DCC Basics: Decoders

This isn't really the place for a tutorial on DCC. There are plenty of books and magazine articles for that, not to mention online information. The NMRA has a number of pages aimed at beginners, for example, and you can no doubt find many others.

But there is some basic information worth noting as an introduction to the other pages in this section. This material will focus on the mobile DCC Decoder (the part of the Digital Command Control system that goes inside a train). For additional information about DCC decoders, see the DCC for Model Trains subsection of the Model Trains section.

The Role of the DCC Decoder

In a DCC system, the command station takes the operators commands (via a throttle), encodes them along with the power and sends both to the rails. The decoder’s function is to control how the power on the track is delivered to the motor and accessories on the train by interpreting the commands sent to it. Because each train has a unique number, a decoder can tell which commands are being sent to it and ignore the others, and this allows more than one throttle to control more than one train. In fact, the command station remembers the last command sent to a train, and keeps re-sending it, so an operator can unplug their throttle, or change it’s address to control another train, and the first train will keep running. This allows one person with one throttle to control multiple trains.

Decoders in trains are called “mobile decoders”, other decoders may be used to control things like signals, crossing gates, or turnouts, and these are called “accessory decoders” (or “stationary decoders”). The way the two are controlled by the user is different, but fundamentally they work very similarly.

The Mobile Decoder

A mobile decoder (often just called a “decoder”) is a small circuit board with at least four wires (and often more). Two connect to the track, and two connect to the motor. Most decoders will have additional "function output" wires to control lights, and a sound-equipped decoder will have two wires for connecting to a speaker. With function wires, typically one wire is a "common", and each function has a single wire, with a light, for example, being connected between the function wire and the common wire. Mobile decoders can also be “function-only” decoders. These would be used if you wanted to control, for example, lights in some car other than the locomotive, such as a dining car or a flashing-rear-end (FRED) device on the tail of a train. They’re also very handy in multiple-unit passenger trains, where there are two cabs with head- and tail-lights, one or both of which will be separate from the motor car.

Note: decoders are a sensitive electronic device and easily damaged by handling. Keep them packaged until use, and follow good ESD safety precautions when handling them.

Decoders can be wired permanently to a train, or may be connected via a plug and socket arrangement, so that the decoder may be installed or replaced by a non-technical train operator. When a train is supplied without a decoder, often a socket is provided and used to connect to an interim circuit board that is removed when a decoder is installed. However, both wire-in and socketed decoders are not commonly found on Japanese prototype trains, so decoders must be wired in by the purchaser, or in the case of Kato's "DCC Friendly" trains, special decoders may be installed (see the Kato DCC Decoders page for more on that).

A decoder typically comes from the factory set to answer to address "3" (the “default address”), and with a default set of behaviors. These can be modified by storing numbers into what are known as Configuration Variables (abbreviated CV). While this isn't particularly hard, most DCC systems seem to have been designed by and for computer programmers, which can make actually figuring out what you need to do rather unnecessarily hard for people who aren't computer programmers.

Decoders pre-installed in a train may have CVs pre-set to reflect characteristic behavior of the train, or require programming. See the manual that came with the train.

Motor control in a DCC environment is, technically, quite complex. You don’t need to understand how a decoder controls a motor, although that can make it easier to set up things like speed tables and Back-EMF. If you want to know more, I delve into this on the Back-EMF page in my Decoders section.

The Accessory Decoder

A decoder can also be used independent of a train, such as attached to a turnout or a signal, so that the operation of these devices can be controlled via DCC. These decoders are called accessory decoders or stationary decoders, and behave similarly to mobile decoders in many ways, but are addressed independently (thus you may have both mobile decoder #3 and accessory decoder #3 at the same time). The default address for a mobile decoder is “1”.

Control of these is generally done by selecting a “switch” number on the throttle, and issuing a “close” or “throw” command to it. The switch number is actually the address of the decoder, although some decoders can have more than one address because they can control more than one output (accessory decoders don’t have “functions” the way mobile decoders do).