Getting Started with DCC

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What is DCC?

Getting Started with DCC

A Starter System

Mobile Decoders

Command Buses

How DCC Systems Work

Making Your Own DCC Components

Decoder Installation

 

This page describes how to make a start with a Digital Command Control system. DCC is different from a conventional cab control setup. It is not necessarily better or worse: that is up to you to decide. This page identifies some of the differences, and some of the steps you will need to go through to have a DCC system up and running.

Choosing a System

There are several DCC systems available from manufacturers in each of the UK, Europe and the USA. These are all different from each other in terms of the features offered, their expansion capability and their price. The most significant differences are in the central control units; the decoders all offer similar facilities (their differences are largely physical).

It is possible to buy components from different manufacturers and use them together. However the user does need to beware that only certain parts of the system are guaranteed to be interoperable by the DCC standards. For example, the standards define a minimum set of command packets to mobile decoders; these decoders can be purchased from several manufacturers and these command packets should work. However, the standards do not guarantee that one controller will be able to program decoders from another manufacturer. Most of the compatibility problems are likely to involve older products: the newer ones stand a much better chance of working in a "mixed" system. In this respect we have an advantage here in the UK over the US: there is very little "old" technology available. However, as a guideline, purchase components from just one manufacturer until you are confident that you understand the issues involved.

Decide what you want from a DCC system. Do you need minimum up-front cost? Do you want a system which can do almost anything, and to which you will add addition interfaces (eg controllers for points, signals) as your system expands? Do you one day want to interface to a computer for automatic control of your layout?

Most DCC systems are, initially, confusing. Each system comprises of several components, some of which may all live within one box. To a newcomer, terminology such as Throttles, boosters, command stations, ops mode programmers, feedback bus etc can be confusing. Most manufacturers sell packages containing all you need to get started. Choose one of these and use it as a starting point.

Most users are likely to consider cost as an issue which affects selection. It is worth remembering that the total costs involved will be dependent on the size of locomotive fleet to be covered. For a fleet of 10-15 locomotives, the decoder cost could be equal to that of a high end controller system; for fleets larger than this the decoder cost will dominate. However, it is also the cost which is most easy to spread over a period of time.

Details of manufacturers and distributors

Most of the starter sets will require a low voltage AC or DC input: they generally don't have an AC mains input. (Since most are from the US, this may not be a bad thing here in the UK). It will be necessary to provide a suitable power feed. This may be a transformer (12-15V AC at 100VA) or a DC supply (12-15V DC at 5A continuous is likely to be OK). I have not tried it, but Maplin sell a power supply with 13.8V output at 5A continuous rating which appears suitable for around £30.

Wiring for DCC

The nature of the DCC system implies that a layout needs to be wired differently for full use to be made of it. In general, the wiring will be simpler than a conventional block cab control setup. However there are some specific issues which need to be borne in mind.

General

Most of the wiring issues are well covered elsewhere, and I don't intend to cover them here. For a good reference to the subject, read Allan Gartner's DCC page which covers all of the issues.

In outline, there are a few simple considerations. These are all based on the fact that a DCC system is potentially powering 5-10 locos from one wire, and therefore has a higher current rating. DCC wiring in general requires a lot fewer wires than conventional cab control, and requires less planning.

  1. Ensure that all wiring can withstand a current of 5A (which is the continuous output of most DCC boosters): this is a lot more than commonly used in model railways and demands different practices. Use fatter wire than normal: the 1mm2 wire sold in DIY shops for domestic lighting circuits (1 mm2 Twin and Earth, or T&E) works well. Strip the red and black wires from the sheath and use them.

  2. Instead of having a common rail and then several switched feeds to the other rail (as would be used for cab control), all track should be permanently powered. This goes against the grain but since DCC can control each locomotive individually, there is no need to isolate them (as would normally be done for example in sidings).
  3. Provide a wired feed to the track every second track section (i.e. each piece of track is fed through at most one pair of fishplates). The usual arrangement for this is to run a pair of wires (e.g. the red and black wires from the T&E) under the baseboard following the track plan. Solder connections to the rails at appropriate points, and connect them to this pair of wires under the board. (Use junction blocks, or simply remove a segment of insulation and solder to them).
  4. Ensure that all pointwork is properly fed and that there are no "high resistance" paths involved. (See Allan Gartner's page for specific details).

Programming Track

One section of track needs to be provided where locomotives can have their decoders programmed. Many programming operations are now possible on the "main" line; but setting decoder addresses or reading back register contents will still need a programming track. This is a piece of track (commonly a siding of some kind) which can be switched between the "main" power feed and the special programming track power feed. It should have a DPDT switch to select its power feed between these two sources.

If your command system doesn't have a separate programming track feed, it may use its normal output feed for both purposes. If this is the case it needs to be possible to isolate the rest of the layout so that when programming only the programming track is powered.

