Power Conversion

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Summary: Power conversion determines the mode a multifunction decoder operates in, other than DCC mode

Short Definition
Ability of a DCC decoder to operate on a non-DCC layout

Power Source Conversion (CV12)

Known as Alternate Power Source in the NMRA DCC Standard. This CV is Optional.

Multifunction decoders can operate on non-DCC or layouts using other forms of command control. The most commonly supported power conversion type is Analog Conversion where a DCC-equipped locomotive can operate on a traditional analog layout.

Many users set their decoders to NMRA Digital Only. This prevents the locomotive from running on a Direct Current layout. The purpose for setting it to only operate on NMRA DCC is to prevent runaways. If the decoder loses the digital signal for a defined period of time (30 mS), it will switch to the Alternate Power Source as defined in CV 12. Distorted DCC signals on the track can cause this, often related to poor wiring practices.

See Analog Conversion for more details on how this process works.

List of Power Conversion Types Available in CV 12

The NMRA DCC standard recognizes the following power conversion types (although none of them are mandatory).

This is not a fixed list, as the NMRA will consider requests from manufacturers to add additional power conversion modes.


These the assigned codes for Power Source Conversion that are found in CV 12.

These are presented in decimal form
1 Analog Power Conversion
2 Radio
4 Zero 1
8 Trix
16 CTC 16 / Railcommand
32 FMZ
These values come into effect when CV 1 is set to all zeros.

Using a Primary Address (CV 1) of "0" enables this feature, overriding the Alternate Power Source (Bit 2) value in CV29. A combination of a Primary Address of 0 and CV12 = 1 will result in an immediate runaway when DCC power is connected to the rails.

See DCC History for more on various command or carrier control systems.

See Also