Digital Command Control (DCC) is a method of running your trains that doesn’t require the constant operation of block toggle switches for throttle and/or block selection as used in Direct Current (DC) systems. You can control all of your locomotives with one hand-held throttle.
A schematic of a DC system with just two throttles and four blocks is shown in Figure 1 below. Rotary switches are used in place of the DPDT (double pole, double throw) switches if there are more than two throttles. An additional DPDT/rotary switch is added for every additional block, with a corresponding increase in the complexity of the wiring.
A DCC system (Figure 2) is, in simple terms, one block that can theoretically have over 200 locomotives controlled by up to 120 throttles simultaneously. However, for practical purposes it’s a separate electrical block (called a power block) for about every 4 to 6 locomotives on the layout - necessary to evenly distribute the electrical power and prevent a thermal overload - and the number of throttles is dictated by the number of operators, or, to put it another way, the number of operators is dictated by the number of throttles.
Larger model railways, or small ones with a lot of locomotives will likely have more than one electrical block, each having its own power booster (but not command station – one is all you need). Each power block provides balanced voltage and amperage to the whole layout without creating an overload. This allows for the running of more locomotives at the same time as well as cutting off power to the specific block if an electrical short occurs, usually caused by a derailed locomotive or freight car, allowing the rest of the railway to keep running without interruption. If you look carefully at the Nottawasaga Model Railway's exhibtion layout you'll see that we have two power blocks and provision for 14 tethered ('hard wired') plus almost any number of wireless throttles.
Referring to Figure 2, you need a power supply, a command station, throttle(s) and, if your railway is quite large, one or more boosters.
Each locomotive contains a small electronic chip known as a decoder. The decoder receives instructions from the throttle, via the command station, rails and locomotive wheels. Every decoder has a unique address, which you choose. When you send a command from the throttle it is translated by the command station into a digital signal which consists of the decoder’s address, instructions on how fast to go, in which direction, which lights to turn on or off, etc. There are many other features that we’ll deal with later.
When you first buy a decoder or a locomotive with the decoder already installed, the default address is 03, but you can program in a new one, either two or four digits long. Most modellers use part or all of the locomotive’s body number , e.g. loco number 6022 could be programmed in as either 22, 60, 602 or 6022.
You can use a regular transformer to supply low-voltage AC to the Command Station and Booster. One that delivers 2 amps at 14-18 volts AC or DC will be quite adequate if you’re running no more than 3 or 4 HO scale locomotives at any one time, and 5 amps if up to about 10 locomotives. Some starter sets, notably the Digitrax ‘Zephyr’ and NCE 'Power Cab', come with a power supply. (see the Glossary for a definition of a starter set.)
The Command Station is the intelligence of the DCC system and is responsible for communication between the throttle and all decoders. The Command Station communicates with decoders by transmitting packets of data to them through the bus wires and rails.
These data packets contain the decoder address, the instructions (faster, slower, reverse etc.) and what is called error detection, which, to keep it simple, makes sure the signal is properly read by the decoder.
A typical Command Station can transmit about 200 packets of information a second, which means that if there are, say, 10 locomotives on the layout, each one will receive about 20 data packets every second - more than fast enough for locomotive control.
Beware: In the absence of any digital signal to the contrary, a locomotive will continue to obey the last instructions received. If, for example, you set the locomotive in reverse at 10 scale kilometres an hour with ditch lights flashing, the horn blowing and the bell chiming, it will continue to do so until you tell it otherwise, even if you un-plug or otherwise disconnect the throttle from the layout. Of course, this means you can also run two or even more locomotives at the same time from the same throttle, although you will find that one is usually enough.
The booster's job is to take the low-current data signals generated by the Command Station and boost them to a high current signal needed by the locomotives to operate their DC motors. The signal is transmitted at a frequency between 15kHz and 43kHz and is carried by the rails but, because of its much higher frequency, it and the power voltages (60Hz) ignore one another.
Many DCC systems have the primary booster built into the command station or throttle.
Additional boosters, running from their own power supply, are often installed to increase the amperage on a layout. Model railway locomotives can draw a lot of power, especially when first starting to move (something to do with Newton’s laws of motion), and more so if equipped with sound. If too much power is drawn at one time the power supply’s circuit breaker will activate, shutting down the layout. A second power supply, routed through a booster and electrically separated from the first power supply, will do much to eliminate this problem. However, a booster is normally only needed on large layouts with many locomotives running at the same time. Because the second power supply is electrically separate from the first, so is the digital signal. A booster must therefore not only provide power to its section of track but also the digital signal, which it boosts (hence the name) after receiving it from the Command Station via the communication cable. The communication cable is the only connection between the different power blocks on a layout. The schematic Figure 3 below shows the basic wiring.
