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Model railways in solihull
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DCC –Digital Command Control
It is a control system that enables several locomotives to be independently controlled on the same section of track, along with locomotive lights, sounds, couplings, carriage lighting & End of Train lights.

DCC will allow for point motors & signals to be controlled from a hand held controller.

How does it work?
Who cares! Do you really want to know about digital “Packets” & their transmission? All systems of NMRA DCC consist of a Power Supply Unit; a Command Station; a Booster; one or more Cabs/Throttles & Decoders (mobile or stationary). These may all be separate items, or they may be combined into one single unit (excluding decoders, of course).

Power Supply Unit
This is the transformer that you plug into your wall socket. There are higher current demands placed on a DCC layout compared to a conventional layout, so transformers have an output capable of carrying up to 10Amps.

On a large or complex layout it will be necessary to add more power to the layout. Boosters perform this task.

Command Station
The brains behind the whole system. This is a mini computer that communicates between you & all the decoders on your layout, ensuring that all decoders obey your instructions.

This is your controller. It will probably look more like a calculator than the type of model railway controller you may be familiar with. This is because it will enable you to control much more than just basic speed & direction.

These fall into two main groups, Mobile & Stationary. Mobile decoders are as you may have guessed, decoders fitted into locomotives & rolling stock. These will control speed; direction of travel; lighting effects & sound etc. Stationary decoders live on the layout itself & control items such as points, signals & level crossing barriers/gates.

A Function is an accessory output on a decoder. It is switched on or off by pressing the appropriate function number on a cab/throttle. Their use is limited by two factors, electrical current & your imagination. Generally, functions are use used for lighting effects. All decoders will have a range of lighting effects “built in”, many of which will only be used by modellers of overseas railways. As a general guide each function will have about 100mA of current available for use. A 12V grain of rice bulb will consume about 80mA whilst a LED will consume about 20mA. All mobile decoders will have between one and six functions available for you use.

Most cabs/throttles are capable of controlling thirteen different functions. So, what are the remainder used for? The answer is sound effects. Sound decoders offer a range of locomotive “noises” such as; steam/diesel whistle/horn; steam hiss/beat or diesel power unit; break squeal & couplings etc. This is in addition to lighting effects. F0 is universally used as a master light switch to control the head/tail lights. When switched “on” the appropriate lights will switch on or off automatically depending on the direction of travel.

Speed Steps
With a conventional DC speed controller you have an infinitely variable number of “speed positions” between “Off” & “Max”. With DCC this is not the case. Initially there were 14 “speed positions” (Speed Steps), which gave crude, but adequate speed control. Developments then allowed for 28 Speed Steps. Decoders now “believe” they have 128 Speed Steps. They still only have 28, but are being “conned” into behaving as if they have 128 Speed Steps. This allows for very fine control of the motor speed.

Speed Tables
For the ultimate in realistic control. Do you know the operating characteristics of your favourite prototype locomotive? If the answer is “Yes” then why not replicate it in model form using a “Speed Table”? For example, the American General Electric Dash 9 locomotive has a low amount of power applied in “notch 1”. Further low increments in power are applied in notches 2 & 3. However, huge amounts of power are applied in notches 4, 5 & 6 with further smaller increments available in notches 7 & 8. This prototype “speed curve” would be very easy to replicate using a speed table.

DC Working
Most DCC decoders will also function on conventional DC. However, they do not like “feedback” type controllers & will not start until at least 5 volts is applied to the track.

A lot of lies & untruths have appeared in the monthly comics concerning this topic. Capacitors are provided in model locomotives to reduce interference with radio & television equipment to within acceptable limits. Unfortunately, capacitors & the DCC signal do not get along too well! At best, capacitors will have a minimal effect & in a worst case scenario the capacitor could short-out the signal altogether. Manufacturers of electrical goods must take reasonable steps to prevent their goods from interfering with radio & television. It is NOT illegal for you to remove these items. However, if the goods then interfered with neighbours’ radio or television, you could be prosecuted.

