2. Common questions
4. Power Supply
5. Cabs & throttle controls
6. Command station
9. Accessory decoders
10. Mobile Decoders
11. Computer Control
12. DCC Terms
What is DCC?
Digital Command Control (DCC) is a scheme for controlling locomotives on a model railroad layout that allows more than one locomotive to occupy the same electrical section of track. Each of the locomotives has a decoder between the track and the motor, an electronic circuit that reads messages sent over the track and controls the amount of electricity delivered to the motor. A command station/booster places both the power and the messages on the track using a scheme where the actual power modulation encodes the digital bits of the messages. Devices that are fixed in one spot, such as switches and lights, can also be controlled by DCC.
Several other digital control systems have been developed, but DCC is most common. It was originally developed by Lenz and was adopted by the National Model Railroad Association (NMRA), a very influential group in model railroading.
DCC essentially uses alternating current or bipolar DC to provide power and transmit data, while traditional systems use direct current for power. However, the DCC current does not follow a sine wave. Instead, the control system quickly switches the direction of the DC current, resulting in a square wave. The length of time that the current is flowing in each direction provides a method for encoding data.
A short, mid packet, example of a DCC signal and its encoded bit stream.
To represent one, the time is short (nominally 58µs for a half cycle). A zero is represented by a longer period (nominally 100µs for a half cycle).In a segment of DCC-powered track, it is possible to power a single analog model locomotive by itself or in addition to DCC-aware engines through a method known as zero stretching. In this scheme, zero bits on the track can be extended to create a net effect where current appears to the engine to be flowing in one direction or another. However, because the power is actually closer to AC, many DC motors heat up much more quickly than they ordinarily would on an analog segment, and some motor types can be seriously damaged with only a brief encounter with DCC track.
The DCC protocol is the subject of two standards published by the NMRA: S-9.1 specifies the electrical standard, and S-9.2 specifies the communications standard. Several recommended practices documents are also made available.
DC or DCC?
The key advantage of using DCC over traditional analog systems is the elimination of electrically isolated blocks of track to allow the operation of multiple locomotives on the same layout.
What is the NMRA?
The National Model Railroad
Association, or NMRA, is a non-profit organization for those involved in the hobby or business of model railroading. It was founded in the United States in 1935, and is now active in Canada, Australia, Great Britain, and the Netherlands. It was previously headquartered in Indianapolis, Indiana, and is now based in Chattanooga, Tennessee.
The best-known activity of the NMRA is the defining of standards, and advisory documents known as Recommended Practices (RP), for model railroad equipment. Many standards defined by the NMRA are widely followed by the industry and modellers, including their HO scale track and wheel standards (S-3, S-4) and related RPs (RP2, 8, 10-15, and 25) and their Digital Command Control (DCC) standard set (S-9.1, S-9.2, and RP9.1.1 through RP9.3.2). The RP25 wheel design in particular has been critical in ensuring the reliability and interoperability of US-prototype model railroad equipment, since practically every manufacturer of such equipment uses wheels conforming to the RP. This is in marked contrast to the British market, in which there is no accepted standard among manufacturers. In Europe, the NEM standard is widely followed, but this is generally further away from prototype accuracy than NMRA standards, and the NEM wheel design is particularly so.
The NMRA also considers the education of model railroaders and encouraging learning and improving modellers' skills to be part of its mission. The NMRA runs an achievement program to encourage these skills, and runs modelling contests.
Promotion of the model railroading hobby is also part of the NMRA's purpose.
The NMRA is home of the A.C. Kalmbach Memorial Library, which houses railroad-related books and videos, was recently designated as the official railroad library of the state of Tennessee. The library is adjacent to the Norfolk Southern Railway Georgia Division and the Tennessee Valley Railroad Museum. It is named for Al C. Kalmbach, founder of Kalmbach Publishing, whose original magazines are Model Railroader and
website [source: wikipedia]
You only need two wires to use
DCC. True and false. The DCC BUS uses two wires to connect to the
track. These wires (red and black) can be simply connected to a piece of
track and your trains will run, but in reality, it is best to use a DCC
BUS that follows your track and 'feeds' it with power every few feet. You
then have separate power feeds for stationary decoders; Feedback BUS;
Booster connectors. See What is a BUS for more info. It
can basically be as simple or as complicated as you make it.
