Dave's DCC Tips #2

Short and Long Addresses Explained

The topic of DCC loco addresses causes much confusion. Terms like short address, long address, 2 digit address, 4 digit address and consist address are bandied about, but confusion continues.

What do these terms mean and does it really matter? This article seeks to clarify the situation, as confusion can lead to the inability to control our locos, or arguments between owners of different brands of DCC systems.

The NMRA DCC standards define only two types of addresses that loco decoders can respond to. One is called the short or primary or baseline or 7 bit address, the other type is called the long or extended or 14 bit address. For consistency we will use only short and long address terminology as these are the terms most used in the standards.

The terms "2 digit address" and "4 digit address" are often used in DCC circles, but these terms are both technically incorrect and quite misleading. As we shall see, short addresses can have 1,2 or 3 digits and long addresses can have 1, 2 ,3, 4 or 5 digits.

The NMRA DCC decoder standards define the following permissible address types and ranges:

1 to 127 for Short Addresses. The short address is stored in CV1.
Short Address 0 is the Broadcast Address. All locos must always respond to the broadcast address, no matter what their normal address is.
0 to 10,239 for Long Addresses. The long address is stored in CVs 17 & 18.
1 to 127 for Consist Addresses. The consist address is a short address and is stored in CV19.

In order to understand why there are both short and long addresses and why the seemingly strange ranges, we need to diverge to take a slight history lesson. Early multiple control systems allowed a small number of locomotives (between 2 and 16 or so) to be independently controlled, but were not compatible with each other. When the NMRA decided to define a standard, they eventually decided to base the standard on a system developed by Lenz. The Lenz system used an instruction packet containing 7 bits of address data (addresses 0 to 127). The NMRA defined an Extended Packet Format that could use a new long address range from 0 to 10,239 but decreed that all command stations and decoders would also have to continue to support the original Baseline Packet Format.

As a result of all this historical brouhaha, almost all modern decoders follow the NMRA standards and support two possible addresses, the long address and the short address.

You can programme any address number between 1 and 127 in the short address CV1 and/or any address number between 0 and 10,239 in the long address CVs (17 and 18) However, only one of these addresses can be active at a time. You need to set bit 5 of CV29 if you want to use the long address and clear bit 5 of CV29 if you want to use the short address.

Every packet (baseline or extended) sent by a command station has information coded into its first byte that enables a decoder to determine whether the command is being sent to a short or long address. So the decoder always knows which address (short or long) to match to.

The problem is that DCC command station manufacturers have restricted the number of addresses that their command station can use to send commands to decoders. To give some examples:

NMRA allows 1 to 127 for Short Addresses, 0 to 10,239 for Long Addresses and 1 to 127 for Consist Addresses.
NCE allows 1 to 127 for Short Addresses, 0 to 9,999 for Long Addresses and 1 to 127 for Consist Addresses.
MRC allows 1 to 127 for Short Addresses, 128 to 9,999 for Long Addresses and 1 to 127 for Consist Addresses.
Digitrax allows 1 to 127 for Short Addresses, 128 to 9,983 for Long Addresses and 1 to 127 for Consist Addresses.
ZIMO allows 1 to 99 for Short Addresses, 100 to 10,239 for Long Addresses and 1 to 99 for Consist Addresses.
EasyDCC allows 1 to 99 for Short Addresses, 100 to 9,999 for Long Addresses and 1 to 99 for Consist Addresses.
Lenz allows 1 to 99 for Short Addresses, 100 to 9,999 for Long Addresses and 1 to 99 for Consist Addresses.
Atlas allows 1 to 99 for Short Addresses, no Long Addresses and 1 to 99 for Consist Addresses.
Bachmann EZDCC allows 1 to 9 for Short Addresses, no Long Addresses and no Consist Addresses.

What does this mean in practice?

It is possible for us to program addresses into our decoders that will be usable with our DCC system but we may not be able to control the same loco with a different brand of DCC System.
When discussing DCC addresses with fellow modellers we need to use correct terminology to avoid confusion. We should abandon the use of the terms "2 digit address" and "4 digit address" as these are both incorrect and misleading.
We need to be aware that our understanding of the subject may be coloured by the limitations of our particular DCC system and that fellow modellers' views maybe coloured by the limitations of their particular DCC systems.
Some DCC systems have a very limited range of loco addresses that can be controlled and that we should always check this before purchasing a new system.
Systems that allow control of overlapping short and long address ranges will have system-specific ways of differentiating between short and long addresses when selecting locos. For example, with an NCE system you select short address 3 by entering 3. You select long address 3 by entering 03 or 003 or 0003 (but the correct name is long address 3, not 03 or 003 or 0003).

More information on this subject can be found at Loco Address Confusion by Mark Gurries. Some of my material was quoted from the information given on Mark's site.