DCC Glossary

DCC Glossary

Accessory decoder
A decoder that is not intended to be installed in a locomotive, but remains in a fixed location and controls accessories such as signals or track switches/turnouts. Also known as "stationary" decoder.

The numeric identification code by which a decoder recognizes commands directed specifically to it. It is also the identifier that a transponder broadcasts. The address is usually unique for each decoder, but this is not a requirement.

Address 00
The special address used to send speed and direction commands which the command station uses to operate conventional, non-decoder equipped locos. Conventional non-decoder equipped locos are sometimes called analog locos.

Advanced consisting
Operating and controlling several locomotives as if they were a single entity by sending speed and direction commands to a common address. Also called decoder assisted consisting. See also "Consist".

Aliasing or aliased addressing
The method used by a command station to run trains with a 2 or 4 digit addresses stored in the command station when the decoder has its own separate 2 digit address. This feature is not part of the Standard or RPs.

Alias roster
A list of alias addresses and their associated decoder addresses stored in a command station.

All live turnout
A turnout where throwing the turnout does not change the rail polarity. This type of turnout is also called a nonpower routing turnout.

Ampere (also Amp, A)
A measure of the amount of electrical current used or required by a device. This is a flow measurement.

The height of a wave func-tion.

A term used to describe conventional DC control where the loco responds to the magnitude of the track voltage.

Analog control
Conventional track voltage (NMRA Standard S-9) typically varying between zero and twelve volts for speed control and polarity reversal for direction control.

Analog mode conversion
It is how most DCC decoders can run on layouts that use conventional control.

Analog signals
Voltages and/or frequencies which convey information and are not digital.

Architecture (or system architecture)
Arrangement of components and/or the method used by a DCC or other computer based system for communication among the various components connected to the system. System architecture is determined by each individual DCC manufacturer.

Rotating frame which supports the field coils of a motor. In common usage the term applies to the entire rotating part of the motor.

A combination of lights or positions on a signal which has a defined meaning. This meaning is the signal's indication.

Automatic analog mode conversion
It is when the decoder handles this change automati-cally when there is no DCC signal present. Some decoders must be pro-grammed to address 00 for this to take effect. This feature is not part of the Standards.

Automatic polarity reversing
Control circuits which sense opposite polarities at rail gaps and automatically reverse the polarity of the rails to allow smooth continuation of the motive power. Applications include: reversing loops; wyes; and turntables.

Automatic reversing booster (ARB)
Booster connected to a reversing section that is configured to handle automatic reversing. ARBs are always used in conjunction with another booster connected to the remainder of the layout that is configured to run as a normal booster. Automatic reversing can also be implemented on DC layouts.

Automatic reversing device (ARD)
An electronic device which is connected between the power bus and a reversing section to perform automatic reversing.

Automatic train control
The process by which sensors, receivers and coded pulses sent through the track enforce the speed restrictions of signal indications in the prototype. (Sounds a lot like command control, doesn't it?)

Automatic train stop
The process by which a train is stopped automatically if it fails to obey a restrictive signal indication.

Some locomotive decoders can sense the rotational speed of the motor and automatically adjust future digital pulses to the motor to maintain a desired speed. Also called load compensating decoders. Back emf is the voltage developed by the spinning motor armature as it acts as a generator. EMF is short for electromotive force. Back emf measurements are used for speed stabilization.

The amount of information that can be transmitted between the command station and decoder(s) (or another communication link ) in a certain amount of time.

Basic consisting
Operating and controlling several locomotives as if they were a single entity by sending discrete speed and direction commands to each locomotive in the consist. See also "Consist".

Measurement of bits per second transmitted or received.

The base two number system. All binary numbers are described by the two digits, 0 and 1.

A wave function which goes from positive to negative and back. The DCC signal is a bipolar wave form.

Bipolar signal
The electrical waveform of digital packets transmitted along the rails is known as a hi-polar signal. Positive pulses followed by mirror image negative pulses are the key characterization.