Reverse Loops

A reverse loop is a piece of track which runs away from the main line and than back onto it through a circle. It is well known that these loops if not isolated from the main line will short the two rails together. A conventionally wired reverse loop will have both tracks "gapped" at either end to isolate it; it is then separately fed. In conventional cab control, it is normal to drive a loco onto a reverse loop, stop it, switch the polarity of the feed, and drive the loco off the other end in a "reverse" direction. With DCC there is another way.

DCC track has no "polarity" because the signal is essentially AC. The reverse section still needs to be isolated and can be handled in three ways:

  1. Feed the reverse loop from a switch or relay, commonly controlled by the points accessing into the loop. The loco will not need to be stopped on the loop but may hesitate if the switch or relay doesn't change over quickly. This is a low cost approach.

  2. Feed the reverse loop from its own booster which handles auto reversing. This is a process where the decoder knows that it is feeding a reverse loop, and will try to clear a short circuit by swapping its output polarity. In this arrangement, when the loco is driven onto the reverse loop the reverse loop power feed will be either the same as the main track, or will be reversed. If it is reversed a short will occur. The booster will immediately swap the polarity of its output signal, correcting the short. The loco will not be affected by this and will continue. At the other end of the loop the same issue will arise. If the short occurred on the way in, nothing will now happen; if nothing happened on the way in the short will happen now.
  3. A third "middle ground" is to use a DCC accessory known as a reverse loop isolator or similar. This is a unit which connects between the booster and the reverse loop. It includes a DPDT relay to swap over the feed to the reverse loop, and a current measuring circuit. When it measures a high current draw, it swaps the sense of the relay to reverse the loop's power feed. These units are a lot cheaper than a new booster.

Accessory Decoders

Accessory decoders are special controllers for accessories such as points and signals. They allow, if you wish, these functions to be controlled through the DCC system. Most DCC controllers allow points etc. to be set by them. Some of the accessory decoders have switch inputs too, allowing a conventional local panel to be provided for point or signal control. The advantage of using DCC is that you can use both methods side-by-side and use the DCC controller or the local panel as appropriate.

Power Districts

A typical DCC booster will have an output current capacity of 5A. This will run 5-8 "OO" gauge locos simultaneously. By this I mean 5-8 locos moving at speed: there could be 50 other locos stationary on sidings etc. If your railway requires that more locos than this are moving simultaneously, then it will be necessary to break the layout into separate "power districts" each fed by its own booster. (All are still controlled by the same controller; it is only the power feed tot he track which is affected). Each district will be limited to 5-8 locos operating at once, but the whole layout can have many more. Locos will run across the gaps between districts without problems as long as the system has been wired correctly.

Installing Decoders

Each locomotive on a DCC layout will need a mobile decoder to be installed into it. The issues associated with installing decoders are covered on a separate page: Decoder Installation.

Decoder Programming

Once a decoder has been installed, it needs to be programmed. This is the process whereby the controller assigns specific information to it. Modern decoders allow a wealth of features to be selected. The list is likely to be confusing to the first-time user. In the first instance, the decoder address needs to be assigned. I suggest that this is all that needs to be programmed until you have a little confidence.

Decoders are addressed using either two or four digit numbers depending on which system has been chosen. The two digit system will allow up to 100 locomotives to be on the rails and ready for use: this is probably sufficient for most needs! Use the last two digits of the locomotive cab number as the decoder address.

Finally, one piece of advice: keep records! Unless all your locomotives have the same decoder, you will soon find that there are subtle differences between them. When you come to program the decoders (eg to set speed tables, acceleration etc) you will need to know which decoder is in which locomotive. Don't discard the programming information which comes with them.

Controlling Accessories

One of the advantages of DCC is that it can be used to control accessories as well as locomotives. This makes it possible, for example, to control points and signals through the same system as the locomotives. This is an option - you are free to control them by other means if you wish - but it may be very useful if your ultimate aim is some form of computer control.

Accessory decoders are available from several manufacturers (see the manufacturers' links page) and are generally programmable to allow a number of different functions to be controlled. For example, they can usually control both current-pulse motors (e.g. the Peco point motor) or continuously fed motors (e.g. the Tortoise motor). Some accessory decoders are programmable to allow external sensors to cause output events. The Digitrax DS54 allows this, as does CML Electronics Limited's DAC10

Using your Layout

At last - this was why we started this in the first place!

A DCC controller will generally allow several "throttles" ie hand-held controllers for one locomotive to be plugged into it. Each controller will have a way of choosing which locomotive it is controlling (by choosing its number), and will be able to set speed and direction. It will be able to control any accessories - e.g. lights - on the loco. It is usually possible to control more than one loco from a hand controller. This may involve setting one loco going, then "letting it go" by selecting a second or subsequent loco. To capture control of the first one again, simply select its number.

DCC operation is in a lot of respects more "prototypical" than cab control.

©CML Electronics Limited 2008
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