FAQ:Is DCC power AC?

DCCWiki, a community DCC encyclopedia.
Jump to: navigation, search
See more FAQs
General information
DCC Categories DCC
Related Articles:
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