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.
Booster
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.
Cab/Throttle
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.
Decoders
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.
Functions
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.
Capacitors
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.
Installation
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.
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