All recent European (& possibly non European) manufactured mobile decoders will have built in circuitry to reduce radio/TV interference. It really is good practice to remove all capacitors from your locomotive, even if this involves a major strip-down. It is also wise to de-solder these items & store them in a safe place, rather than to simply snip them from the circuit board. If you ever wish to sell that locomotive you do NOT have to include radio/TV interference suppression because the model is now “second hand”, but it would be courteous to reinstate these devices.

Multiple Working – (Also termed as “MU-ing or “Consisting”)
There are three methods to achieve this. The first & most obvious method is to give all the required decoders the same address number. Another method supported by some (but not all) Command Stations is “Universal Consisting”. This method involves “telling” the command station that decoder “A” is working in multiple with decoder “B” (& decoder “C” etc). The main disadvantage with Universal Consisting is that the information is stored only in your Command Station, not the decoders. So, for example if you were to take the MU’d models onto a friends’ layout, you will have to set-up the MU on their Commend Station. Finally, recent decoders now enable you to give a decoder a “sub-address”. All decoders that have the same “sub-address” will work together in multiple.

The DCC publicity machine has scored an own goal here. On all but the simplest of layouts, it is a bit more involved than just taking two wires to the track. As previously stated, DCC uses more electrical current compared with conventional DC. So, the first consideration will need to address the size of cables to be used. If your layout is wired using “Telephone wire” then you are in trouble! Ideally, conductors will need to be between 1.5mm & 4mm to be able to carry the necessary current.

If you have any reversing loops or “Wye” junctions then you will need to install special commercially available modules that will automatically reverse track polarity & prevent the electrical problems associated with these features.

A “problem” may or may not be highlighted by your Pointwork. Overseas modellers are fortunate in that the all important relationship between rail & wheel is fully governed & adhered to by a set of recognised standards.

Unfortunately, here in the UK we are not so lucky. Anything that is remotely circular in shape is deemed fit by the manufactures to serve as a wheel & whether or not it will remain on the rail is down to good fortune.

If the switchrails & closure rails are of opposite polarity to the adjacent stockrails, then this will cause a short circuit if it is touched by a passing wheel-flange. With conventional DC control brief short circuits such as these will have no effect & will probably pass unnoticed by the operator. However, with DCC brief short circuits could be fatal to the electronics, so the command station will shut-down until the short circuit has passed.

The cause of this problem lies solely with an offending wheelset & not in any way with the DCC system. The answer of course, is to make sure that your wheels conform to a recognised standard.

Alternatively, arrange for the switchrails & closure rails to be of the same polarity as the adjacent stock rails. There is plenty of published material that explains how to perform this task, so I do not intend to cover that here.

If your layout is an “oval” of any form, then it will be a good idea at some point to electrically isolate both rails in each & every “oval”. The DCC signal will travel in both directions around your “oval” & will eventually “meet”. Due to the time taken to travel the signals may not be in “synch” with each other & theoretically could cause “confusion” to the mobile decoders. Isolating both rails will prevent this problem.

Certain suppliers will try to sell you reels of copper tape to use as a “BUS”. Their intentions are honourable, but how do you turn copper tape around corners? Stay well away from copper tape. Instead use your two main feed cables to form a “BUS” along the route of your layout. It is a good idea to solder “droppers” from the track to the “BUS” about every three or four feet. It is also a good idea to identify the “BUS” with purple tape. This is the BS standard colour for Data Cables.

Installation onto an existing layout should not cause too many headaches. You will have already done the hard work. Leave all your section switches in the “ON” position & arrange for all sidings & headshunts to be permanently “ON” as well.

High-frequency track cleaning & coach lighting units are a definite “no-no” & must be removed prior to installation of DCC.

Decoder Installation
Obviously it would be an impossible task to provide installation instructions for every single model locomotive that has ever been produced. However a few rules & guidelines may be helpful. If there are mechanical or electrical issues with your locomotive, then resolve them prior to installing a decoder. DCC will not compensate for a poor running locomotive.

