NMRA: Standards

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Summary: The best-known activity of the NMRA is the creation of Standards, and documents known as Recommended Practices (RP), for model railroad equipment. Many standards defined by the NMRA are widely followed by the industry and modellers, including their HO scale track and wheel standards (S-3, S-4) and related RPs (RP2, 8, 10-15, and 25), the Digital Command Control (DCC) standard set (S-9.1, S-9.2, and RP9.1.1 through RP9.3.2), and NMRAnet standard set (S-9.7) in development.

The main reason for the creation of the NMRA was to encourage inter-operability between manufacturers, which was a serious issue in the early days. Your trains may or may not have worked on a friend's layout. Today, thanks to the efforts of the NMRA, that is not a problem.

Digital Command Control

The NMRA Standards web page is the authority. The Standards, RPs, and TNs listed link to their page, and those links can change without notice.

This is a partial list of Standards related to DCC. For the latest revisions, see the NMRA Website.


S–9.1 DCC Electrical Standard

  • S-9.1 DCC Electrical Standard - (revised April 2021)
    • S-9.1.1 The NMRA DCC WG is currently working on this Standard. It is anticipated that this standard will address connectors that are currently being used by manufacturers, one sub-standard per connector. For a list of all the connectors, use RP-9.1.1 as the current reference.]

S–9.2 DCC Communications Standard

S–9.3 DCC Bi-Directional Communications Standard

Layout Command Control

S-9.7.x Layout Command Control (LCC)

Recommended Practices

Technical Notes

Layout Command Control

Standards Under Review


The following standards are currently under review and released for comments:

  1. S- 21MTC Decoder Interface
    1. TN- 21MTC Decoder Interface

Standards to be Voted On

  1. S-9.1.1 Decoder Interfaces
  2. S- Six & Eight pin Decoder Interfaces
  3. S- JST-9 Decoder Interface
  4. S- Next 18 & Next 18s Decoder Interfaces

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