A DCC Decoder is the on-board control device for locomotive speed and direction.
A DCC decoder is the control device installed in a model train locomotive which controls the trains speed and direction as well as forward and reverse headlights. In conventional control systems a locomotives speed is controlled by a D.C. voltage which is varied by a throttle control similar to the effect of an accelerator pedal on an automobiles speed. As the throttle is turned, the speed increases in a linear manner. Unlike an accelerator pedal, however, a model loco does not begin to move until the speed control knob is increased to some level above 10% of its rotation. At 50% rotation the engine is moving at about 100 scale miles per hour and at maximum throttle it approaches 300 smph. A real locomotive has a throttle which is increased in incremental steps and for each increase, due to inertia, the engine gets up to speed slowly rather than responding immediately to the setting. Digital command control produces the effects as described for the prototype. All DCC equipment must be able to transmit a minimum of 14 speed steps which is more than a real locomotive. Mid-level and top-level DCC components are also capable of producing 28 and 128 speed steps in both directions. The speed control mode for a DCC decoder is set in CV29, the configuration register, and will be covered in detail later. Once a decoder equipped locomotive has been programmed into the system it can be selected and controlled by the throttle. The throttle sends coded messages to the decoder and tells it which speed step has been selected and the decoder applies the correct amount of voltage to the locomotive motor.
The DCC decoder’s two main functions are to decode the instructions it receives from the command station and rectify and apply the proper speed step to the locomotives motor. There are many DCC decoder manufacturers and it is important to know what specific features will meet your requirements. A beginner, with a small budget, may want to start with a very basic system that only has 14 speed steps and a limited number of functions until he becomes familiar with how the system works. If one then decides that DCC is for him, he can progress to the more fully-loaded systems. The standards set up covering the NMRA approved decoders will guarantee that any unit you purchase will work with a future system. Some years ago it was necessary to perform a major operation to install the components needed in a locomotive for DCC operation but most of today’s motive power comes as “DCC ready” and requires little more than buying a decoder and plugging it in. If, like me, you own a few older engines, you might be forced to modify your locos (especially brass) to make them “DCC ready”.
Most decoders come with a specification sheet describing the operational parameters and special features as well as the default settings for the device. Usually the primary address(CV 1) is set for 3 and values are suggested for the configuration register (CV 29). Some installation guidelines may also be listed as well as maximum electrical current ratings. All models use a standard color code suggested by NMRA RP-9.1.1 which is listed below:
RED wire to Right Side Rail pick up.
ORANGE wire to Red Motor Lead.
BLUE wires to Common Power for headlights.
WHITE wire to Forward headlight.
YELLOW wire to Reverse headlight.
GRAY wire to Black motor lead.
BLACK wire to Left Side Rail pick up.
Simplified DCC Decoder Block Diagram
Decoders are available for all model train scales from N to G gauges. One of the most significant factors for choosing the best device is its current rating which will be discussed in more detail in the section on decoder installation. It should be re-emphasized that standards for decoders are specified by the NMRA but no standards exist for the interface of digital command control stations or communication protocol. The decision for the best DCC system for your layout should be made carefully since only the decoders will be interchangeable. I should mention here that I have installed at least three different makes of decoders in my locomotives and have experienced no problems.
