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DCC

NMRA Digital Command Control Glossary of Terms

This Technical Note is under review by the NMRA DCC Working Group

 

Introduction

The NMRA Standards and Recommended Practices for Digital Command Control (DCC) contain many new terms and concepts for the average modeler. This glossary defines those terms which are not commonly used, and defines commonly used terms in relation to DCC. This Document is still under review, and has not been finalized.

Glossary

0 bit
Digital signal representing the OFF state; used to make up DCC packets.
1 bit
Digital signal representing the ON state; used to make up DCC packets.
Accessory Digital Decoder
A Decoder which provides power and operation control of one or more layout accessory devices according to input from this NMRA standard. Accessories typically include non-train devices such as turntables, cranes, animation devices, etc.
Address Data Byte
The first data byte of the packet normally containing eight bits of address information.
Address Partition
A range of primary addresses for different types of digital decoders.
Advanced Acknowledgment
A method used by a digital decoder to reply to commands from a digital command station in the presence of other digital decoders.
All-in-One Command Station
A single unit that incorporates the Cab, Command Station and Track Power Station in an integral unit.
Baseline Packet Format
A minimal format that provides the minimum inter-operability between different systems.
Basic Acknowledgment
A method used by one digital decoder to reply to commands from a Digital Command Station.
Booster
Abbreviation for Power Booster (or Power Station).
Broadcast Packets
A special packet format which all digital decoders can act upon regardless of their address.
Bus
A set of wires that distributes power, commands, or signals around the layout
Cab
A unit which presents to a human operator the controls necessary to operate a model locomotive, track switches, or layout accessories. Such controls may include both input devices (knobs, buttons, switches)and output devices (lamps, displays, audible sounding units).
Cab Bus
A Bus used to connect all types of Cabs, except Wireless Cabs, to a Command Station. Wireless Cabs are indirectly connected to a Cab Bus via their companion Wireless Cab Base.
Command Station
A self-contained unit that accepts layout control inputs, generates the appropriate DCC packets, and outputs them in an NMRA DCC compliant signal to the power station.
Computer Command Station
A computer running software that outputs an NMRA compliant signal to the Power Station.
Computerized Cab
A Cab which is implemented by a personal computer or workstation.
Configuration Variable
Parameter stored it a digital decoder used during operation.
Consist
A group of digital decoders that have been linked together; for example, several locomotives MUd together would form a consist.
Consist Address
The address data byte all digital decoders in a consist will respond to.
Consist Control Instructions
A command subset used for consists and digital decoder control.
Control Bus
A Bus which connects a Command Station to its Power Stations.
Data Byte
Each data byte contains eight bits of information used for address, instruction, data, or error detection purposes.
Data Byte Start Bit
A bit that precedes a data byte and has the value of "0".
Decoder Control Instructions
A command subset used to modify the behavior (or configuration) of a digital decoder.
Decoder Slave
A unit which is driven by a Decoder and which provides additional power for the operation of additional electrical load in parallel with the original Decoder load. Typical use is for 2nd, 3rd, etc. motors in a multi-motor locomotive or permanently-coupled locomotives.
Digital Command Control (DCC)
Method of controlling multiple trains and accessories using digital communications packets to send commands.
Digital Command Station
Device(s) that supply DCC packets to the track.
Digital Command Control System
System made up of a Command Station, 1 or more Cabs, 1 or more Power Stations, and 1 or more Decoders.
Digital Decoder
A unit which accepts address and command information presented in the digital packet format from a Command Station and directly controls and drives a motor, solenoid, relay, lamp, or other device.
Digital Decoder Idle Packet
A special packet format used for timing purposes.
Digital Decoder Reset Packet
A special packet format used to reset digital decoders to their initial power-on state.
Direct Addressing
A method of accessing all of the configuration variables. Each configuration variable is specified by its number.
Dynamic Configuration Variables
Configuration Variables that are changed many times during regular operation of a digital decoder.
Error Detection Data Byte
Byte of data used to check the validity of all byte in the packet.
Extended Packet Format
A complex packet format that support different types of decoders, additional functions, addresses and speeds.
Fixed Cab