A hand-held with the same basic controls as a DC system throttle, plus, usually, an alpha-numeric display. It also has controls for activating lights, various train related sounds, etc. Unless it’s a secondary ‘utility’ throttle, it is also used for programming the decoders.
The primary throttle display shows the address of the loco currently being controlled, the amount of throttle being applied, either as a percentage of full throttle or as one of up to 128 speed steps, and its direction; or, if in the "programming mode," the function being programmed and the value of that function. Utility throttles - usually those without an LED display - cannot be used for programming.
Some DCC throttles allow control of only one locomotive at a time while others allow control of two locos simultaneously. Your choice could be governed by whether you typically operate alone or with a group. You can have as many throttles plugged into your DCC system as you have people to operate them, and just about all manufacturers now have wireless throttles, which you’ll find with increasing frequency as more-and-more model railroaders convert to DCC.
Wireless and ‘tethered’ throttles can be used on the same railroad at the same time, provided they come from the same manufacturer. However, be aware that wireless throttles can be simplex or duplex and the wireless receiver must also be simplex or duplex. Duplex systems can both transmit and receive data to/from the decoder, whereas simplex can only transmit from the command station to the decoder. The two systems are not compatible with one another.
Decoders are miniature wonders in power and sophistication. Some decoders are designed to directly replace the light board in a locomotive, and these are the easiest to install. Some also have mini plugs attached that slot into mirror image plugs already in the locomotive. These are referred to as 'Plug & Play". Other decoders have wire leads and require soldering. Decoders designed for use in N and Z scale locos have been compacted to the smallest-possible package as there's very little spare room under small-scale hoods or tiny tenders; in these cases the locomotive or tender frames are often machined or otherwise ground down to provide space for the decoder.
The address for each locomotive is programmed into the decoder via the primary throttle. To do this, the locomotive is placed on a dedicated Programming Track which is connected to the Command Station. The Command Station writes the chosen address into the decoder; henceforth, the decoder will respond only to that address. When the decoder detects its address in a data packet it stores the direction, speed and function data while checking the "Error Detect Byte" to ensure the validity of the data; if the data is good, any requested changes will be made immediately (e.g., speed up, slow down, stop, change direction, turn on the headlight and so on). When the locomotive is on the programming track having its address changed it's also a good time to program in some "Configuration Variables" (CV’s), such as:
We use the term AC voltage to describe the power that runs DCC, but it is, in fact, something called Pulse-width modulation (PWM). This is a modulation technique used to encode a message into a pulsing signal. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors.
The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load.
In other words, the maximum voltage is applied to the locomotive motor for a percentage of the time. DC, on the other hand, provides a percentage of the maximum voltage all of the time
If, for example, we want the locomotive stopped, we apply max voltage 0% of the time (in DC, it's zero volts); if we want the locomotive to creep, DCC applies the maximum voltage but for, say, only 5% of the time (in DC, we apply 5% of max voltage); if we want the locomotive to move at approximately half of top speed DCC applies maximum voltage 50% of the time (in DC, it's 50% of max voltage all the time); for top speed, DCC applies maximum voltage all of the time (as does DC).
Figure 4 shows the DCC motor control waveforms graphically. The red line is the voltage that’s applied to the locomotive motor, the blue line is time and the black lines – solid and dotted - are voltage.
Caveat Coreless motors cannot be used without a special decoder. Without the iron to dissipate the heat created by the PWM system, a coreless motor will burn out.
Nearly all, if not all locomotive manufacturers now offer their products with DCC decoders already installed, but older locomotives require some expertise to retrofit. Many retailers or experienced model railroaders can install them for you if you don't have the confidence to tackle the job yourself. All decoder manufacturers offer plug-&-play decoders that can be swapped with the lighting board in many diesel locomotives so equipped.
Because the track and its associated wiring are transmitting not just power but also data, good quality track work and wiring are very important to reliable DCC operation. If your layout is made up of sectional track, you're going to need good soldering skills; to ensure good electrical continuity each section of track, including turnouts, should be soldered to the next or have drop wires to the bus.
