DCC offers an extremely flexible method of control. The decoder is the interface between controller & locomotive. This interface carries out all commands, so it is a clever, complex device.
Decoders handle not only the basic “run” commands…. They control sophisticated lighting and very flexible, useful operating functions. Add to this the fact that as with any other technology, the manufacturers can’t help themselves when it comes to naming, and there are many terms that need to be understood to get the best from your DCC system:
This page will try to explain these functions in clear, simple terms.
Please note that the number of functions varies decoder to decoder. The “function count” is usually either part of the decoder number (ie, TCS M4 is a 4 function micro decoder) or it may be added as a “byline” to the decoder name (ie Digitrax DX143 four function decoder).
Note 1: There are two ways of defining a function – Using the descriptive name and the CV (configuration Variable) that controls it. Familiarity makes some simply quote the CV, others use the name, so I’ll try to mention BOTH where it is helpful.
Note 2: Many CV’s are common and “fixed” in use by the NMRA standards, but others are “at the choice of the Brands”. Where a function is ALWAYS the same, or is always on the same CV, I use the abbreviation “Std”. Where its use or the term are proprietary or a Manufacturers choice, I will quote “Mfr” and the manufacturers name where it is known.
As with ALL pages on this site, we are happy to modify or expand on any point where requested. If you have any comment on this data or a suggestion for improvement, please do NOT hesitate to let us know!
The Basics: (The wires and their basic function)
Black and Red Wire: (Std)
These go to the power source (power bus) and are the connections that provide both power and the means of communication to the decoder. For a loco decoder, the convention is Red to right rail, black to left rail.
For an accessory decoder, connect to track power OR to a separate booster and “accessory power bus” if the layout is a large or complex one with lots of accessories (This minimizes potential problems due
to excess loading of the track bus)
Orange and Gray Wire: (Std)
These usually go to the motor in a locomotive, or they can be connected to ANY device that needs speed or “Voltage” control” such as a turntable motor, traverser or higher power dimmable lighting.
If the start and max voltage are adjusted, they can even be used to power things like a high current relay. (These wires can be “voltage controlled” by separate CV’s (Start volts = CV2, top volts = CV5).
Blue Wire: (Std)
This wire is the COMMON positive wire for ALL accessory functions:
White, Yellow wires: (Std)
These two wires are traditionally allocated to lighting, but can be utilized for ANY function that operates within their “load” ability. For example, most decoders are set up to use white plus blue for front light, yellow plus blue for rear light – but their actual use is YOUR choice!
Lights are generally turned on and off with the controllers “f Zero” or F0 function.
Lights can be configured to automatically reverse with direction, be on and off at the touch of a function button, to flash, to dim, or to do ANY prototype lighing function. Check your decoder info for a “how to” on these functions.
TIP – no matter which decoder brand you are using, take a look at the excellent “plain English programming guide” and decoder literature in general from TCS for a well laid-out description on how to program lighting)
Green, Brown, Purple wires (if fitted): (Std)
These are additional function wires. They operate the same as other wires, and can be turned on and off using function buttons or features on your controller. Use for ANY operatring function that has a current draw within the spec for these functions.
TIP: you can parallel TWO functions to operate a higher functionwith a current draw above the ability of individual function wires – just “remap: the function on-off to the same function button on your controller and connect the two chosen functions to one side of the accessory, and the blue common to the other. This sort of “parallel” function application is most common for smoke units etc.
The FUNCTIONS: (The basic CVs and their actions)
Basic Setup: (Std)
This is a general function that carries out several tasks in a single area of the decoder memory:
It is addressed on ALL decoders via CV29. Read decoder literature for the correct CV values.
*It sets the direction relative to the “forward/reverse” command.
*It sets the “speed steps” for the decoder to 14, 28 or 128.
*It enables or disables “dun on normal DC” within the decoder.
*It enables or disables programmed “speed tables”
*It enables / selects whether the decoder will respond to 2 or 4 digit addressing
Primary or “Short” Address: (Std)
This is the decoders “two digit” address. It is usually set at “3” as an ex-factory default. It can be changed by modifying CV1
Advanced or “Long” Address: (Std)
This is the decoders “four digit” address. It is usually NOT set as an ex-factory default. It can be changed by modifying CV18 and 17 (Most competent controllers will do this very simply for you by entering a single direct number when setting up the decoder.
