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Q: What is DCC?

Main article: Introduction to DCC
Main article: DCC History

DCC is an acronym for Digital Command Control. There have been various Command Control systems since 1946, Digital is used to differentiate Digital Command Control from the Analog Command Control systems.

Analog Command Control systems, sometimes called Carrier Control, superimposed their signals on a Direct Current voltage. Using various tones, phase shifts or pulses they could achieve a limited range of control over a locomotive. Unfortunately, none of the systems were compatible leading most hobbyists to shun Command Control.

Digital Command Control uses a digital signal on the track at all times, unlike older analog systems which superimposed their signals on a Direct Current voltage.

It is a method for taking input from a throttle (think: train controller) and transferring it to a command station, which sends it out as a digital packet to the entire track layout. The locomotives on the track listen to all the digital packets, looking for their address. Once a locomotive sees a command addressed to it, it performs whatever function it's told to do - such as stop, speed up, slow down, turn lights/on and off, or turn on/off various engine sound effects.


The NMRA established a Digital Command Control standard in the 1990s. Today DCC refers to NMRA Digital Command Control, in which systems bearing their logo are compatible.

See the DCC History page for further information on Command Control systems.

Edit FAQ  Categories: Beginner

Q: Can I run large scale trains inside?

The track gauge is the same, the scales are not.

Yes. There are sets from many manufacturers. The most common one are the Christmas sets. Before you dive in, you should spend a little time deciding what your goals may be before investing in these sets. Sets usually contain items from only one manufacturer. Do you like everything in the set? If not, perhaps youâll save money in the long run purchasing everything separately. The big downside of a set is its power supply. It is only enough for that train around a small loop of track. As you expand youâll need a better transformer or power supply. Most of these sets are not DCC equipped.

You might be better served by getting a DCC Starter Set and adding trains and track separately if you plan on running more than one train.

Edit FAQ  Categories: Large Scale

Q: Are large scale beginner sets available?

Yes. There are sets from many manufacturers. The most common one are the Christmas sets. Before you dive in, you should spend a little time deciding what your goals may be before investing in these sets. Sets usually contain items from only one manufacturer. Do you like everything in the set? If not, perhaps youâll save money in the long run purchasing everything separately. The big downside of a set is its power supply. It is only enough for that train around a small loop of track. As you expand youâll need a better transformer or power supply. Most of these sets are not DCC equipped.

You might be better served by getting a DCC Starter Set and adding trains and track separately if you plan on running more than one train.

Edit FAQ  Categories: Large Scale

Q: What are the minimum height & height clearances for large scale layouts?

See Standard Dimensions article for details on clearances and tolerances.

Edit FAQ  Categories: Large Scale

Q: What is a command station?

Main article: Command Station

Command Stations (sometimes called a Central Station) are the very heart of a Digital Command Control system. They receive commands from a throttle network (such as Digitrax's Loconet), process them, and decide if it needs to make a standardized digital packet to send to all the decoders on a DCC system. They don't do the actual work; they tell other things to do the work. Please see command stations for full details.

Edit FAQ  Categories: Beginner, Command Station

Q: What are the NMRA Digital Command Control Standards?

In the late 1980s, the National Model Railroad Association (NMRA) began investigating a standard for command control systems. All the analog based command control systems available on the market had limitations which inhibited expansion, and they were never compatible amongst themselves. The NMRA's command control committee decided that digital was the way to go, and the best way was to use a 100% digital signal on the track.

What caught their attention were the digital systems used in Europe, originating mainly from Germany. One system in particular was promoted by Marklin. The NMRA would examine two digitally based command control systems, from Marklin and Keller Engineering. After examining the commercial offerings, a standard emerged, the Digital Command Control System.

In doing so, it created a basic standard which is compatible across most Digital Command Control manufacturers. This allows us to use a brand XYZ decoder which is controlled by brand ABC DCC system.

Compatibility of Digital Command Control components is defined at the track level. This allows decoders from different manufacturers to work together, but permits manufacturers to innovate on the user interface, throttle, and command station capabilities. Certain parameters of the decoder are defined by the NMRA, but the designers and manufacturers may add additional features (such as sound), providing they do not impair the basic decoder operations.

The complete (and rather technical) NMRA DCC Standards can be found on the NMRA web site.

Edit FAQ  Categories: Beginner

Q: What is a decoder?

Main article: Decoder

A decoder is a device which listens for Digital Packets addressed to it, and then performs whatever action is requested.

There are three types:

  1. Multifunction Decoder,
  2. Function Decoder, and
  3. Accessory Decoders.

Some multifunction decoders are very simple devices providing only motor control and a small selection of lighting options, while others are very complex with multiple special effects, tunable motor response, and sound effects available.

