DCC

Digital Command Control

 

Part 1..... Getting started with DCC
 

The purpose of this page is to help anyone interested in starting up with DCC either with a new purpose built layout or converting an existing analogue one.

Many people consider DCC to be a "black art" that needs a knowledge of electronics and technical wizardry to be able to get to grips with. Nothing could be further from the truth and a great deal of "Bunkum" is expounded on the subject that tends to frighten people away. Hopefully I will be able to convince you that it is not a subject to be feared and can be put to use by anyone capable of putting a layout together.

What is it and how does it work.

 

DCC or Digital Command Control to give it its full title is a method of controlling your layout by sending signals along the track that can be picked up by a locomotive and make that locomotive do just what you want it to. With a standard analogue layout you would control the locomotive by altering the direction and strength of a Direct Current applied to the track to make the the locomotive speed up, slow down or change direction. With DCC there is always a full Alternating Current applied to the track and superimposed on this Alternating Current is another signal that is read by a decoder (chip) placed inside the locomotive.
This "chip" inside the loco is always listening to the signals on the track waiting for one meant especially for that loco and when it recognizes an instruction addressed to itself then it follows the instructions sent by the controller to tell it to speed up, slow down, change direction or indeed other things such as turning lights on or off.
Since each locomotive on the track will only respond to an instruction addressed specifically to itself then it becomes possible to have several locomotives on the same piece of track and all can be independently controlled.

So what do you need to get started.

 

There are 4 basic parts to a DCC system.
1. A power supply that will deliver enough current to run several locomotives at the same time. Typically between 2 and 5 Amperes.
2. The "Master unit" which is the brains of the system and is basically a small computer that "encodes" the signals from your controller and superimposes them on the continuous AC fed to the track.
3. A control unit (often a hand held or walk-about unit) that may have just buttons or buttons and a knob that lets you control a loco.
4. A decoder (Chip) fitted inside each of your locos that you want to run on the DCC system.

There are several available proprietary units that you can purchase, each containing items 1 to 3 listed above. I am not going to try and recommend any particular brand and will only state that in most cases you get what you pay for. In general the more you pay the more facilities the unit you acquire will have.

OK so you have bought a DCC starter set and are ready to get going. If you have an existing layout that is controlled by analogue then all that you need do to make it work on DCC is simply to replace the DC analogue power supply and controller(s) with your new DCC unit. If you have section switches then just leave them all switched on and that's it, you can now control your chipped locomotives on your layout.
Sound much too simple doesn't it? Well there is just one small problem that will only become apparent if you have locos fitted with either sound or lights. Many layouts use the action of a point to isolate a loco by producing a dead section when the point is switched away from say a siding. If you have a loco that has lights or sound then when that section or siding becomes dead the lights will go out or the sound will stop.
Not a major problem but possibly not what you want to happen.

If you were building from scratch you would simply ensure that all of the track has power to it all of the time and then there would be no danger of having lights or sound going off other than when you you wanted it to happen by using the controller to actually switch them off.

Programing

 

Many people are put off by thinking that you have to a computer nerd to be able to program your locos but in reality all the work is done for you by the system itself. Most of the units you can buy make it really simple and it is just a matter of pushing a few buttons to set up your loco.
Typically you would want to arrange just a few of things for each loco on your system and these would be :-
1. The loco address, where you would use say the last 4 digits of the loco's running number if your system supports 4 digit numbering. If not then depending on which unit you bought you could be limited to 2 digits or numbers 1 to 9 with the cheapest system.
2. The start voltage that is applied to the loco when you turn the controller to its first step. (typical values here are from 1 to 5)
3. The acceleration rate. A bigger number here makes your loco start picking up speed more slowly and more realistically. (typical values are between 5 and 20)
4. The deceleration rate. Again a bigger number here makes your loco take longer to slow down even if you wind the speed down to zero. (typical values are between 5 and 20)
5. The maximum voltage. A smaller number here will reduce the maximum speed that your loco will reach at full throttle. (maximum value here depends on the make of the decoder) 

The only one of the above that is important is No 1 the loco address, as you will need each loco to have its own unique number. Your new chip or ready fitted loco will normally be factory set with an address of 3. All of the others can be left at the default settings that came on the decoder chip and your loco will still work. However you can get a more realistic and to my mind better control by setting the other variable to suit the particular loco.

