Digital Command Control Tutorial - Power

DCCWiki, a community DCC encyclopedia.
Jump to: navigation, search

Summary: The DCC signal contains both power and data. The data is represented by the time the signal is High or Low. DCC does not have polarity, it has a phase relationship. Being digital, there is no concept other than High or Low. It consists of a series of pulses which represent the digital data. The pulses of data also power decoders.

Share this page

Introduction to DCC Power

Power Concepts for Digital Command Control.

Let's start off by saying:

You don't need to fully understand the technical details of how DCC works to make DCC work.

If you don't understand anything in the sections on Some DCC Details and "DCC Power", don't worry. Come back in a few months and see if you understand it then. If not, don't worry.

Do you know the technical details behind high-definition satellite TV broadcasting? Does that keep you from enjoying your favourite show? Does it create a barrier to changing channels or turning your home theatre on?

All You Need to Know About DCC Power

  1. There is a binary signal on the power bus, which is sent from the booster to the train.
  2. There is full power on the rails at all times while the booster output is turned on. Voltage does not control loco speed.
  3. There is the signal phase present on the rails, where one rail is energized (ON) while the other is held to 0 or OFF.
  4. The rails are identified as A and B, whose phase relationships can be handled by auto reversers when A meets B so you don't need to flip toggle switches.
  5. The phase of the rails does NOT control the direction of the locomotive.
  6. The signal is present at all times, in the form of the DCC packet shown below.
  7. DCC does not employ a DC voltage with the DCC signal riding on top, or any carrier signals. The rails have a series of pulses on them at all times.
  8. Power and the DCC commands are one in the same.
Simple On/Off binary signal, with one rail always the inverse of the other.

DCC track power can be seen as:

  • Power for motors/lights/sound/animation.
  • A digital timing/control signal to tell decoders what to do.

Myths about the DCC

There are a number of erroneous myths circulating about Digital Command Control that refuse to fade away. Most are the result of applying analog concepts to a digital technology..

  • DCC is Alternating Current[1]
  • DCC is a form of Alternating Current
  • DCC has Polarity.

Correct Answer: None of the Above. The DCC signal on the track is a binary signal, where it is either ON or OFF. One rail is always the inverse state of the other.

Many references on the Internet state that Digital Command Control uses AC (Alternating Current). DCC Multifunction Decoders are not compatible with AC power, such as command control systems offered by Marklin for their 3-rail AC trains.

Some DCC Details

Digital Command Control is a digital technology, which relies on the presence of a positive voltage, or no voltage. It has no concept of negative voltages, as the Low state is defined as 0V.


The power on the track is not simple analog (Direct Current), but a Digital Signal. The DCC signal contains both power and data. The data is represented by the time the signal is High or Low. DCC does not have polarity, it has a phase relationship. Being digital, there is no concept other than High or Low. As shown in the oscilloscope trace toward the right, it consists of a series of pulses which represent the digital data

The DCC signal is not an Alternating Current sine wave, nor is it a special form of AC. The DCC signal switches quickly between states, and varies the time period the rail is High or Low to convey information to trains on the track. Contrary to popular belief, there is no negative voltage present on the rails.
There are many who claim there are both positive and negative voltages, citing the Peak-to-Peak amplitudes as seen in the 'scope trace. This is often a case of erroneous information being repeated so often that it's accepted as a fact. See DCC Power for more details which explain how this is not possible. It is not an analog waveform, where frequency, phase, polarity, or amplitude have meaning.

The reason the oscilloscope trace appears to have positive and negative components is that the reference for 0V is constantly changing. The scope's probe is clipped to one rail and its ground connected to the other. The zero reference point floats, or changes in value, so when the reference point is high the other rail appears to be less positive and a negative going trace is drawn. In fact, the trace is not showing voltage, but which direction the current is flowing.

Rail A must always be the inverse of Rail B. The booster takes the digital data from the command station and amplifies it, which is then applied to the rails. This puts a very robust signal on the rails, allowing the locomotive's decoder to pick data off either rail eliminating the need to place the locomotive on the rails in a specific alignment. Another advantage is that one rail is not held to a constant voltage, eliminating contaminants attracted via the space charge forming around the rails.

Unlike Analog electronics, digital electronics have no concept of polarity. They have only two states: High and Low. This is called Binary, as there are only two defined values. The binary nature of DCC means no negative voltages present on the rails. The signal on one rail is always the inverse of the signal on the other rail. Thus, when Rail A is High, Rail B is in the inverse state, or Low.

Multifunction Decoders

Multifunction decoders are installed in vehicles. These decoders receive the DCC signal from the rails, read the data, and act on it. The DCC signal contains data in the form of the Address, Commands, and a Checksum byte.

