a computer, you can even save the speed tables you like and load them into other decoders quickly and easily via the computer.

Can I MU Locomotives?


DCC systems offer three choices for consist control:


The Basic Consisting method is to reprogram all the locomotives in a consist to the same address and run them on one throttle. In this case all the locomotives must be headed in the same direction, head to tail, head to tail, head to tail.

Advanced Consisting stores the consist information in each decoder. The locomotives can be added to and deleted from the consist in any orientation head to head or tail to tail. This method requires that all locomotives in the consist be equipped with decoders that support this feature. This method allows you to set up a consist that will be "transportable" from one DCC layout to another but you must be sure to always put the locomotives back on the track in the same order and orientation you programmed them for or you can get some unexpected results.

"Universal" Consisting stores the consist information in the command station and allows you to consist locomotives with any DCC decoder as well as an analog locomotive. The locomotives can be added to and deleted from the consist in any orientation head to head or tail to tail.

The number of locomotives you can consist varies widely from system to system.

Loco Lighting and Other Features of DCC Mobile Decoders


In addition to address and motion characteristics, most DCC decoders control constant directional lighting and in some cases offer additional function outputs. DCC decoders usually have at least 2 functions available (sometimes these are set up as directional lights so that your headlights go on and off automatically when you reverse the engine). Large-scale decoders have as many as 8 functions available. Some decoders have special effects lighting built in so that you can activate additional locomotive lighting like Mars lights, ditch lights, cab lights, etc. Additional functions can be used for smoke units for steam locomotives, sound units, and much more. These extra locomotive functions are accessible from full-featured command stations. Some DCC decoders include a mobile decoder and sound decoder in one unit.

Programming Decoders and Wiring a Programming Track

There are several ways to program your decoders. Most DCC Command Stations have built in programmers that send programming information as a broadcast message to any decoder that is listening. This means that you could reprogram all the locomotives on the track with one simple keystroke. To prevent this, it is useful to add an isolated programming track to your layout and program decoders as follows:

1. Run the decoder-equipped locomotive you want to program onto the programming track.

2. Throw the switch to disable the rest of the layout.

3. Switch your command station to program mode and follow the manufacturer's instructions for programming the decoder.

4. Switch the layout back on and drive away.

Some DCC Command Stations offer a separate programming output so that you can program decoders without shutting down the rest of the layout as described above. Also some systems offer operations mode programming which allows you to send programming information to a specific decoder on the layout. Another programming option is a stand-alone programmer or a computer based programmer.

Is It Difficult To Install Decoders?

Now that DCC has been around for a few years, locomotive manufacturers are beginning to build locomotives that are more "decoder friendly." This makes installation much simpler than it used to be! Many new HO locomotives are equipped with the NMRA standard medium plug. DCC manufacturers also build decoders that replace the factory-installed circuit board for many HO locos. If you have one of those, it's just a matter of plugging in your decoder and programming it. Most other HO locomotives allow relatively easy decoder installation. Do the easy ones like Atlas/Kato Diesels and Athearns first. Then as your skill increases, tackle the more difficult engines like Rivarossi Steam engines and small yard engines.

N-scale & narrow gauge installations are more difficult because of the limited space available for the decoders. N-scale locomotive manufacturers are working on making their future releases decoder friendly. Kato's C44W-9 has a light board that can be removed and replaced by a clip in decoder made especially for that locomotive. There are decoders that replace the light boards in the Kato PA's and E8's. Still another N scale DCC decoder is made for the Atlas GP40-2 and U25B's. If you are using other Atlas or Kato engines in N scale, it's probably a good idea to start with locomotives that have replacement frames available. These make N-scale installations easy because you don't have to make room for the decoder or the wires, you simply replace the frame and solder in the decoder. Other N-scale locomotives don't require replacement frames but you will need to modify the weights to fit the decoders inside.

Since almost all narrow gauge installations are in steam locomotives, space is tight! You'll want to consider installing the decoders in your tenders where there is usually more room. Sound is another issue that many narrow gaugers want to incorporate in their operation and this requires even more room inside the locomotive because of the need to install a speaker, too.

In G-scale locomotives, there is almost always plenty of room inside to install DCC decoders and sound units, too. It is usually easy to see where the wires to and this makes large-scale installation easy. Beware that large scale locomotive manufacturers don't follow any wire color conventions when they build the locomotives so, it will be important for you to closely examine your locomotive and determine "which wire does what" before you start your installation. Unfortunately, many large-scale locomotives were not made to be taken apart so, getting the locomotive disassembled is often the biggest challenge you will face in large-scale installations.

Decoder Installation


Because every engine is different we will cover only the basic concepts involved in decoder installation.

Read the Instructions and Plan the Installation


Each manufacturer provides instructions with decoders. Read them! Take a close look at the operation of the locomotive you want to convert when it is running on regular DC. Installing DCC decoders will not improve the mechanical operation of your equipment! Prior to installing the decoder is a good time to audit the mechanisms and give them a good tune up (since you already have the shell off). Be very careful when you take you locomotive apart, don't lose any of the little parts that tend to fly off in every direction. If you decide not to install a decoder in a given engine but plan to run it on a DCC layout do the tune up anyway. If you are working with Athearn diesels, the November 1993 issue of Model Railroader (Page 106) has an excellent article on tuning up these engines.

