The NMRA Standard

The lack of compatibility between manufacturers was a major factor prompting the National Model Railroad Association (NMRA) in the USA, in co-operation with the manufacturers, to negotiate a common standard available to all modellers.  References to DCC have generally become accepted as referring to this standard but other, incompatible, systems can justify this description; the Marklin/Motorola System is an example.  On this website all use of the term DCC assumes the NMRA standard.

The NMRA Digital Command Control Standard defines a basic communications structure at the track level for digital control signals via the rails. The standards specify a communication protocol between command station and decoder but do not lay down standards for the control system components. The data needed to operate each decoder is sent in packet format on the rails in the form of a modulated square wave. This baseline packet format allows for interoperability among equipment made by different companies that support the standard.

Interoperability is the most important advantage of the standard. Interoperability means that if you have a DCC compatible decoder, you can run it with any DCC compatible command station. This is very important since the major part of your investment in any DCC system is in the decoders. We have all heard the horror stories: “I have a fortune invested in this equipment and now I can’t even get spare parts let alone expand my system!!!” Any system that is available from more than one source is not as likely to disappear and leave its users stranded. Also, having equipment available from multiple suppliers creates competition in price and features to the benefit of the end user.

The Standard comprises two sets of rules:

  • Mandatory requirements which all items must comply with and which are surprisingly few in number.
  • Recommended Practices which must be complied with if the capability/function is provided by an item.

When reading about or seeking DCC products it is important to remember that to achieve Conformance Certification or be described as DCC Compatible does not mean that it has all the features that DCC can offer. Low cost locomotive decoders, for example, may not have the facility to change maximum speed, alter speed tables or fine tune some of the characteristics but they will conform if what they do is in accordance with the standard.

In summary any manufacturers decoders can be operated by any manufacturers DCC systems but control system components are rarely compatible between different makes.

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