Before you start to install your decoder, some thought will need to be given to the location of the decoder & the routing of the cables. Make sure that the decoder cannot come into contact with any metallic surface. It is a wise decision to apply electrical insulation tape to the area where the decoder will be housed. The decoder can be secured in place with a sticky foam pad. Also make sure that all cables are safely secured away from moving parts (gears & worm wheels etc).

Most recent model releases now have the NMRA eight pin decoder socket fitted. Remove the dummy plug & insert your decoder plug making sure that pin1 (orange) is correctly orientated & away you go. It really is as simple as that.

If your locomotive does not have the NMRA eight pin socket fitted, then decoder installation will be a little bit more involved. The motor absolutely & without exception MUST be totally & completely electrically isolated from the track. How you achieve this is dependant on the model in question. It may be as simple as severing the pick-up wires, or as with some split frame chassis (especially in 2mm scale), it may involve removing large quantities of metal from the chassis.

“Red & Black to the Track. Orange & Gray –The Other Way” If you can remember this when you solder your wires together you won’t experience any problems. All NMRA DCC compatible decoders will have their cables colour coded in accordance with the relevant NMRA Recommended Practice. Don’t cover your soldered joints with insulation tape. Eventually, the adhesive will fail & the result could be disastrous for your decoder. Use heat-shrink tubing instead, & remember to thread your cable through the tubing prior to soldering!

Having now completed your decoder installation either via the NMRA eight pin socket or having “hard wired” it, before reinstating the body it is time to take the locomotive to the programming track to test you decoder (see next section).

Decoder Programming
Run to the hills! I’m going to mention that dreaded “CV” word shortly. ARRGHHH!!! This is where the fun really starts. There are several forms of decoder programming, but you’ll probably only ever need to use two of them & they are: “PAGED MODE” & “OPERATIONS MODE”.

Paged Mode
Or “Broadcast Mode” as it is also known. As the name implies any programming performed in Paged Mode will be “broadcast” & apply to all decoders on the layout. Therefore this method of programming is performed away from the main layout, on a designated programming track. This may be a separate length of track on your work bench, or could be a siding on the layout that can be switched via a Double Pole Double Throw switch to switch between Ops mode & Paged mode.

After each & every new decoder installation your first port of call must be to the programming track. Paged Mode programming uses low current to write to the decoder & read back CV (it’s that word again!) values. After successfully verifying that you are able to read & write values to the decoder you may exit Paged Mode programming & get on with the business of driving trains.

Operations Mode
Any programming performed in Operations Mode (or Ops Mode, for short) will apply only to the decoder being addressed. The advantage of this mode of programming is that it can be performed on your layout without interrupting the running of other trains.

Configuration Variables
Once again the DCC publicity machine has scored an own goal. I’m quite sure the inventors of DCC could have come up with a less intimidating phrase than “Configuration Variable”. From this point on the phrase “Configuration Variable” will be substituted with “CV”.

Imagine you have a cupboard, within that cupboard there are several shelves. On the first shelf you choose to store teabags, on the second shelf you store coffee & on the third shelf you store sugar. Should you so choose; you may store tea, coffee & sugar on the first shelf, chocolate biscuits on the second shelf & leave the third shelf empty, or any combination that you choose. In this very simplistic example the 3 shelves represent “Configuration Variables” & the tea, coffee, sugar & chocolate biscuits represent the “Values” that you store in a CV.

A CV is nothing more complicated than a memory location within the decoder. Each memory location will contain a numeric value. The value of the number stored in a specific CV will cause the decoder to behave in a certain manner. There are over one hundred CVs available for use, but not all are used on every decoder & you’ll probably only ever need to alter a few of them. The NMRA dictates the use of certain CVs to perform specific functions; manufacturers are free to use the remainder for whatever purpose they choose.

The main CVs that you’ll need to use are listed in the table below.

