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Welcome to the DCC Faq. Please add items here which are commonly asked in forums and discussion pages.

What is DCC?

DCC is an acronym for Digital Command Control. There have been various Command Control systems since 1946, the word Digital is used to differentiate DCC from the Analog Command Control systems in use. 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. See the DCC History page for further information on Command Control systems.

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.

Please see Introduction to DCC for more details.

What is a command station?

Command stations are the heart of the DCC 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.

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.

What is a Decoder?

A decoder is a device which listens to digital packets addressed to it, and then performs whatever function is requested. There are two types - mobile decoders and stationary decoders. Some decoders are very simple devices providing only motor control and/or a small selection of lighting options, while others are very complex with multiple special effects, tuning motor response, and sound effects available. Please see the decoder page for full details.

What can burn out a decoder?

  • Overvoltage. For example, booster is set to incorrect scale.
  • Current draw exceeds decoder ratings (wiring short or other overload condition) Poor wiring can cause this.
    • Be careful when first testing a decoder installation. Put the DCC device on a test track (programming track or section with a 100-ohm series resistor for current limiting) instead of the mainline for testing. If all functions work, and it can move (assuming motor output drive from decoder) at low speed steps, then you should be relatively 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.

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 recieve 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 it's 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.

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.

128 vs 126 Speed Steps

The number 128 comes directly from the use of seven binary bits to set the locomotive speed plus one more bit used for indicating locomotive direction within the command. Together they make up a total of eight bits or one byte which is a standard unit of binary information for computers.

Of the 128 speed steps, two are reserved for special commands such as Emergency Stop. This leaves 126 usable speed step commands to control the locomotive normally. Thus any throttle or cab with a display will only show 126 speed steps at full speed and not 128.

For more information on 126 versus 128 speed steps, see the speed steps article.

How do I measure Digital Command Control voltage?

Because Digital Command Control uses a high frequency waveform, you cannot use a simple AC/DC meter. The cheapest solution is to build a simple circuit. Here's one circuit diagram, and here's a second one. Although it won't be 100% accurate, it will get you pretty close. A second solution is buy a DCC specific Volt/Amp meter from various DCC resellers, such as the RRampmeter from Tony's Trains.

How do I control track switches?

You'll need some turnout motors and a mobile decoder. See Turnout Motors for details.

Add your question here, someone will answer...sooner or later. ;-) TazzyTazzy 14:18, 12 January 2006 (EST)

Limit speed of a loco?

For those of us with children, or grand children around, that like to run trains at top speed, there is a quick an easy way to slow the train down.

Simply, program the decoder CVs to use a lower "max" voltage, thus limiting the speed. You'll need to change two CVs - CV05 (max voltage) and CV06 (midpoint voltage).

CV02 should remain unchanged from "normal speed" to "limited speed". Some people have an extra locomotive just for these situations and always leave them set for slower speeds.

How much track can my booster/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. Whats 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.

Are the Voltages on the Rails Dangerous to Children?

The maximum voltage regarded as safe for human contact is nominally anything less then 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. However, your booster should detect the short and cut the current. But, 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.

Selected Sections of the NMRA DCC FAQ 1.9

Some sections may reflect a time when Digital Command Control was still in development or early prototyping stages. (It is dated 1998).


Q: How are reverse loops and wyes wired?

A: Reversing loops can be done in the classical way for command control. The polarity is correct when the train enters the loop. While in the loop the polarity of the rails is reversed. Note that unlike conventional systems, it is possible to reverse the polarity of the reverse loop itself while the train is running in it - the locomotive will keep moving in the same direction.

Q: What is a digital circuit breaker?

A: Should you want to combine analog and digital operations on the same layout, a device needs to be placed on the analog side to prevent the voltages from combining when you go over a gap. This can be accomplished by using a large choke or by a digital circuit breaker. A digital circuit breaker must be used if the “0” bits are elongated (the DC component of the signal is non-zero).


Q: Why use Packets? Just what is a packet anyway?