Function decoders are similar, except they lack the motor control functions.

Accessory decoders are used for turnouts, signals and other animated items.

Please see the decoder page for full details.

Edit FAQ  Categories: Beginner, Decoder, Multifunction Decoder

Q: What can burn out a decoder?

  • Overvoltage. For example, booster is set to incorrect scale.
  • Current draw exceeds decoder ratings (wiring short or another overload condition). Poor wiring can cause this.
    • Be careful when first testing a decoder installation. Put the vehicle with the newly installed decoder on a test track (programming track or section with a 100-ohm series resistor for current limiting) instead of the mainline for testing. This will limit the current, preventing damage if the decoder was miswired. If all functions work, and the vehicle can move (assuming motor output drive from decoder) at low speed steps, then it should be safe to place it on the mainline. As always, follow the manufacturer's instructions for testing in addition to the above.
  • Overheating
    • Imagine running a black loco in direct sunlight and hauling a heavy load at nearly the limit of the current output of the decoder. Now imagine the decoder only having current limits, and not thermal limits built in. Your imaginary decoder should running extremely hot. As you know, when things run too hot you'll let the magic smoke escape from the decoder. Avoid excessive loads for extended periods of time. If in doubt, check temperatures on newly installed decoders. If you find it running too hot, upgrade the decoder to handle larger currents and/or increase air flow around the decoder.

Edit FAQ  Categories: Decoder

Q: Can One Command Station Control Multiple Layouts?

Short answer: Yes. It's possible to control two (or more) separate layouts using a single command station. For instance, if you wanted to operate an N scale layout inside and a G scale outside. Most systems combine the command station and booster into a single unit. Either case, you connect one layout as normal, that is, connect the command station/booster to one layout. Then, you simply purchase a second power supply and booster for the other layout. The second layout will receive it's commands through the throttle network (LocoNet, XpressNet, etc). This allows you to have same, or different voltages for different layout sizes.

Assume we have an N scale layout in the garage. Since power requirements are low, we purchase a DCC system that outputs about 2 to 3 amps, and the voltage is set for N scale. We setup this layout as described in other parts of this website. A year later, we want to setup a G scale, or garden railroad in the backyard. To do this, all we do is purchase another power supply and booster for the second layout. Because the power is independent of the layout, we don't need to worry about the higher voltage from the garden railway making its way to the N Scale layout. To get the commands from the command station to the garden railroad booster, we simply connect the throttle network (such as ExpressNet, or LocoNet) to the booster. We now have two railroads being controlled from any throttle, at any location, with a cost savings by not having to purchase two command stations.

Edit FAQ  Categories: Beginner, Command Station

Q: Can I Quickly Change from DC to DCC and Back Again?

Yes, simply wire for multi-cab control as you normally would for DC. However, do not use common rail wiring. Also, use #14 or better bus wire (see Track wiring for details). When you want to change from DC operations to DCC, simply hook up the booster in place of one of the cabs and switch all blocks to that cab. When you're ready to go back to DC, simply flip to the block toggles back to the DC cabs.

Running Analog and DCC power on the same layout is dangerous, as a mistake will destroy something. Run your trains using only one method at a time.

Once you experience Digital Command Control, you will soon forget about analog operations.

Edit FAQ  Categories: Beginner

Q: How do I Control Track Switches?

You'll need some turnout motors and an accessory decoder. See Turnout Motors for details.

Edit FAQ  Categories: Beginner, Stationary Decoder

Q: Are the Voltages on the Rails Dangerous to Children?

The maximum voltage regarded as safe for human contact is nominally anything less than 32 volts. The maximum "safe" voltage was 48V, but that has been reduced over the last few years. For this question, AC and DC are considered to be the same risk. Anything over 48 volts is considered "high voltage" and as such, is not allowed in this application.

However, there is more danger from heat generated by shorting the tracks with metal objects, such as bracelets or necklaces. Fortunately, your booster should detect the short and cut the current. Before that happens, the metal may get warm or even hot. The child may be scared and even slightly burned, but not seriously harmed. If operating properly, the booster should disconnect the track power the instant a short occurs.

This is yet another reason to test your wiring to make sure your booster's short circuit protection works on all sections of track. The simplest method is a called the quarter test, where a coin is used to short the rails together. If the reaction is not instantaneous, the wiring to that section of track needs work.

Edit FAQ  Categories: Beginner, Power

Q: Can the digital decoder send information back to the command station?