CV No Description
CV1 This where a “two digit” (1-127) decoder address is stored
CV2 Motor starting voltage (not used very often now)
CV3 Acceleration rate (momentum)
CV4 Braking rate (momentum)
CV5 Max Speed
CV17 “Four Digit” (128 & above) decoder address
CV18 “Four Digit” (128 & above) decoder address
CV19 Decoder sub-address. Used when creating multi-loco/unit “Consists”
CV29 Basic decoder set-up. Controls normal direction of travel; two or four digit decoder address; dual mode operation (DCC & DC) & other items.

If you have access to the internet, there are free to use CV calculators available. These will provide you with a list of CV values based on the information that you enter into the calculator. For those who really are unable to grasp programming there are computer software packages available to purchase that will perform the CV calculations & then programme the decoder for you.

How much? I’m not paying that! Regrettably this seems to be a very common & somewhat blinkered response. Take a good look at the technology of what you are purchasing. The Command Station & Decoders are actually mini computers. The Throttles/Cabs are far more sophisticated than their DC counterparts. Think about the operational potential that DCC can unleash; the ability to drive the trains as per the prototype. No more sudden emergency stops because you failed to energize a section switch; the ability to stable locomotives buffer to buffer; the ability to add a loco or two to a train & then just as easily remove them again (for example, when banking up an incline). Is the cost worth all this extra operational potential? Only you can decide that.

Which Manufacturer is “Best”?
Providing a manufacturer has made their product NMRA “compatible” then it comes down to a matter of personal choice. At track level, manufacturer “A” will work just as well as manufacturer “B”. Stay away from the very basic entry level units, such as the Bachmann product. These are aimed squarely at the junior market & you will “outgrow” them in minutes. I personally, would advise against the *ZTC 511. This unit is grossly overpriced, does not support Operations mode programming & there are compatibility issues with certain decoders.

Zimo decoders are widely regarded as being the “Rolls-Royce” of decoders. TCS decoders are proving to be very popular. The instructions are written in an easy to understand manner & include a table for recording CV values. However their biggest flaw is the lack of short-circuit protection. They rely solely on the Command Stations’ ability to shut-down quick enough to prevent damage to the decoder. I wouldn’t recommend the use of NCE decoders; they have a lower specification to TCS at a similar price.

Digitrax are a high specification decoder. However, the price is somewhat inflated if purchased from within the UK. The Lenz Silver decoders are excellent value for money. They have a very high specification & include short-circuit protection for both the motor & functions. However, they assume the user is fully conversant with the binary numbering system.

*The ZTC 511 has very recently had a software upgrade. These concerns may, or may not have been resolved.

Where to Purchase
Sadly it is unlikely that your friendly local model shop will stock DCC. The situation is improving & more model shops are starting to stock DCC products. The retailers that do stock DCC generally only stock one manufacturers’ product range.

Some traders at the larger model railway exhibitions are also now stocking DCC products. Alternatively, you can use mail order either from a magazine advert or via the internet. It should be noted that generally DCC is supplied without a Power Supply Unit. This may need to be sourced independently. If you are purchasing Digitrax mobile decoders, consider sourcing them from the USA. For example, the DH163 decoder retails in the UK for about £27. Due to the favourable exchange rate, it can be purchased from the USA & airmailed to your letterbox for around £20.

I hope this brief guide has proven useful. It isn’t my intention to “convert” anybody & it certainly is not my intention to preach either. However, it is my intention to dispel a few myths & to show that you don’t need a Masters Degree in Computer Science to successfully incorporate DCC on your layout. Remember that the authors of the articles that appear in the monthly comics are journalists. They over-complicate matters solely to fill column space (a bit like I’m doing here!). I f you can programme your video recorder, then you will take to DCC like a duck to water. This guide is based solely on my own experiences with using DCC as a total beginner.




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© Solihull Model Railway Circle 2000-2017. Whilst every care has been taken in the preparation of this website the publisher, Solihull Model Railway Circle, cannot be held responsible for the accuracy of the information contained in the website, nor for any consequence arising from such information. The articles included and the views expressed on this website are those of the writers and do not necessarily reflect the views of Solihull Model Railway Circle or its members or advisors. This website is intended to be a resource, but initially it is for promoting the Solihull Model Railway Circle.