Cost

 

How much will it cost to get started with DCC.
At the time of writing this (October 2010) you can get the basic starter set which will allow you to control 9 locos for about £50 and the fully fledged all singing all dancing unit that will allow you to address up to 9999 locos and configure prettywell anything on any loco for £200 with the ability to purchase extra throttles so that more than one person can run trains.
Loco decoders or chips that will suit most 00 locos can be had for as little as £9 each. (miniture ones where space is limited are more expensive at approx. £20) However you will need one chip for each locomotive.

Summary so far

 

So you have seen how easy it can be to convert an existing layout to DCC but why should you bother going to the trouble and expense of converting?
I will list the Pros and Cons I have found with DCC.

Pros

 

1. Very much smoother control of your locos especialy with slow running.
2. The ability to have several locos on the same section of track and have individual control of each.
3. There is no need for section switches to isolate locos.
4. If starting a new layout then wiring is greatly reduced.
5. The ability to have lights always on even when trains are stopped.
6. The availability of sound for your engines.
7. Modern locos now come "DCC ready" which means they have a socket inside that you just plug a decoder into. 

Cons

 

1. A momentory short circuit anywhere on the layout will bring everything to a halt.
2. Initial expense of "Chipping" your fleet.
3. Some older locos need to be "Hard Wired" to fit a decoder.

So thats probably as far as I need to go with the basics to give you an idea of what is involved, but if you are like me I was absolutely hooked once I had fitted a chip to a Standard Class 2 Ivatt and found how smoothly it ran.

Digital Command Control

 

Part 2....Wiring your layout
 

The purpose of this page is to help anyone starting a new layout with DCC.

This article will assume that you are using standard 00 gauge flexitrack and either Insulfrog or Electrofrog points to construct your system.

Which point system to choose.

 

You have the option of using either Insulfrog or electrofrog points for your new system or indeed a mixture of both. You will probably have heard talk of electrofrog points being "unfriendly" for use with DCC but that is just a rumour. You may have also seen articles showing that you need to solder wire straps from the switch blades to the running rails on insulfrog points or that you need to have auxiliary switches to change the frog polarity on electrofrog ones. You do not need either of these to make your layout work but they can aid electrical pickup to your locos by ensuring that you are not relying totally on the switch blades making good contact.
On my DCC layout "Livsey Lane" which uses Electrofrog points throughout (apart from the 2 double slips) I have not fitted auxilary switches and have had no problems with running at all.
With Insulfrog points fitting the wire straps from the switch blades to the running rails can actualy cause a problem if any of your stock has wide metal wheels. More about that later.
I personally would always choose Electrofrog points for a layout as there is no "dead" section at the frog so slow running is much better with far less chance of stalling on the point if running slow in sidings or on a shunting layout.


The rules for feeding power at points.
Power to the track should always be fed from the "Toe" end of the point which is the end of the point where switch blades make contact and never from the "Frog" end which is where the two tracks emerge.
With Electrofrog points you should always use insulating rail joiners on the two rails that come from the frog. The reason for this is because of the frog itself is permanently connected to both of these rails and whichever outlet from the point is not selected will have its two running rails connected together. This would short circuit the tracks beyond the point if the insulated joiner was not used.
Although that rule does not strictly apply to Insulfrog points it is a good idea to also use insulated joiners on the two frog rails here as well. As mentioned above older stock with wider wheels can cause the two rails at the exit to connect together momentarily causing a short circuit that will bring your whole system to a stop. For this same reason you should not solder the switch blades to the running rails if you have older stock with wide metal wheels.
See pictures and text below.