On the track, each pulse is repeated, once on Rail A, then on Rail B. The multifunction decoder samples one rail to extract the data. This works regardless of the orientation of the locomotive. If one rail carried pulses of a negative amplitude, the decoder would only see 0V, and would not be able to extract any data if the decoder was sampling that rail.

The multifunction decoder controls direction and speed, regarless of the vehicle's orientation or track voltage.


Analog control systems have Polarity; the Digital Command Control equivalent is Phase.

Main article: DCC Power/Phase Relationships

DCC does not rely on the analog concept of polarity to control direction of travel. The multifunction decoder controls the direction (and any related wiring functions) independent of phase.

All boosters on a layout must operate in phase, and all track segments must be in phase. This means that all Rail A outputs on the boosters are in the same state, and all Rail B ouputs are in the inverse state.

If at some point the power bus wiring is reversed, a phase mis-match will occur. In a single booster district, a short would be registered. If a Power District is out of phase a metal wheel bridging the gap between two districts short will create a short. When the rails across a gap are in phase, there is no potential (voltage) present, as they are both in the same state. When there is a difference in phase, a potential will be present across the gap. By measuring across the gap with a voltmeter or a lamp, phase mismatches can be quickly identified. This often is an issue with reverse loops and turnouts.

It is not possible to create situations where the voltage is doubled. This issue is caused by inadequate wiring.

Turnouts and Phase Issue

There are two places where the phase is important: The frog and the switch rails. See the pages on turnouts for more details.

Reverse Loops

When the track loops back onto itself, Rail A will meet Rail B through the turnout's stock rail. For those applications, a method of inverting the phase of the loop is needed. This can be done automatically, or manually using a toggle switch. The multifunction decoder onboard the locomotive will continue as if nothing has happened.

Digital Command Control Signals

The DCC signal produced by the Command Station. This shows the binary On/Off nature of the signal, and each cycle consists of an equal On and Off period.

The digital waveform is created by your DCC system's (command station). It is sent to a booster (or multiple boosters for large layouts) where the DCC Commands are amplified to track power levels. The resulting higher voltage digital waveform from the booster is applied to the rails. The nominal transmission values are 58µS for a "1" and 100µS for a "0".

The multifunction decoder in the locomotive picks up the signal from the rails. The multifunction decoder has circuitry to process the DCC signal. The microcontroller the multifunction decoder is built around examines the data, and if it is addressed to it, acts on the instructions contained in the waveform. Additional circuits supply power to the motor(s), under control of the microcontroller which is responding to the information encoded in the waveform. Each decoder has an address, and will only respond to commands addressed to it.

Wireless DCC controllers have an RF transmitter in the handheld (throttle), and an RF receiver that controls the DCC controller (command station). But the DCC Signal and power still goes through the booster and the rails.

A Digital Command Control Packet

A DCC Packet is a defined group of signals. The technical term is a broadcast protocol. In the case of a broadcast protocol, the data is sent to all devices. Every packet consists of a minimum of 38 bits.

The packet always begins with a preamble of at least twelve bits representing a value of one are transmitted. No other DCC command can consist of twelve sequential bits set to equal 1. The preamble is considered to be floating, and provides sync information to the devices, making the entire system self-synchronising without additional clock signals.

When a decoder sees the preamble, it immediately takes note. After the preamble the data bytes begin. The first segment is the address byte, the next is the instruction byte. The NMRA DCC standard only defines a few basic instructions. The final segment is the error correction byte. A typical packet consists of three or four data bytes, or 24 to 32 bits. The final segment is the packet end bit, equal to 1. Each byte is separated with a zero bit.

The command station will either transmit idle packets, or retransmit previous packets. No data, no track voltage. The command station will have logic in it that will determine the sequence and repetition of packets to ensure that the bandwidth available is used efficiently.

Conversion from Analog/Direct Current to Digital Command Control

Conversion of a layout from Analog direct current (DC) operation to Digital Command Control usually means rewiring. The reason is a fundamental difference between wiring methodology: Direct current layouts rely on multiple low current power supplies located around the layout, with each powering a single block/train at a time. Since one power supply is routed to a block in the immediate vicinity, they do not require heavy wire to carry large currents, as they do not supply power to multiple trains a great distance from the power source.

DCC gives you the ability to run multiple trains simultaneously from one booster, and most modellers will soon begin doing that because is it easy and possible. Which means more current draw, requiring wiring that can handle that.

What Is Next?

Continue to Getting started with a Starter Set.

  1. Alternating current is the current that flows in a circuit stimulated by an alternating voltage. DCC signals are not continuously changing in amplitude and polarity.