"Will I need to rewire my DC layout?" If you already have 14ga bus wires running under your layout and have track feeders to both rails at least every 36 inches then you likely won’t need to do any major rewiring, other than to eliminate the block toggle switches. If your wiring doesn't meet these criteria you’ll need to re-do some of it.
If you're wiring a DCC layout from scratch, buy some 14ga (12ga for larger layouts) twisted conductor wire in two different colours. These will be the "bus wires" that connect to the command station's terminals, and booster(s) if installed, and run all around under the layout. For track feeders you can use solid or twisted copper wire (22ga is fine), soldered to the rails and bus wires every 36 inches. Keep the feeders as short as is practical to eliminate as much electrical resistance as possible.
The tracks ‘downstream’ of a turnout must be electrically isolated from the turnout. While it’s acceptable to isolate just the inside rails, good practice calls for both inside and outside rails to be done.
On a DC system we usually handled reverse loops, wyes and turntables with DPDT toggle switches (or relays) which reversed track polarity before the train exited the loop, wye or turntable. In DCC we use a ‘reversing module’ which will automatically reverse the polarity for you. The cost for a single reversing module is around $30 but modules that control four loops, wyes and/or turntables are available. Reversing modules from one manufacturer can often be installed on a railway where the DCC system is made by a different one.
If you choose, you can use a separate booster to power your reversing loop(s); many have "auto-reversing" capability which, whenever they detect a short across the rails, such as would occur when a locomotive bridges a rail gap between out-of-phase power blocks, automatically reverse the polarity of the track for you. A separate booster used as a reversing module is very useful where the 'tail' of a wye has many tracks and sidings
All DCC systems have advantages and disadvantages over one another, with a myriad of various features that you may or may not want. However, provided the system is NMRA (National Model Railroad Association) compliant the system you buy really depends on the maximum number of operators you’ll have running trains at any one time which in turn is directly related to the depth of your pockets.
Keep in mind that with the exception of decoders, components from one manufacturer rarely, if ever, are compatible with those from another. An NCE throttle will not work on a railway controlled by either a Lenz, Digitrax or MRC command station, and vice versa. However, nearly all components in any given system are compatible with other components from the same manufacturer. If, for example, you plan to use your throttle on a club layout as well as your own, and the club uses the Digitrax Super Chief, you can buy Digitrax’s Zephyr system and an additional UT-4 utility throttle and both can be connected to the club layout - the Zephyr command station as a 3 amp booster and the throttle as, well, another throttle.
However, you do NOT need to buy your decoders from the same manufacturer as your DCC system; just about all readily available decoders are NMRA compatible. But like a car, it’s the extras that add to the cost. Basic decoders control direction, speed and directional lights. Prepare to pay more if you want to separately control locomotive ditch lights, lights that emulate an open firebox on a steam locomotive, sound, momentum, Back EMF, etc.
You're not going to get into DCC for less than about Can$250, which will get you a starter set such as Digitrax’s ‘Zephyr’ or NCE's 'Power Cab'. You're going to need the following:
Hooking it all up is fairly simple; each manufacturer will provide the needed operational manuals and hookup instructions.
If not already installed, each locomotive will need a decoder and someone to install them if you don’t have the skills or confidence to tackle the job yourself .
It’s not for the Nottawasaga Model Railway to suggest which system you should buy, so we’ll limit ourselves to giving you the names/models of the various starter sets from the most commonly used manufacturers and their MSRP. You may find these starter sets at a lower cost from individual dealers and retailers.
Buy from a dealer that you feel comfortable with, and can give you knowledgeable support when you have questions or when problems arise.
Some excellent sources of information are the web sites of the manufacturers, some of whom are:
Tony's Train Exchange - www.tonystrains.com - is not a manufacturer, but their website contains a lot of information and tips about DCC.
Decoders start from around C$25 for something very basic which will control speed, direction and lights, to 'the-sky's-the-limit' (well, almost) for a high quality programmable sound decoder. If you want to try your hand at installing your own, start with a less expensive one. It can always be replaced with something better later.
ADDRESS - The address is used by the Command Station to communicate with a specific Decoder. It can be either 2- or 4-digits, depending on the system and selected mode of addressing, and is typically part of the locomotive's road number. Addresses are unique, except in the case of a consist.
AUTOMATIC REVERSING - Provided by a circuit which senses opposing polarities across a rail gap and reverses the polarity of (typically) the reversing section. Used with reversing loops, wyes and turntables.
BACK EMF - A decoder feature which enables the speed of the train to remain constant, whether on flat terrain, climbing or descending a grade.
