Decoder Basics

A while ago, James wrote a post about installing a decoder in an Athearn locomotive. I thought I would expand on that topic a bit, and talk about the different decoder types, what they are used for, and basic programming for them.

Decoder Types

Different types of decoders are made for different uses. Sound decoders attach to a speaker to play realistic sounds, motor decoders power a locomotive, and function decoders toggle different auxiliary functions such as lighting. Sound decoders usually have motor leads and function leads, so a single decoder can be used to power the sounds, motor, and lights. Sound decoders are by far the most expensive, usually in the $100 price range. However, sound only decoders are available as well, so you don't have to remove an existing decoder from a locomotive if you want to add sound to it. This is a more complicated project, one that I have never done, but Model Railroader Magazine recently had a short article about it in their June 2011 issue.

Motor decoders are used to connect between the track power and the motor of a locomotive. This allows the locomotive to operate. These usually have 4 functions, but decoders with more functions are available as well. These can be used to control headlights, lighted number boards, marker lights, ditch lights, or any other cool features on the locomotive you can think of. These decoders are more affordable than sound decoders, generally in the $20-$30 price range.

Function only decoders do not have hookups for motors or speakers, they are strictly used for functions. Functions are strictly on or off, so they are no good for connecting to a motor. These can be used in a locomotive with a motor decoder if more functions are needed, or in passenger cars for lighting, or even in structures for interior or exterior lighting. I generally hook up structure lighting to a separate power source, so the DCC power only goes to trains, but if you only run one or two locomotives then there's no reason why you can't hook up lighting to the DCC system. A project I've wanted to try for a while is adding DCC lighting to a passenger car with a function decoder, where function 1 controls interior lighting, function 2 controls the marker lights on the A end, function 3 controls the marker lights on the B end, and on observation cars, function 4 controls the lighted drumhead. If I ever do this project, I will write a blog post about it. Function decoders are the cheapest and are generally under $20.

Installation and Programming

National Model Railroad Association, or NMRA, standardized the colors for decoder wiring. No matter what decoder you get, the colors all go to the same place. Red and black always go to the track power, orange and grey always go to the motor, blue is always the positive lead for functions, and all other colors are the functions. DCC ready locomotives come with either an 8 pin plug, a 9 pin plug, but decoders can be installed in locomotives that aren't DCC ready. Click here for a tutorial on that process. I always get 9 pin decoders. They plug right into a 9 pin socket, and they come with an adapter so they can also plug into an 8 pin socket. With a locomotive that is not DCC ready, if you cut the 8 pin end off of the adapter and solder the wires to their appropriate locations, you have successfully installed a 9 pin plug. That way if your decoder ever burns out, replacing it is as simple as plugging a new one in.

For step-by-step instructions on programming, please look at the instruction manual for your DCC system. Every system is different and I cannot possibly explain the programming procedure for each system, but I can talk about the different features you can program into your decoder. Every decoder works on a system of configuration variables, or CVs. Each CV is assigned a number, which you bring up so it knows which one to program. Then, each CV has a value, which depending on the CV, tells the decoder what to do. Different CVs do different things depending on the decoder, so consult the instruction sheet for your decoder to find out what the common ones are. More complete CV charts can be found online for more complex programming. However, the more common CVs are standardized, so your DCC programming can be more user friendly. For instance, the CV for the decoder's address is always the same CV on every decoder, so a DCC system can automatically call up that CV and tell you that your are changing the address.

The address is basically the decoder's name, but it is actually a number. It is generally a good idea to program the address to be the same as the locomotive number, so you can look at the locomotive and know what its address is. This is important for operating the locomotive. The address can be a value between 0 and 9999, so your choices are not at all limited.

Another common CV is the acceleration/deceleration controls. They are always the same CVs, so most DCC systems can program them easily. When you turn the throttle up or down, these CVs delay how quickly the locomotive responds to the action. That way they slowly go faster or slower, like the prototype. A real locomotive will not go zero to 60 in 3 seconds, nor do they stop on a dime! Operationally, this takes some getting used to, but it enhances the realism in your locomotives.

Finally, there is the starting voltage/top voltage CVs. These are also the same in every decoder. Some decoders also have a middle voltage. This is used for setting a top speed. Most model locomotives can go a lot faster than their full scale counterparts. This is a more complicated procedure, and I will only go into the basics. By using a scale speedometer or by calculating scale speed yourself, you can figure out how fast a locomotive is going at a certain voltage. You can then look up the top speed of the real locomotive you are programming, and program the top voltage so the model cannot go any faster than the real thing. Starting voltage is generally zero, but some locomotives won't start until they have a higher voltage. When the starting voltage is set to zero, the locomotive may go through several speed steps before moving. By figuring out the voltage where the locomotive starts moving, you can then set the starting voltage to just below that point. That way, the locomotive responds to low speed steps.

Other CVs include operation of headlights, marker lights, ditch lights, and other functions. These are less common CVs, and so they may not be standardized. You will have to look up what they are in your decoder instructions to program them. Most CVs exist for the basic operations of the decoder and should not be changed. If you mess up and don't remember what the value of certain CVs used to be, you have not ruined your decoder. Most decoders have a CV where you can set the whole thing back to factory settings and start over. Again, this is not a standardized CV, so consult the instructions.

Decoder programming is not as complex as it may sound. With a little practice, you will quickly become a pro at programming decoders. I find it is a good idea to keep a spreadsheet for your decoders, with each CV value, its factory standard, what you have reprogrammed it to, and what that CV does. This will keep everything organized and will simplify future programming. And that way, if you buy a new locomotive and want to speed match it with another one you already have, you can look at the spreadsheet and know what to set as your top voltage!