Older and non-current technology controllers may require a more complex procedure.
Factory Reset: (Std)
Sometimes its easier to just “go back to basics” rather than re-set a decoder bit by bit. Generally, you will be able to return a decoder to “ex factory” settings, and an address of “3” by entering a value of 2 into CV 8 or CV 30, then turning power off and on again.
Decoder Lock: (Mfr) (TCS)
This is a VERY useful function where you may need two decoders in one loco (say, a full decoder plus a lighting decoder in a DMU/EMU or passenger consist) or where you wish to make sure one decoder in a two decoder loco will not be changed when the second decoder is programmed.
Decoder lock is activated via CV64 (for example, for the TCS “fleetlighter”, add “8” to CV64, program as per requirement, then put “0” in CV64 to lock it.)
Consisting / Consist addressing: (Std)
This is user selectable. Consisting” is the act of making two or more loco’s act as a single unit. If this is activated, then it will over-ride other addressing and you will NOT be able to change some decoder parameters unless the decoder is taken OUT of consist mode.
See specific controller or decoder data for procedures. Consist addressing is held in CV 19. (You might want to address some changes in lighting and direction when in consist mode).
Start Voltage: (Std)
This can often be “over-ridden” by selecting a “brand dependent” function like “dither” or BEMF. It is addressed via CV 2. Basically, Start voltage sets the “first speed step” for the decoder.
Speed curve or graph: (Mfr)
This is useful to make dissimilar loco’s run at similar speeds across their speed range. Adjust my setting Top volts with CV5, mid volts with CV6
Dither: (Mfr) (TCS)
This a very successful slow speed and smooth running adjustment found in TCS decoders. It allows adjustment of the width and size of a programmed pulse to encourage even “sticky” locos to run smoothly at slow speeds. It is adjusted by varying CV 56 and 57.
For a really easy to understand programming guide for dither, click on the “plain English programming guide”.
BEMF: (Mfr) (ESU) (Lenz)
A sophisticated form of slow speed and “cruise control”, BEMF acts by sensing the load on the motor and automatically adjusting decoder response to keep a consistent loco performance. BEMF is at its best in decoders which allow adjustment of the BEMF response.
We highly recommend the ESU LOkPilot and Lenz Gold Series as excellent examples of high quality BEMF application.
Momentum: (Std)
This allows the decoder to simulate the “mass” and momentum of the prototype – it is a sort of very flexibly applied “electronic flywheel” effect. Adjust the way a loco starts gradually (Acceleration) with CV3. Adjust the way a locomotive slows and stops (braking) with CV6.
Speed tables: (Std)
These are created by adding specific values to a whole series of CV’s, generally CV67 to CV94. They are laborious to make manually, and if you intend to implement programmed speed tables we highly recommend that you read our section on “Automation and computers” and take a look at the “decoder Pro” project at http://jmri.sourceforge.net/
Kick Start: (Mfr)
This gives a “burst” of power to get sticky loco’s moving. It is often over-ridden by other manufacturer specific functions such as “dither” or BEMF. For TCS users, this is usually adjusted via CV58 and CV65. Other brand users please check your documents.
Lighting Control: (Mfr) (Std)
Some aspects of light control are quote standard, others are Mfr dependent. Generally the more sophisticated decoders offer an almost unlimited range of options. Don’t be afraid to play with these functions – some spectacularly good results are possible!
Lighting control is usually programmed using CV49~51, 63, 59 and 60. However do check decoder by decoder. There are a LOT of options here – read carefully before doing anything, take it step by step and you WILL be delighted with the results!
Dimming: (Mfr)
Dimming via a decoder usually means a rapid on-off cycle, which “dims” the lamp (LED or Incandescent) by turning it on and off for a greater or lesser proportion of time. Not all decoders do this equally well… some result in a clear “flicker” effect. We highly recommend the TCS “X” series decoders as having by far the best dimming ability of ANY decoder we’ve used.
TCS adjust this via several CV’s depending on the “effect” being utilized.
