Trains4Africa
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NMRA : Primary Address Range

NMRA Standard CV Definitions for Mobile Decoders

This Document forms part of the “DCC for the Engineer” series, and lists the complete NMRA Standard Configuration Values (ref: S-9.2.2)

Select-and-Run

With Digital Command Control there is no need to worry about where the train is located, which cab you’re using, or flip switches to get a cab to talk to a loco. To run a loco, you simply pick up a cab, select the loco address (number) you wish to run, and control it. This allows you to easily and quickly control the train’s speed, direction, lights, sounds, or devices such as couplers or animation.

NMRA Address Ranges

The NMRA defines the address structure for 2 and 4 digit address, the actual implementation is determined by the manufacturer of the system. Not all systems implement this in the same way in their software, so there may be some issues running locomotives set up for 2 digit addresses on one DCC system when using another system.

All DCC Command Stations, Cabs, and Decoders support two-digit (Primary) addressing as per the NMRA standard. Even if your equipment has four-digit addressing, you don’t have to use it.

It is possible for command stations to restrict the available addresses, but the NMRA DCC Standard requires that item be identified on the package and in the instructions.

Note: The NMRA Addressing scheme allows for some overlapping of addresses, and different DCC command stations handle this overlap differently. For example, some command stations do not allow you to use the low addresses (0-99 or 0-127) in four-digit mode.

The NMRA defines two ranges of addresses for mobile decoders:

Primary Address Range

  • The first is 1 through 127.
  • This is called the Primary Address.
  • It is a one byte (eight bit) address which in Hexadecimal (see Binary) can have the value of 0116 to 7F16 (1 ..127 decimal) – hence it is called a two digit address.
  • Other terms used for the Primary address are “Short Address”, “Baseline Address”, “7-bit address”, and “2-digit address”.
  • It is called a 7-bit address because the first (most significant) bit in the first byte of a DCC packet is always a zero when it is addressing a mobile decoder in primary address mode.
  • The MSB of this byte is used for other purposes.

 

Extended Address Range

  • The second range of addresses is from 0 through 10239 (000016 to 27FF16 – hence 4-digit address). This is called the Extended Address. Other terms used for the Extended addresses are “4-digit address”, “14-bit address” and “Long address”. It is called a 14-bit address because 6 bits are transmitted in the first byte of a DCC packet in extended mode and 8 bits in the second byte. To reach FFFF16 (or 6553510) would require all 16 bits and then there wouldn’t be any bits left to indicate which addressing mode is in use.
$27FF is 0010 0111 1111 1111 in Binary