A logical value, a binary digit, that can be either a one or a zero.

Booster is the electronic device that combines and amplifies the DCC commands generated by the command station with power from the power supply. The booster sends the DCC commands as electronic signals along with the track power to the decoders to deliver both power and DCC signals to the DCC devices on the layout. A DCC system may have more than one booster. Boosters are also sometimes called power boosters or power stations. Also known as "Power stations" or "Power Boosters"

Braking sections
Track segments where the power supply is set up so that DCC trains stop automatically.

Broadcast packet
A specially encoded digital packet that will be acted upon by all decoders that receive the packet. Commonly used in service mode programming and for stopping a locomotive in front of a red signal.

A set of wires that serves as a conduit for electrical signals and distributes them around the layout.

Is a group of eight bits.


A device used by model railroad engineers (operators) to control motive power and accessories by sending electrical or electronic instructions to the locomotive (via the DCC command station).

Cab bus
The bus used for cab-to-command station communication, and viceversa.

Carrier Sense Multiple Access with Collision Detection (CSMAICD)
A signal and message handling system used by networks such as Ethernet and LocoNet.

Cascaded route
Operation of one or more turnouts by a function cell when it sends a request for the command station to initiate the required action.

Network architecture used to manage communications among devices on a network. It usually uses event driven communications. LocoNet uses this type of architecture.

The state of a turnout or the decoder which controls it, where the routing is through the straight leg or set for the main line.

Command Station
The command station receives electrical signals (operator instructions) from the cab. The command station then creates NMRA DCC digital packets in accordance with NMRA specifications to achieve the desired results and transmits these packets to the power station(s).

A decoder status, used by Digitrax, which means that, although the decoder is active, it can be selected by any throttle.

Common rail wiring
Method of wiring conventional layouts. The track feeds for one rail are connected together to one output of the power pack. The other rail is gapped and the track feeds are connected to the power pack through block control switches.

The rotating contact on the armature which transfers power from the motor brushes to the field coils.

A claim made by a manufacturer that their product will generally work with other compatible devices in areas where both devices support a given function.

Same as conformance. A compliant product is one that has passed NMRA tests and earned an NMRA Conformance Warrant.

Configuration register
Configuration variable (CV) 29. The configuration register soft switches control some of the most basic aspects of decoder operation. These are normal direction of travel or NDOT, 14/28 or 128 speed steps, analog conversion on or off, speed table on or off and two or four digit addressing.

Configuration variable (CV)
Memory location in the decoder that contains information that controls the decoder's characteristics. A defined piece of information used by the decoder to adjust its operation. This information is permanently stored inside the decoder until the user wishes to change its value.

Products that have passed the NMRA's extensive testing procedures are eligible for a Conformance Warrant if the manufacturer also agrees to fix any discrepancies that might become apparent in the future. Conformance seal is awarded by NMRA for products passing the Conformance and Inspection program for particular NMRA Standards.

Conformance Warrant
An official document awarded by the NMRA to a manufacturer for a specific product that has demonstrated conformance to NMRA Standards and applicable Recommended Practices by virtue of passing all appropriate tests as performed by the NMRA.

Operating and controlling several locomotives as if they were a single entity. For example, several diesels might be connected together to provide more power for a steep grade. Also called multiple unit lashup, "MUing", multiunit consist, or lashup. There are three types of consisting: (1) Basic consisting is where all locomotive decoders in the lashup have the same address. (2) Advanced consisting is where the consist information is stored in CV19 in the decoder. (3) Universal consisting is where the consist information is stored in the command station.

Control Bus
The bus used for transmitting digital packets from the command stations to power station.

Control Digital Packets
A digital packet is a defined sequence of bits that instruct the decoder how to respond. See also bit and byte.

Conventional control (or analog or block control)
This method of model train control uses extensive wiring to control the power delivered to the locos through the rails. It is a system of running the track, not the trains.

The flow of electricity in a circuit.

Current Draw
The amount of electrical flow required by an operating device.