BOOSTER- A device which takes the low-current signal from the Command Station and boosts it to the high-current signal needed by locomotives (and other accessories) to operate DC motors, etc. Boosters are connected directly to the rails of their associated "booster district." Also referred to as Power Stations or Power Boosters.
BOOSTER DISTRICT - A segment of track connected directly to its own booster, electrically isolated from any other booster district. Large layouts may segment mainline trackage due to the large current demands of large fleets of locomotives. On some layouts, the track is segmented via toggle switches to facilitate troubleshooting and may also be referred to as booster districts (although a slight misnomer).
BUS - A heavy-gauge wire used to distribute the command station and booster output around the layout. Each booster would have its own pair of bus wires. "Feeder wires" are used to attach the bus wires to the track.
COMMAND MODULE - The 'brain' of a DCC system. It receives information from the throttle, forms the appropriate DCC information packet and transmits this information to the track either directly or via the booster(s).
CONFIGURABLE VARIABLE (CV) - A set of numbers programmed into the decoder that control its behavior. These numbers remain stored until re-programmed. A decoder address is a configurable variable. For example, CVs allow different make/model locomotives to have their speed characteristics adjusted so that they travel at the same speed at any given throttle setting. There are over 1,000 possible different CV’s, although some are still reserved by the manufacturers and the NMRA for future use. In practice no more than about 50 are changed and many modellers only change the one for the decoder address.
CONSIST - A group of locomotives controlled as if they were one. Allows several locomotives to be connected together to pull a heavy train. Also known as ‘MU-ing’ (from multiple units). With basic consisting, the command station sends the same signals to each locomotive in the consist. In Advanced Consisting, decoder equipped locomotives respond to the address of the consist and their unique address, allowing the Command Station to control the consist with a single command.
DCC READY - A term used by locomotive manufacturers to indicate that either the light board can be replaced with a readily available decoder or a plug is installed into which a decoder can also be plugged.
DECODER - A small electronic circuit board which receives digital packets of information addressed to it in accordance with the NMRA standards. Mobile decoders are mounted inside locomotives and control motor, lights and sound.
FUNCTION BUTTONS - Buttons on a throttle that control locomotive accessories such as lighting and sound.
FUNCTION MAPPING - Changing a decoder’s CV to determine which function button controls which function output.
NMRA COMPLIANT - Indicates a manufacturer has made every attempt to ensure the equipment so labeled meets all applicable DCC standards. Does not imply NMRA (National Model Railroaders Association) conformance.
NMRA CONFORMANCE - The product was submitted to the NMRA C&I Testing and has received an NMRA Conformance Warrant, indicating that the product completely adheres to all NMRA DCC Standards and applicable Recommended Practices.
OPERATIONS (OPS) MODE PROGRAMMING - a.k.a. ‘Programming on the Main’. Changing the CV's of an individual locomotive while it’s on a track other than a programming track. Most, but not all decoders or DCC systems support this.
POWER BLOCK/DISTRICT - An area of a layout connected to its own booster, electrically isolated from other boosters or power districts.
PROGRAMMING TRACK - A short length of track electrically and digitally separate from the main track. It is used to program any CV, including the decoder's address. CVs may also be read back to the throttle from a decoder on the programming track.
PULSE WIDTH MODULATION (PWM) - Used to control the speed of a DC motor by applying pulses of a constant voltage of varying width. The wider the pulses, the faster the motor turns. This is the method of motor control used by DCC Decoders.
SERVICE MODE PROGRAMMING - A more sophisticated mode than Operations Mode Programming, allowing any CV (including addresses) to be programmed and CVs to be read back. Used in conjunction with a Programming Track.
STARTER SET - Not to be confused with an elementary train set, such as a child might get at Christmas and run on the floor. A DCC starter set consists of the basic components necessary to start running a DCC model railway, and can be as simple as a power supply, command station and booster, combined into one unit or, at the other end of the scale, three or four separate items, including an 8 amp booster and duplex wireless communication between throttle and locomotives. Most model railroaders will buy a starter set to suit their current and foreseeable future needs. Just about every starter set can have additional components added, provided they’re from the same manufacturer, and be upgraded from tethered (a.k.a. plug in throttles) to simplex or duplex wireless.
THROTTLE - Sometimes ‘Cab’ in the USA. The device used by the operator to send speed, direction and lighting information to the locomotive. It's usually handheld, facilitating walk-around operation. Larger layouts usually have a throttle for every operator running a train.