In Service Mode the digital decoder can acknowledge a packet with a brief pulse of current. This can be used to find out the digital decoder’s address, for example – send a packet asking the digital decoder if it is address 1 and wait for a response. If no response, try address 2, etc. There is currently no defined way to send information back to the command station in normal running mode, although some work is being done in this area to determine what is feasible. See the NMRA Extended Packet Format RP for a description of feedback in service mode.

However, some systems like Digitrax can have limited feedback using their multifunction decoders, which Digitrax calls transponding.

Another technique is RailCom, which can send a limited set of messages regarding speed, load, address and the content of a CV. RailCom requires a compatible decoder and a cutout device to activate it. A typical use is to automatically register a locomotive added to the layout when a compatible command station is used.

LISSY is another method of communication using infrared light.

Edit FAQ  Categories: Beginner, Command Station, Decoder

Q: Can you control things other than locomotive speed and direction?

Yes. The baseline packet in the standard only provides for basic locomotive control since that is all that is essential for interoperation. The extended packet format RP defines packets that can be used to control 32,000 different accessories such as lights, sound, turnouts, etc.

Edit FAQ  Categories: Beginner, Decoder

Q: How much Track can my Booster or Command Station Power?

Many people just getting into Digital Command Control often wonder how much track their command station and/or booster can power. However, as long as you have an adequate bus and feeders, the length of track isn't an issue. What needs to be considered are just two factors:

  1. The number of locomotives (loads) you will be running simultaneously; and
  2. The number of accessory loads on your booster.

It then comes down to how many trains your system can power, not how much track. Simply add up all the power needs of the trains you want to run and make sure you have enough boosters distributed throughout your layout.

Keep in mind sound decoders demand more power to operate when you are making calculations. The fact that many decoders have a one-amp output does not mean that they will be at one amp all the time either.

Edit FAQ  Categories: Beginner, Booster, Power

Q: For my large scale layout, can I leave the power transformers out this winter?

It all depends... How severe are your winters? Are you in Tampa or Buffalo? What are the manufacturer's recommendations?

Anything electrical needs to be protected from the elements. Are the transformers in weatherproof boxes? Who rates them as weatherproof?

Heat, cold, and sunlight will accelerate deterioration of anything left outside, especially plastics. UV will damage any unpainted plastics, so they must be painted.

Edit FAQ  Categories: Large Scale, Power

Q: Can you run a digital decoder equipped locomotive with a conventional 12-volt DC power pack?

This is an optional feature of multifunction decoders provided for by the standard. If a multifunction decoder supports analog operation, and it does not see a digital signal on the track, it reverts to analog operation and can be controlled as if no digital decoder were present. Decoders can be confused by some forms of very narrow pulsed power and extreme care must be taken to ensure that the packs maximum output is less than the decoder’s 24 volt maximum.

If the multifunction decoder was configured for NMRA DCC Only, it will not work on a DC layout.

The NMRA DCC Standard does allow for the option of using alternate power sources, including analog DC. See NMRA DCC Electrical Standard 9.1 for more details.

For most multifunction decoders, the minimum voltage required is 7 Volts. The decoder will power up. but motor control is not possible at this voltage. The decoder can operate low power devices such as the lights at this voltage.

Some manufacturers caution against using an analog pulse power throttle with their decoders, as these may damage the multifunction decoder.

Edit FAQ  Categories: Beginner, Power

Q: How does the standard work with computer control of the layout?

Computer control can be accomplished by using a command station with a computer interface, or by generating the signal directly with the computer and using a power station to drive the track. The interface between a computer and a command station is outside the scope of the standard.

Edit FAQ  Categories: Beginner

Q: NMRA Standards State Track Voltages Range from 7 to 20 volts. How are Speed and Direction Controlled?

During operation, the voltage provided by the booster does not vary.

The DCC multifunction decoder within the locomotive controls speed and direction by varying the amount of power provided to the motor. The voltage of the DCC signal described in the standard is to allow for the different power needs of the various scales. For example, a typical Z scale booster will supply 10 volts, a typical HO scale booster supplies 14 volts and a G scale booster will put 20 volts on the rails.

The NMRA DCC Standard allows for a range of voltages on the track, as well as minimum and maximum voltages that a multifunction decoder must tolerate.

For controlling live steam locomotives, one would probably use the minimum 7-volt signal.

Edit FAQ  Categories: Beginner, Booster, Booster District, Decoder

Q: Is a non-DCC install locomotive affected by running on a DCC power track?

It is not recommended to leave locomotives not equipped with a multifunction decoder on track powered by a digital command control signal for extended periods of time.