Daisy chain topology

Network wiring plan where each new device connects to the previous device and through the chain of devices to the controller.

Stands for Digital Command Control. One of several methods of controlling and/or operating a model railroad layout. The control information is provided in the form of a digital signal instead of a standard analog (DC or AC) power, overlaid with control information. NMRA DCC is a specific form of Digital Command Control specified by the NMRA as a non-proprietary international specification and is implemented by a significant number of manufacturers worldwide. On the most basic level, DCC encompasses systems and products that are interoperable with the basic NMRA DCC Standards and RPs. In addition, DCC includes other related technologies that are designed to enhance and extend the basic capabilities outlined by the NMRA.

Electronic device that receives the DCC signal from the com-mand station through the track, decodes it and tells the locomotive, turnout or other equipment, it is controlling, what to do. Decoders come in a variety of sizes and specifications. See also "Accessory decoder", "Locomotive decoder", "Mobile decoder', "Stationary decoder", and "Slave decoder".
- Mobile decoders are installed in locomotives to control their movement and, in some cases, other functions such as lights or sound.
- Function only decoders are installed in equipment that moves, but function only decoders do not control movement. Rather, they control other functions like lights, sound, smoke or animation.
- Stationary decoders control fixed equipment like turnouts, lights, signals, sound and other immobile animation devices. These are sometimes called accessory decoders.
- Terminology note on decoders:
- Sometimes decoders are referred to as throttles or receivers. This comes from carrier control terminology. DCC man-ufacturers use the term throttle for the handheld that to sends input commands to the system. The term receiver is not used because a decoder does more than simply receive a signal, it actually decodes the signal and determines what actions are needed.

Detection section
A section of track gapped on one or both rails and connected to an occupancy detector.

Digital Command
See 'DCC"

Direct home wiring
Method of wiring layouts where each power district and its booster is electrically isolated. The track within each power district may use common rail wiring for detection or power management.

Direct CV programming
A high performance form of service mode programming for manipulating the values of a decoder's CVs.

Direct programming
Form of service mode programming defined by the RPs.

Ditch lights
Lights mounted on a loco's pilot or low on the hood to illuminate each side of the track just in front of the loco. When the horn is sounded they flash alternately increasing the visibility of the loco, especially at grade crossings.

The slope of the graph of speed vs. load for a locomotive. This is one of the variables that is used in scaleable speed stabilization calculations by the decoder. Speed stabilization is used to manage the effects of load on loco speed.

Dynamic braking
Action of converting the mechanical energy and momentum of a moving train into electrical energy by using traction motors as generators. The electrical energy is dissipated as heat by arrays of resistors.


Electrically-Erasable Programmable Read-Only Memory. These computer memory devices are used to store data in a manner that is easily read, but that changes infrequently. Nonvolatile memory which is designed to be changed infrequently, and is used to hold the values programmed for the configuration variables that control the decoder's characteristics. Most decoders use EEPROM to store CV information.

Erasable Programmable Read-Only Memory. These computer memory devices are used to store data in a manner that is easily read, but can only be erased and reprogrammed with special tools.

Event driven
Refers to a strategy of allocating communications resources on a network by sending traffic only when network devices need to communicate.

E unit
Originally an electro-mechanical device which was responsi-ble for reversing locos using AC motors. The unit selects which field coils are used in the motor. Modem devices are usually solid state, but they are still called E units.

Exact feedback
Method of using a number of switches or sensors to deter-mine the exact state of a device.

Fast clock

A clock set to run faster than real time to allow for operating sessions on a model railroad to be run in compressed time. The ratio between fast time and real time is typically 4:1, 6:1 or 8:1.

The ability of a device to transmit information regarding its status back to the command station.

Forward trim
Scaling factor which is applied to all the speed step power values in a speed table for the forward direction of the loco.

Flashing Rear End Device, the light and logic box on the end of a modem train that replaces a manned caboose. Also known as end of train device (EOT or EOTD).

The number of wave function cycles per second.