A: A packet based system is very flexible and easy to extend in the future. Basically, each packet corresponds to a command for a particular digital decoder (receiver). Each packet contains the address of the digital decoder it is intended for, and some data that tells the digital decoder what to do - for example change to speed 10. It is flexible because you can easily add new packet formats if needed, even ones which can address more digital decoders that the baseline packet format.

Q: What is meant by a bi-polar signal?

A: Each bit is divided into two halves, which are “mirrored” around zero volts: Note that the bottom half of the signal (below zero volts) is a mirror image of the top half, but shifted over half a bit width.

Q: Why use a bi-polar signal instead of DC with a superimposed signal?

A: There are a number of advantages to using a bi-polar signal: - The signal is the power, instead of just being superimposed on the power. This means that if you can get power to a locomotive, you can control it. Wiring loops and resistive loss, which can cause problems in carrier systems, do not cause problems with this technique.

• The track stays cleaner, because there is no polarized electro-plating action as there is with DC systems.

• The bi-polar signal simplifies the design as reversing the locomotive on the track has no effect on how it sees the signal.

• It is possible to control an unmodified locomotive - see the section on compatibility for more details.

Q: The standard implies that the track voltage can range from +/- 7 to +/- 20 volts. Is this how speed and direction are controlled?

A: No. During operation, the voltage provided by the command station does not vary. The digital decoder within the locomotive controls speed and direction by varying the amount of power provided to the motor. The voltage range of the signal described in the standard is to allow for the different power needs of the various scales. For example, a typical Z scale command station will place +/- 10 volts on the rails, and a typical HO scale command station will place +/- 14 volts on the rails and a typical G gauge command station will place 20 volts on the rails. If you were controlling live steam locomotives, you would probably use the minimum +/- 7 volt signal. The +/- part is because the signal is mirrored around zero volts.

Q: If a packet is lost due to dirty track, what happens?

A: Command stations retransmit packets to mobile digital decoders as often as possible, so if one packet is lost, another one with the same information will arrive soon after. If power to the locomotive is interrupted for long enough, the digital decoder goes into reset mode - and stops the locomotive. When power is restored it will not move until it gets a new packet.

Q: Doesn’t retransmitting packets all the time use up too much bandwidth causing slow response to throttle changes, etc.?

A: Although simple low end digital command stations that only provide for control of a limited number of engines simply cycle through all the addresses in use repeatedly, higher end systems may use a priority based scheme so that packets that contain values that have changed are sent before packets that are repeats. Thus repeating packets does not necessarily impact the effective bandwidth of the system.

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

A: 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.

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

A: 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.

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

A: 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 extended packet format RP for a description of feedback in service mode.


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

A: This is an optional feature of digital decoders provided for by the standard. If a digital 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.

Q: Can you run an unmodified locomotive together with a locomotive equipped with a digital decoder from a digital command station at the same time?

A: The standard allows for this, although some care needs to be exercised in its use. We refer 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 of the zero bits on the positive side of the signal a positive DC component can be added. Likewise by lengthening the bits on the negative side of the signal a negative DC component can be added. Only the 0 bits can be lengthened in this manner.

The result is a non-zero average DC voltage which will run an unmodified locomotive. However, since the complete signal gets to the motor, the stretched zero side causes the motor to turn, and the following instant the unstretched side (the opposite polarity) tries to reverse the motor. The longer (stretched) side wins, but motors do run more noisily and generate more heat. Some high precision can motors (the ironless core type) may be permanently damaged. (See the following question.)

Q: How is a locomotive that does not have a command control digital decoder installed affected by exposure to the command control signal?

A: It is not recommended that you leave locomotives not equipped with a digital 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 maximum stall rating of the motor, the motor may experience permanent damage. We have found that common open frame motors, and many common can motors are not damaged by the digital signal even after extended exposure. Ironless core type motors can be damaged by extended exposure to a digital command control signals that are greater than 12 volts due to excessive heat build-up.

Q: What affect does analog compatibility mode (zero stretching) have on normal digital operation?

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

Q: Why allow zero stretching at all?

A: 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.

See Also

  • DCC History, an introduction to command control systems.

External FAQs