You need DCC Safe points to run on a DCC
layout. This is nothing special. Point work has to be well done and
connected logically to the layout. Shorts can occur at the frog with
incorrect polarity so it is best to use live frog points, isolate the frog
from the tracks leading out of the points and then switch the polarity of
the frog either with the point blades, but even better, using an accessory
switch connected to the point motor. The frog should have the same
polarity as the inside rail of the exit track that the point blades are
set to. Always feed the current for the point from the toe of the point or
the single track leading into the point.
What about my DC blocks, will I have to
get rid of them? Well you could leave the DC blocks in place and turn
them all on. A revering loop where a track comes back on itself and one
track ends up becoming the other has to be wired with a reversing loop
module otherwise it will cause a short. These will be covered in another
part of this FAQ.
I will have to
convert my whole fleet of locos to DCC... I have hundreds what will I do?
Well in reality do you run every loco on your layout. Most people end up
running a handful of well running, favourite and popular locos. If you do
run all your locos, you can always leave a line on the layout isolated
from the DCC circuit and controlled by DC. If you use some nifty switches,
you could share the line between DC and DCC.
2. Common questions
What do I need to run DCC?
A DCC Command centre (power and
DCC signal - a few years ago these were separate)
A DCC controller
You can add
extra boosters to increase capacity; You can add extra controllers to
allow more people to run trains or to have specific controllers assigned
to specific trains; You can add DCC stationary decoders to operate points
and signals; You can add DCC reversing loop modules to manage reversing
loops and turntables.
How much does DCC cost?
DCC is as cheap as the cheapest system on
the market or as expensive as the most expensive system with all the bells
and whistles. You can pick up a good second hand system on eBay for under
£50 or you can by a new top of the range system for a couple of thousand
What DCC system should I get?
If you are comfortable with the concept
of DCC, if you like technical challenges and if you can afford it, get the
best system that you can afford. If on the other hand you are unsure about
DCC, get a simple system that won't break the bank and that will allow you
to grasp the concepts of DCC. Always try and get a system that can be
expanded by adding other modules or components. Roco Multimaus, Lenz
Compact, Hornby Select and Bachmann EZ Command are all simple systems that
allow to some degree expansion.
What wiring is required?
Two wires to connect the DCC command
system to the track. See section below for other wiring requirements. Make
sure that you BUS wires are rated to carry at least 20 amps at 20 volts.
What is a BUS?
In the DCC system architecture, a BUS
is a subsystem that transfers data or power between
DCC components. Unlike a point-to-point connection, a BUS
can logically connect several peripherals over the same set of wires. Each
BUS defines its set of connectors to physically plug devices, cards or cables together.
Early computer BUSes
were literally parallel electrical BUSes
with multiple connections, but the term is now used for any physical arrangement that provides the same logical functionality as a parallel electrical
On our DCC layout, we have a few
1. DCC Signal /
Power BUS - this is what powers the track locomotives and sends the DCC
signal to the decoders.
2. DCC Command BUS - the network of controllers that control the locos.
3. DCC Feedback BUS - a network of feedback modules that allow you to
monitor the location of your locos.
4. DCC Booster BUS - a means to increase the power on your layout by
connecting multiple power boosters.
DCC systems use different BUS architecture. The image below shows the
XpressNET BUS and power feed architecture. LocoNET, used by Digitrax and
Fleischmann DCC systems uses a different BUS that combines the Command
BUS, Booster BUS and Feedback BUS.
4. Power Supply
How much power do I need?
Your power depends on your current draw.
Locos draw power, points and signals - if fed from the track power - also
draw power. Coach lighting is perhaps the worst culprit. A loco can draw
anything from 200mA to 1A depending on the type of motor and load on the
engine. Some locos draw even more and require special decoders. Most loco
decoders handle up to about 1A.
if a loco draws say 300mA and you have 4 locos running at once, your total
current draw is 1.2A.
A DCC system
will supply 3 to 4 amps on average to the track. Some supply 5A or even
10A. The more amps, the more locos you can run and the more coach and loco
lights can be used.
Even if the DCC
BUS is used to send the switching signals to the point decoders, point
motor mechanisms can usually be powered by a separate power supply. Street
and house lighting too should be used with a separate power supply.
Is DCC dangerous?
If you stick your tongue on the track you
will get a shock - don't do that though as it could send a shock to your
brain. At about 14 to 16 volts and at 3 to 5 amps, the track voltage won't
kill you. It will tingle your fingers if you press down on the track -
don't do that either...