Function cell
A group of electronic components within a stationary decoder which controls the logic for a pair of decoder inputs and outputs.

Function mapping
The ability to specify (i.e. map) which function buttons on a cab activate which specific decoder function outputs. This defines which decoder wires are active for each user input.

Function output
A decoder controlled switch that can be turned on and off by a user's cab action.

Handheld (cab)
A portable cab used by the model railroad engineer (operator) to control one or more locomotives. Simple handhelds may have speed and direction controls only. Specialized cabs may also control accessory functions. Fullfeature handheld cabs have further capabilities such as programming. See also "Cab".

Unit of frequency, cycles per second.

Base sixteen number system. The digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.

Railroad employee at an engine terminal who moves the engines from where they are stored to where the train crew picks them up. The hostler usually couples the locos to the outbound train.


Decoder status, used by Digitrax, when the decoder is not active and can be selected by any throttle.

Decoder status, used by Digitrax, when the decoder is active on a throttle and cannot be selected by another throttle.

Process by which the turntable bridge is aligned with selected approach or stall tracks.

The meaning given to a signal aspect.

One of the most important features to look for in a com-mand control system. This allows you to use your DCC compatible equipment with other DCC compatible equipment. Interoperability means that your DCC decoder made by DCC manufacturer A will work with a command station made by DCC manufacturer B.

Interrupt request
When computer's input or output device requests the computer to stop what it is doing and take care of that device's communication needs.
Ironless core motor
Small ultrahigh performance motor developed as a precision instrument or servo motor, but occasionally used in model railroad applications. These are sometimes called Micromotors. Supersonic decoders or ballast lamps must be used when installing DCC decoders with this type of motor to prevent damage to the motor.


Kilobits per second (also kbps) refers to the speed of data transmission on various buses. For example 2.2kbps transmission passes data at 2200 bits per second. See also bus and byte.
Kick start
An entry in a speed table to regulate a burst of power sent to a motor when the decoder first commands it to start (at speed step 1).


Local Area Network is a computer term referring to the interconnections by which various computers and devices communicate with each other in a geographically small location, as opposed to a Wide Area Network (WAN) like the Internet.

Light Emitting Diode, a solid state electronic device that con-veils electricity to light, without heat.

Local route
The operation of a number of function cells within a single stationary decoder and without intervention from the command station.

Peer-to-peer local area network (LAN) system architecture used by Digitrax to carry DCC and other commands across Digitrax command control systems.

Locomotive decoder
A decoder intended for controlling a locomotive (or other similar device). See also decoder, and mobile decoder.


A sequence of userdefinable commands, that is accomplished with a single button press.

Manual block system
Method of train control used by the prototype and run from block station to block station. Trains require explicit authority to enter each block. This is also used as the backstop method if an automated signaling system fails.

A network architecture where a central controller manages access and communications to remote devices. It usually uses polled communications.

Maximum voltage (or Vmax)
Defined in configuration variable 05. It limits the maximum voltage sent by the decoder to the motor, effectively limiting top speed.

A miniaturized, selfcontained, computer on a single chip. The computer's operating instructions are also stored in this selfcontained chip.

A miniaturized, selfcontained, computer on a single chip. The computer's operating instructions are not stored in this selfcontained chip, but instead are stored in an external device, usually an EPROM or PROM.

One millionth of a second. It is written as µsec.

Mid Point voltage (or Vmid)
Defined in configuration variable 06. It defines the power sent by the decoder to the motor at the middle speed step. This is step 7 of 14, step 15 of 28 or step 65 of 128.

One thousandth of an ampere. One thousand milliamperes (mA) equals one ampere. See also ampere.

One thousandth of a second. One thousand milliseconds (ms) occur every second.

Mobile decoder
A decoder that is designed be able to properly work if installed in a device that moves around the layout using rotating wheels on tracks. See also decoder.

Fundamental property of matter, the product of mass and velocity, which expresses an object's tendency to keep moving at its current speed and direction. Prototype trains have momentum.

