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NMRA Standard

Posted by on Dec 18, 2014 in Miscellaneous, Tony's Tips |

The NMRA Digital Command Control Standard defines the basic communications structure at the track level for digital control signals via the rails. The standards specify a communication protocol between transmitter and decoder without specifying transmitter and decoder hardware. The data needed to operate each decoder is transmitted in packet format on the rails in the form of a balanced 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 any more much less 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 does not cover the actual command stations or control equipment used to operate the decoders or the features they offer. You can buy a full-featured DCC command station or a basic DCC command station. You can spend more money or less money. There is a place in the market for both low end and high-end equipment. You decide what makes sense for you and your railroad.

Because of the DCC standard we have already seen the cost of Digital Command Control systems drop dramatically. In the early days, a “starter” system ran about $1000 and decoders were $95 each. Today a system that does much more than those early systems costs about $325 and decoders can be purchased for less than $30.00.

Today’s NMRA DCC Standard provides a framework for interoperability without precluding manufacturer innovation. Some innovations we have seen that are not required or covered by the standard include: automatic reversing boosters and devices, 128 speed step control, analog locomotive operation, various cab bus systems, a network for layout operation, cost effective decoder harnesses, block detection systems, sound decoders, system upgradability, new “painless” ways of installing decoders and much more to come. The standard is just the starting point!

Recommended Practices (RP’s) are adopted from time to time to give manufacturers additional guidelines for interoperability. Several RP’s have already been adopted to cover the NMRA recommended locomotive plugs, the extended packet format that allows for decoders to receive and process more information, the programming RP and the “fail-safe” RP. The NMRA DCC working group is continuing to work on additional RP’s and refinements to the standard. Once new RP’s are adopted manufacturers will begin to incorporate the ones that make sense in the marketplace. Hopefully, these new RP’s and changes to the standard can be incorporated in a way that will be backwardly compatible with existing equipment.

What does the “DCC symbol” mean? How is it different from an NMRA “Conformance Seal”?

Manufacturers that build interoperable DCC equipment compatible with the NMRA’s DCC Standard use the DCC logo to let customers know that they support the NMRA’s standards effort by producing compatible equipment. Various groups who support the DCC effort, including the DCC working group and the DCC SIG also use the logo. This symbol is not a conformance seal.

The NMRA conformance & inspection program covers all aspects of model railroading interchange, not just DCC. Many people who have heard a lot about the NMRA DCC standard are surprised to learn that the NMRA actually has standards covering couplers, track gauge, wheels and much more. The NMRA conformance and inspection program was relatively inactive until 3 or 4 years ago. Now, the NMRA is working to revive this program. To that end, the NMRA has established a conformance testing program for DCC equipment and for other model railroad products as well. The NMRA is now issuing conformance seals based on the tests they are performing. Let’s briefly review the conformance seals that have been issued for equipment manufactured by DCC companies and “non-DCC” companies. (Since locomotives must conform to more non-DCC than DCC standards & RP’s we have not counted the ones that follow the NMRA plug RP as DCC products.) In 1996 (the first year of the C&I revitalization), 9 seals were issued (8 for products made by DCC manufacturers and 1 for other products). In 1997, 13 conformance seals were issued (2 for DCC and 11 for others). Through June of 1998, 51 conformance seals have been issued (none for DCC specific products although some previous seals were updated). As you can see, the C&I program has grown beyond just DCC.

According to the NMRA, an NMRA Conformance Seal is not an endorsement or guarantee by the NMRA. It is merely a statement that a particular product passes a particular test to determine whether, in the opinion of NMRA volunteers, it conforms to a particular NMRA Standard. It is important to remember that the NMRA conformance tests are administered and defined by dedicated NMRA volunteers who are working very hard to turn the C&I program into a useful tool for NMRA members.

Adapted from “Digital Command Control: The Wave of the Future”, by Zana and A.J. Ireland.