DCC basics

for the Sodumb & Gamorra lumber Co.

last updated: 24 febuary 2003

Digital Command Control as we know it today was developed in the early seventies by Lenz for Märklin. Originally designed for AC Lenz took the development forward and made his own system based on DC; Lenz Digital+. Some years ago Digital+ became the basis for NMRA DCC standards.

Concept

Let's skip the technical bull.

DCC is based on a (stationary) transmitter providing a signal that makes trains move and functions switch. Therefore there has to be a (mobile) receiver to perform the commands given by the transmitter: we call the receiver a decoder.

What does a locomotive need to move in a DCC environment?

A locomotive needs current to feed the motor and the decoder and it needs a signal to instruct the decoder what to do. To provide the loco with both, there has to be 1) a 'CPU' (the stationary transmitter, likewise in a computer it is the heart of the DCC system), 2) a 'booster' to boost up the signal and bring the power to the system, 3) a transformer to deliver the power for both the booster and CPU and 4) a decoder in the locomotive.

What does that decoder do?

Imagine a decoder as a remote trottle built in a locomotive. I's the voltage regulator for the motor; it picks up a constant 16V from the rails, together with a signal (that is your hand on the trottle's knob) and passes it through to the motor in a kind of regulated way. When you change directions on your trottle the CPU reacts by 'translating' this command that orders the decoder to change the polarity of the motor. The polarity on the rails stays always the same!

Does a decoder get confused with several locomotives on the same track?

Signals from the CPU are passed to every decoder on the track. A decoder only reacts to the signal that carries his unique decoder number.

Can I use brand A decoders with brand B CPU's?

As long as they both 'speak' the same language, yes! This language we call 'protocol' and is guarded by the NMRA. As long as manufacturers conform to these rules it doesn't matter which brand decoder with what brand CPU communicates; they will understand each other. To be shure watch for the NMRA conformance sign!

What Brand CPU should I choose?

Difficult questions you come up with! I have only some expierence with brands available in Europe. Mainly there are two directions: AC an DC (some new hybrid decoders can be used in both worlds). The AC world is based on a Motorola chipset and provided by Märklin. Do you have a Märklin layout then stick to the Märklin system or use a Hybrid solution like Uhlenbrock's Intellibox.

The DC world has the most manufacturers on the market and that makes the selection process more complex. So, before you start collecting information, you should ask yourself some questions:

1. How big is (or will be) my layout? (small, big, Club)
2. Is there any need to expand the system in the future?
3. Is NMRA compliance essential?
4. Is service available from the shop or manufacturer?
5. Can the CPU's software be upgraded in an easy way?
6. What kind of trottle do I like the most?

Here are my answers to your questions?

1. The answer to this question says something about the number of trottles, locomotives and turnouts the CPU has to control. Almost every manufacturer has a entry level system, a medium and a professional solution.
2. The answer depicts to the flexibility of the system. Do you need an entirely new CPU every time you expand or can it be expanded from, let's say entry level to professional, by adding components (like a real system)
3. Will you and your friends ride trains on the same layout? Will you buy the best or cheapest decoders regardless the brand? For club layouts I would say 'it matters!'
4. Do you have to bring your equipment for service to a local hobby shop or do you have to ship it to the manufacturer? Also, does the guy in the shop knows what he is talking about (often they really don't).
5. A CPU needs software to run. From time to time this software has to be updated (Lenz recently migrated to version 3) to take full advantage of the latest DCC features; can I download and install this software myself or do I have to bring or ship it to the manufacturer every time a new release is issued?
6. I find the most important selection criterium 'ease of use' of the trottle. This is the part of the system where everything comes together. Can I do all the controll I want with the trottle (using sound requires 6 - 8 functions!), is it user friendly, is it ergonomically shaped? Can I control more than one locomotive without re-programming? Lot's of questions that need to be answered. But there is something else to be regarded:

Front-end compatibility

On the cab side of the CPU, that is between your controller and the CPU every manufacturer can do as he pleases. This connection we call the Bus or network structure of the system. Be prepaired: you cannot use a Lenz controller on a Digitrax CPU and vice versa! So, start with evaluating trottles!

Based on my (poor) experience so far, I come up with this evaluation based on the high-end system of these brands.

p=poor, g=good, e=excellent, *=depends on local circumstances

I hope I will get some nasty comment on this one. My rating is based on technology, engineering, system concept and user friendlyness.