Motorola trinary
Digital command control format used by Marklin AC digital HO and Marklin Maxi decoders. This format is different from the NMRA DCC Standards.

MU lashup
One way that the prototype refers to consists. This is a group of locomotives linked together by cables (MU cables) and controlled as one unit. MU means multi or multiple unit.

See "Consist".

Nested consist

A consist which is part of another consist.

Nested route
A route which is part of another route.

The National Model Railroad Association is an organization of volunteers that, among other things, created the NMRA/DCC Standards and Recommended Practices. Founded in 1935, one of its purposes is to define and manage model railroad Standards related to interchange of equipment in North America. For membership information, call +1-423-892-2846 or check their web page located at www.nmra.org.

A configuration of a stationary decoder function cell which requires it to complete its output action before it will accept new activa-tion input.

Normal direction of travel (NDOT)
The direction a decoder sees as forward when the throttle is set for forward motion. Some diesels run long hood forward, others short hood forward.

The most widely used Standard for N scale modular layouts.

Occupancy detector

A device which senses and provides feedback for the presence of a train or specially equipped cars on a section of track. Also called a block occupancy detector on conven-tional layouts. Not covered by the DCC Standards or RPs.

Unit of measurement for the electrical resistance of an electronic component or device. This is a "fric-tion" measurement. The kilo-ohm, or 1000 ohms, is more com-monly used. An ohm is a small unit, like a cent. Ohms are represented by the Greek letter Omega.

Operating current
The current draw in amps used by a loco, including its motor, lights and other accessories, under normal continuous operation at full load.

Operation (Ops) mode programming
Programming method where programming information is sent to a specific decoder on the layout instead of on the programming track. This method of programming decoders does not interfere with the operation or settings of other decoders on the same track. A programming track is not used, the information sent is directed to a specific address. Not all decoders accept ops mode programming. This is sometimes called mainline programming or address directed programming.


Packet is the organization of bits and bytes into complete DCC commands. It consists of preamble, address, instruc-tion and error detection information with bits to indicate the start and end of the components of the packet. The packet format is defined by the DCC Standards. See also digital packets.

Paged programming
A method used for programming of decoder CVs. It is a method of accessing the configuration variables, four variables at a time. Each set of four variables is called a page. See also Register Programming.

Network communica-tions scheme where messages between devices are not managed by a central controller or server. LocoNet uses event driven peer-to-peer communica-tions.

Physical Register Programming
Another form of service mode pro-gramming defined by the RPs.

The two directions of current flow, plus (+) and minus (-), or potential in an electrical circuit.

The process by which devices are interrogated sequentially, one after another in order, to see if they have information or commands to send to the system.

Positive feedback
Method of using a switch or sensor to determine one of the two possible states of a device.

Power Booster
Booster is the electronic device that combines and amplifies the DCC commands generated by the command station with power from the power supply. The booster sends the DCC commands as electronic signals along with the track power to the decoders to deliver both power and DCC signals to the DCC devices on the layout. A DCC system may have more than one booster. Boosters are also sometimes called power boosters or power stations. Also known as "Power stations" or "Boosters"

Power bus
Main wires that carry the power from the booster to provide power feeds to the power district.

Power district
The portion of a layout that is powered by a single power station. Power wiring, components and equipment attached to that wiring.

Power pack
A source of electrical power. Commercial power packs might also have controls for conventional analog (NMRA S-9) operation. See also transformer.

Power routing turnout
Turnout where only the route selected is live and the rail polarity changes when the turnout is thrown.

Power station
Booster is the electronic device that combines and amplifies the DCC commands generat-ed by the command station with power from the power supply. The booster sends the DCC commands as electronic signals along with the track power to the decoders to deliver both power and DCC signals to the DCC devices on the layout. A DCC system may have more than one booster. Boosters are also sometimes called power boosters or power stations. Also known as "Boosters" or "Power Boosters"

Power sub-district
Wiring, components and equipment that are con-trolled from both power bus wires by their own power management device, for example, a reversing section controlled by an automated reversing device.

