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Summary: While the Digital Command Control signal on the track does not have polarity, it does have a phase relationship between the rails. Direction is controlled directly by the multifunction decoder in the vehicle.

The term Phase is the correct term describing the relationship of the voltages applied to the rails.

The NMRA/NMRA Standards refer to one rail as being positive, which is determined by which direction the locomotive is facing. This term establishes a reference point, it does not indicate polarity. This is strictly for the purpose of maintaining compatibility with Analog DC operation.


Phase in Digital Command Control

Digital Command Control employs a binary signal on the rails, so there is no concept of polarity. No negative voltages are present on the rails, they are either logically High or Low. The multifunction decoder controls both speed and direction, receiving instructions from the command station.


  1. Binary refers to having two possible states.
    1. High indicates a positive voltage is applied to the rails.
      1. This is also the logical High state
    2. Low indicates the rail is held to 0 volts, or to ground
      1. This is the logical Low state
    3. The rail can be in one of two logical states: High or Low. The other rail will be in the inverse state: Not High or Not Low.
  2. A Source supplies voltage from the power supply
  3. A Sink provides the return path to the power supply
    1. Each of the two rail outputs of a booster alternate between Source and Sink, in accordance with the data packets from the command station.

Binary denotes that not more than two logical states are permissible.

The phase of the signal on the rails is important, as incorrect phasing creates a short circuit. As shown on the oscilloscope traces the rails alternate between High (Source) and Low (Sink) states.[1][2]

High refers to the rail energized with a positive voltage (On). Low signifies the rail is held to 0 Volts (or Off).


Phasing is important. When wiring for reverse loops, power or booster districts it is important that the correct phase relationships be maintained.

For a reverse loop a phase inversion device automatically corrects any phase mismatches between the mainline and the loop.

For other applications, the wiring must be done in such a manner that the correct relationships are maintained. To determine if there is a phase mismatch, measure the voltage across the rail gap between two districts. If the meter indicates a difference in potential by displaying voltage, the phases are out of sync between those two segments. If the meter displays little to no voltage, both segments are in phase.

Determine where the issue lies and correct it. A booster district mismatch can be fixed by reversing the connections for Rail A and B at the booster. For power districts, connections at the power management device will require correction.

A Voltmeter displays the potential difference between two points. If both rails are in phase, there will be no difference across a gap, as both voltages are equal.

Reverse Sections

Main article: Reverse sections

Digital Command Control signals on the track do not have polarity, they have a phase relationship. Rail A will be HIGH, Rail B will be the inverse, or LOW. The HIGH and LOW states represent voltages, HIGH being energized to a specified voltage, LOW indicating voltage near or at 0V. When Rail A1 and A2 are in phase, no potential exists across the gap between them, hence no current flows. A short circuit occurs during a phase mismatch: Rail A1 is HIGH and Rail A2 is LOW, providing a return path to the source.

Although there is no electrical polarity, it is necessary to deal with reverse sections on your layout. The phase of the rails is important in DCC. Rail A is aways the inverse of Rail B. If the track turns around back onto itself, the right rail (A) will come in contact with the left rail (B), creating a short circuit by connecting one phase to the other; the same as placing a metal object across the rails.

If the rails are out of phase across a gap, a potential difference between them exists, which can be measured. Should a metal wheel bridge that gap, current flows from the energized rail it the unenergized rail, causing a short circuit. Auto Reverse maintains the correct phase relationships between a reversing loop and the approach track. As the direction of travel is determined by the decoder, the train will continue to move as if nothing happened. If an auto reverser is not used, the booster(s) will immediately disconnect during the short to avoid damage to their outputs caused by excessive current flows.

The same issue can happen if the frog in a turnout is incorrectly phased, the point rails are electrically one unit and not isolated, or with point rails which are insulated from each other, a wheel lacking the the 3º taper bridges those rails at the heel of the frog.


Matching phase is very important with booster districts: Both boosters must be in phase to ensure trouble-free operation as trains move across the gap from one district to the next. Also, only one booster should have the autoreverse mode active otherwise an endless series of phase reversals will occur as the two boosters attempt to maintain the correct phase relationships.


Main article: Turnout

Often there are issues with turnouts manufactured prior to the introduction of DCC. The likely culprit is the gap between the stock and switch rails[3]. When the switch rails are wired together, as often found in older designs, they will both be at the same potential. A metal wheel can bridge the gap between the stock and switch rail, causing a short. Checking wheelsets for proper gauging helps with this issue. Ensuring the stock rail and its adjacent switch rail is in phase by modifying the turnout is another option.

The phase relationship between stock and switch rail is the main feature of the DCC Compatible turnout. It is also important that the point rails[4] are not connected together, and if they are, that each one has a gap between it and the downstream trackage.

Analog Operation

In Analog DC, two conditions determine the direction of the locomotive: Amplitude and Polarity. The amplitude of the voltage determines speed, polarity (direction of current flow) determines the direction the motor rotates. Changing those parameters directly affects the speed or direction of your train. Many multifunction decoders can operate on Analog DC.[5]

See also

  1. Many modellers will make statements referencing a negative voltage on the rails. This is based on observation of an oscilloscope's trace of the DCC signal on the track. It is misleading, as the track is floating, meaning there is no zero reference. Thus, "voltage doubling" is a myth, as it is not possible to have a voltage double when an opposite polarity is not present.
  2. There are no negative voltages, but the current flows from High to Low. Thus, the direction of current follows which rail is high at that point in time.
  3. Sometimes referred to as the points, or inaccurately labelled as point rails.
  4. The Point Rails are a part of the frog, which come to a point at the toe of the frog. Confusion arises from inaccurate terminology, confusing the switch rails with the point rails.
  5. Many modellers disable this feature in CV29. Disabling Analog Mode eliminates the potential for a runaway.