With DCC there are many ways to modify (configure) how each loco will run and work. You can run it as it comes from the factory, or change it in many ways for many different things - even make Athearn run almost as well as a Kato. Most locos will run as they did before DCC without changing anything in the decoder. Some locos will actually run better than before, without any changes, because speed is controlled by pulse power. You don't have to change anything if you don't want to, but if you do want to change the way a loco runs or how the lights work, it can easily be done.

Changing how a loco runs is done with memory locations inside the locomotive decoder. These memory locations are called Configuration Variables (CVs for short). Why "Configuration Variable"? Programming is a term used to describe the process of changing a CV. When programming the decoder, you are "configuring" it to the loco's needs and/or your desires. Since that may be different from loco to loco, and it may change from time to time, CVs can be programmed and re-programmed at any time - making them variable. Hence, "Configuration Variables".

There are many CVs defined by the NMRA (National Model Railroad Association), and many defined by each decoder manufacture. Each CV has it's own purpose, such as loco address, momentum, starting voltages, special lighting effects, custom speed tables, and the list goes on. These CVs are numbered. For example, CV1 is the two-digit base address. CV2 is for start voltage (V-Start is the nomenclature for it), CV3 is for acceleration momentum, CV4 is for deceleration momentum, and so on.

Because decoders are small, and the small processors used in them are limited in clock speed and memory, no one decoder has every single feature. Because of this, different manufacturers have differing ideas about which CVs they will or will not provide. Never assume that the decoder you're buying has the feature you want unless it's specifically stated to have that feature.

Basically speaking, each CV relates to a feature advertised for different decoders. Some decoders may have only a few CVs while others have many. Your operating style determines which CVs you will need. Sometimes there is more than one way to do the same thing. Again, your operating style will determine which way you use the features at hand. Regardless, there are documentation sheets and manuals for each decoder's CVs; telling what each CV in the decoder is for and how to use it.

Some decoders have so many features, with so much processing to do, that some features may suffer. For example, with Digitrax's newest line of decoders, Blinking Ditch Lights suffer because the processor is so busy keeping track of Back-EMF control, transponding, and all the other chores of parsing command packets and keeping the loco's motor and lights going.

When there is a conflict of processor time and/or memory like this, we've tried to provide that information on the page describing that decoder - so you'll know in advance if there are any problems. Your job is to decide which features are important to you and to forget about the rest. Remember, just because a decoder has a feature doesn't mean you have to use it. It's highly unlikely you'll find a decoder with the exact set of features you want with no more and no less. You'll have to decide which features you want and ignore the fact that the decoder may also have others that you won't use.

Once you have decided which features you want, you can then search the list of decoders to decide which one will fill your needs the best.

Following is a list of CVs and the features most widely provided. Each one is linked to a page with a full description so you can decide whether or not you need or want it. Again, there is no decoder with every one of these CVs implemented. So it is incumbent upon you to decide which features are most important to you to determine which decoder best suits your needs.

For more information about any of these, refer to the Decoder Feature Index.

Again, all decoders have some of these features, most have many of them, but no decoder has all of them. It is up to you to determine which features you need or want and to be sure you select a decoder that has those features.