How to set up a DCC decoder

When talking with modellers about Digital Command Control (DCC), they tend to fall into two categories - some can’t get enough of it, looking forward to their next acquisition, but others won’t go near it, claiming it’s too much hassle and plagued with problems. Few people seem undecided.

Converting non-DCC users who have heard tales of puffs of smoke and expensive replacement bills isn’t easy, but you have to ask if the equipment was being used correctly in those cases. This article aims to dispel such myths and encourage you to embrace the prevailing direction of the hobby. It is ideal for newcomers to the technology and it's certainly not as difficult as you might think.

Installing a decoder

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Installing a decoder into this Bachmann Class 08 and setting it up is a five-minute job. The chassis is parted from the top by removing a single screw under the cab.

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At the front of the chassis is an eight-pin DCC socket. Before installing a decoder, remove the ‘blanking plug’ which links pins one, two and eight as a group. and four, five and six as another. It connects each motor terminal to a rail and without it, the locomotive won’t move on DC power.

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The only way to install a Zimo MX645 sound decoder in this compact locomotive is to remove part of the chassis and flywheel with a small hacksaw. Many locomotives have room for such a decoder, but this one hasn’t. It's a 'worst case' scenario, but at least you'll know what to do if you try the same.

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Avoid using glues when installing decoders as they’re messy around electronics. It's better to use masking tape - it’s much neater and doesn’t leave the same sticky residue as insulation tape. Double check that everything fits before going any further. Look out for wires that might pinch, as interference with moving parts and any non-insulated wires is likely to cause short circuits.

Ten rules for installation
1. Because of inconsistencies and changes in standards six, eight or 21-pin socket might be fitted to your model. Check which you have before buying a decoder.
2. Match the decoder power output to the amperage of your locomotive. What a locomotive consumes depends on its size, weight, age and the type of motor fitted.
3. Make sure all decoder wires are kept away from moving parts such as gears and wheels
4. Exposed or unused wires must be isolated to avoid short circuits
5. Keep your decoder away from motors if possible. Decoders like to be kept cool and motors are a source of heat in a confined space
6. If possible, install a suitable 'Stay Alive' capacitor. These reduce stuttering of models over poorly laid or dirty track and improve performance, especially with sound.
7. Decoder installation must be carried out as the final job when building a model. Paint it before installation.
8. Beware of using steel wool around a model fitted with a decoder. Small fragments can create short circuits in a decoder. Keep your workbench clean and free from debris by regularly vacuuming.
9. Make sure you have space for a decoder. If you fit a sound decoder, make sure you have room for the speaker and wires too.
10. Some decoders are surrounded by heat-shrink wrapping. It prevents stray wires from coming into
contact with the fragile circuits - don’t remove it.

Basic Decoder programming

If you’re new to, or considering DCC, you might wonder why you need to program each decoder. Put simply, it customises the running characteristics of each locomotive. Imagine a pigeon loft - this represents a decoder. Each hole changes a characteristic of the decoder, for example its top speed or time to reach its top speed. These holes are known in DCC language as ‘CVs’ or Configuration Variables.
Using the word ‘programming’ shouldn’t provoke an instant fear of computer illiteracy. DCC simplifies many things, but having to program each decoder isn’t one of them. Nevertheless, once you get the hang of it, you’ll be surprised at the difference it makes. 

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Many DCC systems are available and all perform differently when programing. In this case it's a Lenz Set 90. The programming track is connected to terminals ‘P’ and ‘Q’. It should be an electrically separate section, such as a siding.

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To access programming mode, the ‘Om’ button is pressed to get ‘Pro’ on screen. Continually pressing the button will cycle through the menu options.

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If the ‘A’ button is pressed to validate our choice, the command station will ask what we want to access - in this case CVs, noted ‘C’.

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Validating again with the ‘A’ button will ask which CV we would like to read. Reading a CV is like seeing what is inside the pigeon holes referred to earlier. Here, we have selected CV1 which stores the model's address or 'ID' if you like. This will identify it from other decoders. The Zimo MX645 decoder has 428 CVs, most of which we don’t need to touch.

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The Lenz system is good at informing the user what it is doing. When the command ‘read’ on a CV is given, it clearly notifies us on screen.

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The value of CV 1 is now returned, in this case its factory default value ‘3’. That means that this locomotives address is 3 - a default industry standard with DCC. The decoder hasn’t been touched, so all CVs contain their default settings. These are included in the instruction manual, no matter what your make of decoder.

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We need to change this CV because if we run two locomotives with CV1 set to address '3', they’ll both set off at the same time. This is done by putting a number - or value - different to 3 in the CV. Here we choose 8, since it’s an '08', but you can enter anything in this CV from 1-127. The values you can enter will be specified in your manual.

CVs to change

Below is a list of CVs for the Zimo MX645 and the values to which we change for the purpose of this tutorial. These are the most important aspects that should be changed. The result will see our Class 08 run at a realistic scale top speed, no matter how enthusiastic the operator. It’ll have its own address different from the default and will accelerate and decelerate better too.

Expert Tip - Choose a decoder with the right capacity

Fitting an underpowered decoder can result in premature failure. To find how much power a model consumes, place it on the rails against a solid object - such as a bufferstop - and force it to stall. Place an ammeter in series between one rail and a terminal on the DC transformer. This determines what the locomotive consumes in a 'worst case' scenario, should it stall or accidentally run into a bufferstop and its wheels spin. Turn up the power on the transformer slowly from a crawl to maximum speed, noting the maximum amount of current consumed. Add to this figure a ‘safe margin’ of around 20% and this is your locomotive current consumption. Look for a decoder that can handle this amperage on a ‘continuous’ cycle, not ‘peak’. The peak output of a decoder is the maximum current it can handle, but only for short periods of time before it overheats. Most accessories connected to decoders consume little current, especially modern LEDs - the margin of 20% will more than handle them. Beware if fitting a smoke unit, as the resistor heater element and fan motor - if fitted - will consume far more current than an LED. In this case, refer to the manufacturers’ manual for its amperage and add this to the sum total.

More advice

Still unsure and have more DCC-related questions? Ask your local model shop or retailer for advice. Plus, there's lots of DCC information, advice and chat on RMweb