A Cab which is permanently mounted in a specific location.
Function Group One Instructions
A command subset used to control headlights and up to four extra accessories.
Function Group Two Instructions
A command subset used to control more accessories that allowed in Function Group One Instructions.
Instruction Packets
The regular packet format used for most commands.
Locomotive Digital Decoder
A Decoder which implements the specific functions to drive the motor, lamps, and accessories on a locomotive in accordance with the NMRA Standards.
Mobile Digital Decoder
A Decoder which is installed upon, and controls, the operation of a locomotive or other piece of rolling stock; the Mobile Decoder derives its signal and power from contact to the rails.
Operations Mode
Mode used to operate trains on a layout; opposite of Service Mode.
Operations Mode Acknowledgment
A method used by a digital decoder to reply to commands from a Digital Command Station during operations mode programming.
Operations Mode Programming
Mode of programming digital decoders while operating them on the layout with other digital decoders.
Packet
A sequence of bits is used to encode one of a set of instructions that the Digital Decoder operates upon.
Packet End Bit
A bit that marks the termination of the packet and has a value of "1".
Packet Start Bit
A bit with a value of "0" that follows a valid preamble. The Packet Start Bit terminates the preamble and indicates that the next bits are an address data byte.
Paged Addressing
A method of accessing all of the configuration variables, four variables at a time. Each set of four variables is called a page.
Power Booster
See Power Station.
Power Conversion
A mode of operation for digital decoders where DCC packets are ignored and an alternate method of communication of speed, direction, and control is used.
Power Station
A unit which accepts as input a signal from a Command Station and provides as output a power source onto which is encoded a digital packet signal in conformance with NMRA Standard S-9.1.
Preamble
The preamble to a packet is a sequence of a minimum of ten bits each of which has the value of "1".
Programmer
Unit which provides access to the system configuration functions, such as setting locomotive addresses, combining and breaking up Multiple Unit lashups, etc. May be integrated with the Command Station, or with one or more Cabs.
Reset Instructions
A command subset used to return a digital decoder to a known state.
Service Mode
Mode of programming used to customize the operating characteristics of a digital decoder.
Slave Decoder
see Decoder Slave.
Software Command Station
A Command Station implemented by software processes in a computer, typically a personal computer or Workstation.
Speed and Direction Instructions
A command subset used for speed and direction control of a locomotive, or other device.
Speed Table
A table of parameters that specify the motor voltage in response to a speed and direction command.
Stand-Alone Programmer
Unit which provides access to the system configuration functions, such as setting locomotive addresses and other configuration information independent of a layout
Stretched 0 bit
Digital signal representing the OFF state; specifically used to modify the overall effect of a DCC packet upon a locomotive without a Digital Decoder.
Tower Cab
A Fixed Cab which is primarily used for operating turnouts, signals, and related track accessories. A Tower Cab may, but typically does not, also control locomotives. Tower Panels typically include display devices for indicating track switch positions, track occupancy, and layout status. A CTC board is an example of a Tower Cab.
Tower Panel
see Tower Cab.
Track Bus
A Bus which connects a Power Station to a Track Segment. The aggregate length of the Track Segment is limited such that the Power Station can power the maximum number of locomotives to be simultaneously operated on the segment. Note that Stationary and Accessory Decoders may be connected to a Track Bus. The wire gauge of a Track Bus must be appropriate to the rating of the associated Power Station.
Track Segment
The portion of electrically-isolated track powered and controlled by a single Track Power Station.
Turnout Digital Decoder
A Decoder which provides power and position control for one or more turnout motors in accordance with the NMRA Standards.
Walk-around Cab
A Cab which is hand-held and connected by a cable to a plug which may be inserted in jacks at multiple points around the layout. Walk-around Cabs permit the locomotive(s) being controlled by that Cab to maintain direction and speed during the interval while the Cab is unplugged, moved to a new location, and re-plugged. This is because packet transmission is done in the command station.
Wireless Cab
A hand-held Cab which has no cable connection to the layout. Wireless Cabs may use infra-red, radio, or other means of communicating information.
Wireless Cab Base
A companion receptor unit for one or more Wireless Cabs which is fixed to the layout and presents cab information to the Cab Bus.