The mechanical placement of the decoder is important and may involve sculpting plastic and or metal parts to allow enough room for installation. Decoders from different manufacturers have different form factors. You should choose the one that has a current rating appropriate for your locomotive and that fits best in your locomotive. Try to locate the decoder in the coolest part of the body. Your decoders will provide more power to your motors if they are installed away from heat sources inside the locomotive body like motors and lamps. Try to put them where they can shed as much heat as possible.

Obviously, the scale you model will have a bearing on the ease or difficulty of decoder installation. In G scale, there is usually lots of room inside for decoder installation, the trick is removing the shell. Even though decoders are smaller today than ever, it is still a tough job to get them into many N-scale engines. The small size of the HO decoders has made installation possible in most diesels and steam engines. Some of the smaller switchers still present a challenge and some modelers use the smaller N-scale decoders in these with no problems. For N-scale modelers replacement frames really simplify decoder installation.

Determining which Decoder To Use

Measuring Stall Current

The first and perhaps the most important part of decoder installation is being sure you have the right decoder for your locomotive. If the motor's stall current exceeds the decoder's rating you are sure to have problems down the road so, start by using the following procedure to check the stall current of your motor.

1. Put the locomotive without the shell on a regular DC track.

2. Attach a DC current meter (ammeter) in series with one of the track feeds. Some power packs that have ammeters are really ideal for this test.

3. Apply 12V DC power to the track for N or HO. (16V for G)

4. Hold the flywheel or drive shafts to stop the motor from rotating for a couple of seconds.

5. While the motor is stalled, measure the current that the unit is drawing from the power pack. Be sure that while you are taking the measurement that the power to the track remains at 12V to get an accurate measurement.

6. Use the manufacturers' recommendations to choose the appropriate decoder for your application.

Generally speaking, N-scale engines with can motors draw about one amp, HO engines with can motors draw about one amp. Older Athearns with open frame motors and Bowers with Pittman motors draw around 1 3/4 amps. Large scale engines (O, S & G) vary in current draw and some even have two motors, those with can motors may draw less than 2 amps but each should be tested individually to determine which decoder to use.

Test the Decoder

Test your DCC decoders before installation by following the manufacturers' recommendations. Some manufacturers include basic test kits with starter sets; you can easily build your own decoder tester or purchase one of the commercially available models. You can save yourself a lot of troubleshooting time if you perform this test first to be sure that the decoder you are installing is working before you put it in your locomotive. You can do this test for new decoders and for ones that you are moving from one locomotive to another. You will need a test lamp and a protection resistor to perform the test. Instead of using an actual motor, locomotive lights and functions, use a test lamp to be sure the decoder is functioning properly. Use a protection resistor to avoid any damage to the decoder caused by wiring errors. If you are a first time installer, this procedure will have the added benefit of familiarizing you with the decoder wiring before you do the installation

Decoder Diagram

Once you have chosen the right decoder and tested it, it's time to check the installation instructions once more. Pay particular attention to the decoder wiring diagram provided. Be sure you know the purpose of each wire and can identify where it should be soldered to the locomotive. In general decoders follow the NMRA DCC standard recommended wiring colors, but it's always best to check just to be sure.

Note that several different types of light bulbs are used in locomotives and some lamp installations may require that you use current setting resistors to prevent the bulbs from burning out. Be sure to follow the manufacturers' instructions concerning light installations.

Isolate the Motor


For DC permanent magnet powered locomotives, the decoder must be electrically inserted between the track power pickups and the motor brushes. The most important part of any successful locomotive conversion is proper electrical isolation of the motor brush connections, so that they are driven exclusively by the decoder circuitry.

Note: Failure to isolate the motor will damage the decoder.


Once the motor is isolated, you can proceed to follow the manufacturer's wiring diagram for installing the decoder.

Test your installation on DC and DCC

Once you have completed the installation, test the locomotive with decoder installed to be sure it runs properly on DC (if available on your system) and DCC. Address the locomotive, run it in both directions, turn the lights on and off and try out any other functions you installed.

Program your decoder's personality


Refer to your manufacturer's instructions for programming the various CV's to set up your decoder to run the way you want it to. If you are just getting started, it is probably a good idea change the address right away but to run your locomotive in the default settings for a while. This will allow you to learn how to operate your system and become comfortable with all the new variables you have control over. Later you can go back and change acceleration, deceleration, speed curves, etc.

To learn more about decoder installation, choose an installation application note that applies to your operation from our applications note page. You will find examples of decoder installation in various locomotives of various scales on this page.

DCC can add excitement and realism to your layout whether you are an old pro or new to the hobby. It is very reliable and easy to wire and install. You have many different equipment options at every price level. You can run your trains the way you've always wanted to and not worry about your control system. So, catch the wave of the future with DCC!

If you are interested in presenting this clinic to your local group, we will be happy to loan you a set of slides and the script. We will also be happy to arrange for you to have handouts of this clinic available for all clinic attendees. Don't be shy, lots of people have already done this successfully! For more information e-mail your request.

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