Accessory Decoder Addressing

DCCWiki, a community DCC encyclopedia.
Jump to: navigation, search

Summary: Accessory Decoder Addresses are three-digit addresses used to identify accessory decoders


Accessory addresses are used to identify accessory (or stationary) decoders or sub-functions thereof. Accessory Decoders are addressed by three digit numbers. The digits in question are Hexadecimal (Base 16) numbers, not Decimal (Base 10).

Accessory addresses are not simply different addresses that happen to identify accessory decoders instead of locomotives. Locomotives and accessories occupy different address spaces. A real-world analogy would be they live on different streets: 100 Locomotive Dr. is a different address than 100 Accessory Blvd.

Address Range

Addresses are available from 1 to 2044. Each address location can control a pair of functions, usually either two solenoids for turnouts or two LEDs for signals.

Different manufacturers view this range in one of two ways:

  1. ) 511 decoder addresses, each with 4 sub-addresses, or
  2. ) 2044 individual output addresses

Not all command stations support all of the possible addresses in that range.

Accessory decoders are addressed differently than multifunction decoders. Each address defines an individual cell, by changing that cell turnout motors or signal aspects can be activated.

Basic Addressing Structure

In this mode the decoder is configured with a 9-bit address which is considered to be the base address. The decoder can then have up to four pairs of outputs at this address.

Basic Accessory Decoder Addressing
Byte 1 Byte 2
Flags Address Byte Flags Address Command Data

The first pair of flags indicate the address is for an accessory decoder. The second flag indicates a two- or three-byte command. The flags also seperate the first and second bytes.

The Command and Data bits are used to determine the state of and which output is to be used.

A total of 2044 address are available using both the 511 possible address and the four available outputs (D1D0). When used with turnout motors, the Data bits are used in the following manner: The first two (D1D0) indicate the output (one of four), Dx defines which output will be used if configured for a three-wire device.

Extended Addressing Structure

Extended Accessory Decoder Addressing
Flags Address Byte Flags Address Address Aspect 1 Aspect 2
Byte 1 Byte 2 Byte 3

Extended Accessory Decoder addresses are sometimes called 11-bit address as they have two additional address bits.

The second flag is set to zero, indicating a three-byte packet is in use. The third byte is indicated by the yellow background.

Special Addresses

The NMRA reserves addresses 2045 to 2048 for broadcasting to all accessory decoders.


Many accessory decoders use learning to configure their address. You put them in learning mode and use your throttle to send an accessory command to the address you want.

When configuring the decoder using the CV method, the NMRA defines two ways to set the address CVs:

Decoder Address

In this mode the decoder is configured with a 9-bit address which is considered to be the base address. The decoder can then have up to four pairs of outputs at this address.

Output Address

In this mode the decoder is configured with an 11-bit address which is considered to be the specific output pair address. The decoder may have only one output pair.

Comparison of Various Accessory Decoders

Manufacturer Model Output Address Range Output Pairs Learning CV Method
DCC Concepts AD1 1-2044 1 Yes No
Digitrax DS64 1-2044 4 Yes No
SE74 4 Yes
DS78V 8 Yes
DS74 4 Yes
DS44 4
DS52 2
Gaugemaster DCC30 1-127 4 No Yes
Lenz LS100/110 1-256 4 Yes Yes
Zimo MX820E 1-2044 1 No Yes

See Also

Address Range: How addresses are partitioned

External links