Since the signal has an Average DC level of zero (unless you are using analog compatibility mode - see the previous question), the locomotive will not move. However, the motors will make some noise. If the amplitude of the digital signal is greater than the rating of the motor, the motor may experience permanent damage.

Experience has shown that common open frame motors, and many commonly used can motors are not damaged by the digital signal even after extended exposure. Iron-less (coreless) type motors can be damaged by extended exposure to a digital command control signals due to excessive heat build-up.

Edit FAQ  Categories: Beginner, Power

Q: Can an unmodified locomotive operate together with a DCC locomotive at the same time?

The NMRA DCC standard allows for this, although some care needs to be exercised in its use. The DCCWiki refers to this type of operation as analog compatibility mode. The signal is symmetric around the 0-volt level which provides a 0-volt DC component. By expanding the length (period) of the zero-bit applied to one rail, the resulting current flow will generate enough torque in the motor. The long period of the zero-bit results in an average DC voltage which determines the speed of the motor.

DCC waveform demonstrating Zero Stretching

The direction of the vehicle is determined by which rail receives the expanded zero bit(s). Be advised that this technique can create excessive heat in the motor. As a rule, a locomotive without a multifunction decoder should not be left on the layout as the DCC signal can cause overheating of the motor.

Some high precision motors (the ironless core type) may be permanently damaged. (See the following question.)

Edit FAQ  Categories: Beginner, DCC, Power

Q: Does zero stretching have an effect on normal digital operation?

None, unless you use it. When you are running an analog engine, the zero bits are stretched, which reduces the bandwidth of the system.

Edit FAQ  Categories: Beginner, Command Station, DCC

Q: Why allow zero stretching at all?

Zero Stretching in the DCC Waveform

Zero Stretching or Analog Mode is not part of the NMRA DCC Standard.

Some manufacturers added the analog mode to create a compelling reason to adopt Digital Command Control in the early years, as modellers (particularly those with large fleets of motive power) were concerned about the costs of conversion to DCC.

This allowed DC locomotives to be controlled without adding a multifunction decoder. It is not a crucial feature today unlike 30 years earlier when DCC was introduced. It is now a legacy feature that some DCC systems still incorporate.

Analog compatibility mode is only one use for zero stretching. Other possible uses include the following:

  1. Providing a stretched zero after each packet for the purpose of superimposing locomotive feedback to the command station.
  2. Allowing generation of the command control signal with a computer using a standard serial port, which may not be possible without slightly stretching some zeros.
  3. There are probably other uses that will become apparent as time goes on. Note that the above-mentioned uses are still very much in the experimental stages. The point is that preserving the ability to stretch zeros allows for the possibility of some interesting things.

Edit FAQ  Categories: Beginner

Q: Does retransmitting DCC packets consume all the bandwidth and affect throttles?

Although simple low end DCC command stations may provide for control of a limited number of engines and repeatedly cycle through all the addresses in use, higher end systems may use a priority-based scheme, where packets containing values which have changed are prioritised over those that have no changes. Thus, repeating packets does not necessarily impact the effective bandwidth of the system.

More sophisticated command stations use an algorithm to determine packet priority. Packets containing updated commands, such as speed or functions, will be prioritized over those which simply refresh the state of the decoder.

A notable bandwidth hog is Zero Stretching.

Edit FAQ  Categories: Beginner, Command Station

Q: Is DCC power AC?

DCC waveform

Short answer: No

Digital Command Control uses a 100% digital signal on the rails, delivering both power and data in the same signal. The digital information is encoded in the time domain by pulse width, not amplitude. It is not superimposed on a DC or AC waveform, nor does it use a high frequency carrier.

Since the signal is completely digital, the NMRA Digital Command Control waveform is a series of pulses and thus, is not AC. It is not DC either as the phase between the rails alternates to deliver the commands to multifunction decoders installed in the locomotives. This allows the multifunction decoder to reliably receive data from either rail.

Being digital, the signal has two states: On or Off. The NMRA DCC Standard defines the signal on the rails as a Differential Signal. Meaning one rail must always be in the inverse state with respect to the other rail. Differential signalling allows random noise to be cancelled, resulting in a robust signal.

The DCC signal on the track is floating, as it has no fixed reference point.

Confusion arises from the fact that the current flows from Source to Sink. Rail A and Rail B alternate between these two states, creating oscilloscope traces many incorrectly refer to as validation of a positive and negative signal.

Note: To maintain compatibility with analog operations, the NMRA Standard defines the positive rail as the righthand rail as determined by which way the locomotive is facing. This determines the direction the locomotive will travel in.

For a full explanation, see: DCC Power

Edit FAQ  Categories: DCC

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