Glossary
DCC, in plain English.
Digital Command Control (DCC) uses digital instructions to operate model trains and railway accessories. These guides and the glossary explain everything you need to know.
Getting Started
Control and power
- DCC
- Digital Command Control. A system that sends digital instructions through the track while also supplying power to locomotives and other equipment.
- Command station
- The central unit that creates DCC instructions, manages addresses and sends commands to the track and compatible accessories.
- Controller or throttle
- A handset, touchscreen or app used to select locomotives and operate them. It sends commands to the command station.
- Power supply
- Supplies electrical power to the DCC system. It must meet the command station's requirements.
- Booster
- Supplies additional power to the track. Some command stations include a booster; larger layouts may use more than one.
- Power district
- A section of the layout supplied by its own booster or protected power output. A short circuit in one district can then be limited to that area.
- Circuit breaker
- A protective device that cuts power to a section if it detects a short circuit or overload.
- DCC packet
- A digital message sent through the rails. It contains an address and an instruction, such as a speed or function command.
- Short circuit
- An unintended electrical connection between the rails, often caused by a wheel, point or wiring fault. The command station or circuit breaker usually cuts power to protect the system.
Wiring and connections
- Track bus
- The two main wires that carry DCC power and commands around the layout. Feeders connect the bus to the rails at several places.
- Feeder
- A short wire connecting the track bus to the rails.
- Accessory bus
- Wiring used to connect accessory decoders and other layout equipment. It may carry DCC power or separate power, depending on the equipment.
- Cab bus or control bus
- The connection between the command station and throttles or other control devices. It carries control information, and some systems also use it to power wired throttles. Examples include LocoNet and XpressNet.
- Data bus
- A network that lets DCC equipment exchange commands or feedback. Depending on the system, it may connect the command station to throttles, a computer interface, accessory controls or feedback modules. The name and capabilities vary between manufacturers.
- Feedback module
- A device that reports information—such as an occupied track section or a changed switch—to the command station or computer.
- Programming track
- A separate, electrically isolated piece of track used to configure decoders. It allows the system to read settings from one locomotive at a time.
- Insulated rail joiner or isolating gap
- A plastic joiner or gap that electrically separates one section of rail from another.
Locomotive addresses and programming
- Decoder
- An electronic unit that receives DCC commands and controls a locomotive or accessory.
- Loco decoder
- A decoder fitted inside a locomotive. It controls the motor and may also operate lights and sound.
- Address
- The number used to identify a locomotive or accessory decoder. The command station includes the address in its instructions.
- Short address
- A locomotive address from 1 to 127, stored in CV1. Some controllers display short addresses as two digits, but the DCC range extends to 127.
- Long address
- An extended locomotive address. The DCC standard supports addresses up to 10,239, although some controllers or decoders support a smaller range.
- CV (Configuration Variable)
- A setting stored in a decoder. CVs can control settings such as the decoder's address, acceleration, top speed, lighting and sound.
- Programming
- Changing the settings stored in a decoder's CVs.
- POM (Programming on the Main)
- A way to change some decoder settings while a locomotive is on the main layout. Most systems cannot read CV values back using POM. If the wrong address is selected—or several locomotives share that address—you could change the wrong decoder or change several at once without being able to confirm the result.
- CV readback
- The command station asks a decoder to report one of its settings. On a programming track, the system normally reads the decoder's brief electrical response. If two or more locomotives are on the track, their responses can overlap, so the value may not read correctly. Use one locomotive at a time.
- Function
- A decoder-controlled feature, such as a headlight, cab light, horn or sound effect. Function buttons are often labelled F0, F1, F2 and so on.
- Consist
- Two or more locomotives set up to run together as one train.
- Back-EMF or load compensation
- A decoder feature that adjusts motor power to help maintain a steady speed as the load or gradient changes.
- Keep-alive or stay-alive
- A capacitor arrangement that helps a locomotive keep running briefly over small interruptions in electrical pickup.
- RailCom
- A feedback system that allows compatible decoders to send information back through the track, such as their address or other status.
Points, signals and accessories
- Accessory decoder
- Receives DCC commands and operates equipment such as point motors, signal lights, relays or uncouplers.
- Accessory address
- The address used to send commands to an accessory decoder, such as one operating points or signals.
- Point motor or turnout motor
- The device that physically moves a set of points. It may use a solenoid, motor or servo.
- Solenoid
- A coil of wire that becomes an electromagnet when electricity flows through it. The magnetic pull moves a metal part to operate the points or signal. Solenoids usually need a brief burst of power; leaving one powered can overheat it.
- Servo
- A motor that moves to a controlled position. Servos are often used to move semaphore arms or operate points.
- Relay
- An electrically operated switch that can turn another circuit on or off. A relay does not understand DCC commands by itself, so it needs to be operated by a DCC decoder or another control circuit.
- Signal aspect
- The indication shown by a signal, such as red, yellow or green. A semaphore signal shows its indication with a moving arm.
- Signal decoder
- A decoder designed to control signal lights or a signal mechanism. Some signal decoders can operate multiple signal heads and aspects.
- Frog
- The crossing point where rails meet within a set of points. The name is commonly linked to the frog-shaped part of a horse's hoof, though the precise origin of the railway term is uncertain.
- Frog polarity switch
- A switch or relay that changes the frog's electrical connection when the points move, helping prevent a short circuit.
Detection and automation
- Block
- A defined section of track used for train detection, signalling or automated control.
- Block detection or occupancy detection
- Equipment reports whether a section of track is occupied. A current-sensing detector detects the small amount of current drawn by a locomotive or illuminated vehicle in an electrically isolated section. Other systems use optical, infrared or magnetic sensors.
- Sensor
- A device that detects something, such as a train passing, a wheel or a change in current.
- Computer interface
- Hardware or software that lets a computer communicate with the DCC command station so that software can control trains or receive layout information.
- Automation software
- A computer program that can use commands and feedback to run trains, set routes, manage timetables or operate signals automatically.
- Route
- A set of points and signals configured to provide a particular path through the layout.
- Interlocking
- A system that prevents conflicting routes or signal instructions from being set at the same time.
- DCCext
- An extension to DCC accessory control supported by some systems. It can provide a way to select signal aspects using a single address and aspect value, rather than combining several conventional accessory commands.
Standards and network protocols
- NMRA
- The National Model Railroad Association. Its standards define many aspects of DCC equipment and compatibility.
- LocoNet
- A control and feedback network used by some DCC systems to connect throttles, computers, feedback devices and other equipment.
- XpressNet
- A control network used by some DCC systems to connect throttles and compatible equipment.
- CBUS
- A model railway control and feedback system used to connect layout equipment.
- CAN bus
- A type of data network used by some model railway systems, including CBUS.