ifrog.jpg

The above is an Insulfrog point showing the rails A B C D and the places where you would strap the rails A and D to the switch blades to give more reliable running by removing the need for the switch blade to make good contact with the running rails.
Note also the "dead" section that exists where the switch rails meet the frog.
Also note the very small gap between rails B and C. With older rolling stock it is very easy for these two rails to come into contact with each other as the larger metal wheels pass over them resulting in the DCC system shutting down. Even fitting insulating joiners at B and C will not solve the problem but I recommend that they are fitted anyway and then if you do encounter the problem it is very easy to just snip out the straps which will then solve it.

efrog.jpg

Above is an Electrofrog point. the most obvious difference is that there is no "dead" section at the frog as the the switch rails are metal all the way with no gap. To obtain this condition however, it becomes necessary to have rails B and C permanently connected together. This is what causes most people to have problems when using this type of point, but if insulated joiners are always fitted at rails B and C then there is no danger of short circuits caused by power feeding back from other sections of the track beyond the point.
If problems are encoutered with continuity to the switch rails then you can always fit an auxilliary switch beneath the point motor that feeds power from tracks A or D to the frog. i.e. tracks B and C are fed from either A or D depending on which way the point is set.

Wiring the track
With DCC You only need two wires to carry power along the layout and supply current to every section of track. The normal way to do this is to run two "Bus Wires" along the underneath of the layout boards and then have shorter lengths of wire branching off this "Bus" to each place you need to supply power.
Many of the earlier documents on DCC talk about using Copper tape or very heavy duty wire to for this power Bus along the layout. This is overkill and unless you want to operate several points at the same time or lots of locos on the same section then something less hefty is sufficient.
An ideal and easily obtainable alternative is to get hold of or even purchase a metre or so of "Cooker Cable" and split this up into the individual copper wires that formed the cable.
Each of these strands will be hefty enough to form one of the wires of your "Bus" around the layout and as there is no isulation left on the wires once you have dismantled the cable you can solder your smaller track feed wires directly onto the heavier cooker cable strand.

bus.jpg

If your layout is a round and round one then also make the bus wires go all around the layout to complete the circle. This has the advantage if effectivley doubling the cable thickness as current can travel either way round the layout to reach a branch to the track and if there should be a break or high resistance joint anywhere it wont stop the current flow due to the alternative routing. Just like a household ring main.

With a layout that is in sections then a good system is to use the bolts that pull the sections together to serve as the connectors between the sections. On my layout "Livsey Lane" which is in 4 sections I use M6 set bolts and "T" nuts, (captive nuts that have spikes that hold them to the wood) see images below.
The end of the Cooker Wire strand can be trapped beneath the "T" nut on one side and secured behind a large washer on the other. This then gives an electrical connection when the bolt is inserted and tightened up.

Digital Command Control

 

Part 3....Wiring your layout
 

The next part will look at a typical section of track with a siding and where to put power feeds and electrical breaks.

track.jpg

It would be impossible to cater for every possible track combination that the modeller could use but above is an example of a typical cross over and a siding off a main line. The places where insulated fishplates are needed are shown by the red lines. Remember that above I stated that because of the problems that can occur with insulfrog points and wide wheeled stock that it is better to use them even though they are not strictly necessary for that type of point. With electrofrog points however they are essential.

The feeds to the 2 power "Bus lines" are indicated in green and blue and ensure that all track is fed all of the time. The positions of the feeds shown are not critical and can be located anywhere along the section of track fed by that feed. 



Points operation
It is perfectly feasible to have your points operated from your DCC controller as well as your locos by purchasing "Accessory Decoders" that will allow you to enter a number to select a point and then set its direction. You can even arrange for whole "Routes" to switch with a single number.
However if you have a lot of points and possible routes then life can get a bit complicated by having to remember all these "Accessory" numbers as well as the Loco numbers. I have found that a good compromise is to have your Locos controlled by DCC but still use a "Mimic diagram" (track plan) with push buttons that operate my points in the conventional way.

Hopefully the above has been of assistance to anyone thinking of building a new layout especially for DCC use. 

John Essex Heywood Model Railway Group