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DCC: Digital Command Control For Model Railways

While it has been around for some years now, Digital Command Control or DCC is now becoming increasingly popular as more and more manufacturers incorporate it into their new models, along with various accessories such as complete locomotive sound systems.

By Leo Simpson

SO WHAT IS DCC? Well, at one time it stood for “Digital Compact Cassette” but the march of technology has consigned that to a technical curiosity. For model railways, DCC is a “packet switching” system whereby multiple locomotives on a model railway layout can be simultaneously controlled. Each locomotive has its own digital address and its speed, direction and a bunch of other parameters such as inbuilt sound and lighting can all be adjusted remotely.

If you are familiar with the Ethernet protocol, one of the original “packet switching” systems, you are well on the way to understanding how DCC works. Of course, a major difference between an Ethernet system and a DCC model railway system is that Ethernet signals are transmitted over Cat.5 cable while DCC signals are broadcast over the rails in the model railway layout. But we’re getting ahead of ourselves. Let’s backtrack a little.

Originally, it was only possible to run one locomotive on a model railway layout. You connected a variable DC power supply to the rails and you varied the track voltage to control the speed of the loco. This is the way it’s been done ever since electric model locomotives became available, back in the 1930s.

On early model railways, the speed controllers were really quite crude but with the availability of silicon power transistors from the 1960s onward, model railway speed controllers greatly improved, offering much more realistic operation with simulated inertia (also known as “momentum”) and braking. In the late 1970s and early 1980s, the advent of switchmode and pulse-width modulation enabled very realistic low-speed operation of locomotives. The pulsed track voltage was better able to overcome track/wheel contact resistance and motor “stiction”.

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A selection of digital decoders which are designed to fit inside model locomotives. Each has a unique address to “pick off” its own packets of data while ignoring all the other packets. If the locomotive includes sound, it will have a sound decoder as well. In addition, the decoder uses the track voltage to produce a PWM waveform to drive the locomotive.

As well, these electronic controllers were able to monitor the back-EMF voltage from the locomotive motor and thereby provide very good speed regulation, regardless of the load or track gradient. SILICON CHIP has described a number of very good speed controllers incorporating all these features and more.

But as good as these electronic speed controllers are, there is still the limitation that you can only control one locomotive or train at a time. That might be satisfactory if you only have a small circle of track but it rapidly palls if your modelling is more ambitious.

Inevitably, all railway modellers have many locomotives and they want to run more than one at the same time. Of course, you can run two locomotives if you have two track loops on the one layout board but immediately you want to connect those two loops in any way, you run into serious problems.

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Another typical digital decoder, shown here slightly larger than life-size. The decoders are designed to fit inside the model locomotive but can also be hidden inside the tender in the case of steam locos.

On larger layouts, to make operation more realistic, enthusiasts took to dividing them up into blocks (or “cabs” in US parlance), each with a separate speed controller, so you could have an operating locomotive in each block. That meant you could have trains running in different directions on a large layout, as well as shunting operations and so on.

However, that method still only allows one locomotive to operate on the tracks within a block. So if you want to run more locos, you need more blocks and more speed controllers. That rapidly becomes expensive and the necessary wiring and switching to all those blocks becomes very complex and a nightmare when you have to troubleshoot faults.

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