A short on
the track could cause a problem and unless your system has adequate short
protection then it could cause a fire. All modern DCC systems have short
circuit protection and you can buy add-on short circuit breakers to
protect your equipment. Keep metal objects away from your track, keep the
track clean and dust free. Turn off the power to the DCC units when you
are not using the track. Keep an eye on kids when they are using the
How do I get more power?
Power comes from the Power booster in the
command station. This is usually about 3 to 5 amps. If you need more
power, some systems allow you to piggy-back a booster (another power
station) to the first giving more amps to your track. Most systems though
allow you to add a booster to a separate section of your layout that is
isolated from the first section. So if the first section has 5 or 6 locos
on it, the second section with it's own booster can run another 5 or 6
locos on it's own power.
connect the boosters to the main command station using a Booster BUS. This
keeps the same DCC signal constant around the whole layout. You can also
set the boosters to shut down power to the whole layout if there is a
short circuit in the section fed by the booster or alternatively, it can
shut down power only in the section covered by the booster.
5. Cabs & throttle controls
What is a Cab?
Fixed or walk- around cabs?
6. Command station
What is a
DCC command station?
How can I expand my DCC system?
I need more power and need to expand,
What are power districts?
What is feedback?
What is bi-directional layout control?
What is Railcom?
9. Accessory decoders
What is an accessory decoder?
What accessory decoder do I need?
How do I wire an accessory decoder?
10. Mobile Decoders
What decoders are available?
How do I select a decoder for my loco?
How do I install decoders in my loco?
Please see the Model Rail Forum
Reviews section. Most of the locos that have been reviewed have
had decoders fitted and there are plenty of photos explaining the process.
Back to Basics article talks about the main issues that arise when
fitting a decoder.
What about Sound decoders?
11. Computer Control
What are the advantages
of computer control?
Some people use a PC connected to their
layout and DCC system to control points, manage routing, follow the
trains around the track (if feedback circuits are installed) and manage
the locos and rolling stock.
You can have a very complex layout and
the PC can help you simplify it. You can also automate certain parts of
the layout and run trains by themselves leaving you free to do something
on another part of the layout. You can also configure your DCC decoders
using the PC. This is especially useful when mapping speed tables.
See the section below for some available
How do I link my layout
to my PC?
You need a DCC system that has either a
USB or serial connection designed to connect to a computer. Most PC
connection kits come with basic software allowing you to control the basic
functions of a loco and to configure a DCC decoder. 3rd party
software is available that enables you to control everything.
Computer control systems:
12. DCC Terms
What are MU's (Multiple Units or
MU of course stand for multiple
unit or consist.
systems offer three choices for consist control:
Basic Consisting method is to reprogram all the locomotives in a
consist to the same address and run them on one throttle. In this case all
the locomotives must be headed in the same direction, head to tail, head
to tail, head to tail.
Consisting stores the consist information in each decoder that
supports this feature. The locomotives can be added to and deleted from
the consist in any orientation head to head or tail to tail.
stores the consist information in the command station and
allows you to consist locomotives with any DCC decoder as well as an
analog locomotive in any orientation.
is a CV?
CV is short for
Configuration Variable. These are the decoder's settings that can be
'programmed' by the user to determine how the locomotive behaves and how
the various functions of the decoder perform. Once a CV is adjusted it is
remembered even when power is removed. A few examples are the address:
CV 1 - short and CV 17 &
18 - long address - Locomotive address
CV 2 - Minimum Speed (minimum voltage at step 1 of the speed controller)
CV 3 - Acceleration delay (Inertia)
CV 4 - Brake Delay (momentum)
CV 5 - Max speed (maximum voltage at the last step of the speed controller)
CV 6 - Acceleration Curve (voltage at the middle step of the speed
All decoders have the basic
operating CVs and other CVs that may be unique to that decoder. Refer to
decoder's instruction manual for all associated Configuration Variables.
Adjusting the CVs is usually
done by giving a decimal value between 0 and 255 for most CVs, see manual.
The manufacturer has entered
default values for each CV that enables the decoder to run a loco without
any adjustment (programming). To address this decoder you must select
"3" the default value for CV 1, the short address. The decoder
can be reset to default values. For independent running of locos, all
locos decoders must have a different address so a bit of 'programming' is