Power supply
Transformer or power pack that provides electricity to the DCC system.

The action of setting the internal parameters of decoders and other control equipment. During programming, values are set for CVs to determine the personality of locomotives, stationary decoders and other programmable DCC devices.

Programming track
An isolated track section used for programming decoder equipped locomotives or transponder equipped rolling stock.

Programmable Read-Only Memory. A computer chip which can be programmed only once. The contents of this memory are non-volatile. Also OTPROM: One-Time PROM. These computer memory devices are used to store data in a manner that is easily read, but can only be written at the factory before or during assembly. Many decoder manufacturers use PROMs to store the machine code instructions used to run the decoder since it allows them to put the most up-to-date code into the decoder during production.

The definition of the "language" used between two devices. The agreed upon definitions of the packet's format and intended meaning is known as a protocol. The DCC protocol definition is contained in NMRA Standard S-9.2.

Pulse width modulation
The technique of controlling motor speed with voltage pulses of varying time duration (pulse width). The wider the pulse, the more power is provided to the motor, the faster the motor rotates. Also known as PWM.


The sequencing of items to be processed. A programming technique intended to insure that command stations transmit important digital packets first and less important packets later would be a priority queuing configuration. Use of priority queuing permits the bandwidth of a command station to be used most efficiently.

Recommended Practices (NMRA DCC RP)

A set of specifications that are only less mandatory than NMRA Standards by virtue of their slightly less critical subject matter. While the inclusion of features described by NMRA DCC Recommended Practices is optional in any given product, if a manufacturer chooses to include these feature(s) in a product, then the design must fully implement the feature as described in the pertinent RP in order to earn a Conformance Warrant.

This form of computer memory is used to store data in a manner that is easily read and written. Used in command stations and decoders to store information that frequently changes. This is volatile memory used as the working memory for the decoder.

Recommended practices (RP)
Established by the NMRA as an adjunct to the Standards. RPs are not mandatory but if a feature covered by an RP is implemented, it should follow the RP.

Electronic device which performs a similar function to a decoder for a carrier control system. They are called receivers because the early systems used different frequencies for each channel.

An electronic device which converts a bipolar alternating current (AC) into direct current (DC).

Register programming
A basic method for accessing the eight most basic decoder CVs. See also paged programming, direct CV programming and operations mode programming.

Repeaters (power station)
This device cleans up the DCC signal timing and provides power to drive additional power stations.

Resistor wheel set
Set of model railroad wheels where the two metal wheels are not completely insulated from each other. The wheels are connected by a fairly large resistor, which allows a little current to flow. These wheel sets are made to trigger detection sections.

One possible configu-ration of a stationary decoder function cell which allows it to accept new acti-vation input and commence a new action before it completes the current output action in progress.

Reverse trim
Scaling factor which is applied to all of the speed step power values in a speed table for a loco in the reverse direction.

Reversing feature
Track geometry which allows a locomotive to enter and exit on the same rails with the same direction of motion. Examples are reversing loops, wyes and turntables. A loco enters traveling forward and leaves on the same rails still traveling forward. This geometry creates a polarity mismatch at one or the other end of the reversing section that must be corrected for the loco to continue moving no matter whether you use DCC or DC train control.

Reversing loop
Reversing feature which is made up of a turnback curve which connects to itself.

Reversing section
An isolated piece of track within a reversing feature which is set up to handle polarity con-flicts either manually or automatically.RJ12
Standard type of telephone style plug and socket used for six conductor cable.

Read-Only Memory. Also Mask Programmed ROM. These computer memory devices are used to store data in a manner that is easily read, but can only be written at the time the silicon chip is manufactured. This type of device is used for very large production runs to save production cost. See also PROM, EPROM, EEPROM.

Stationary decoders linked together so that they operate on a single command. This is like consisting for stationary decoders.

Routing control
The act of specifying the desired route for a train and programming the DCC system to properly actuate all turnouts (track switches) automatically when the route is chosen. See also Macros.

Rule 17
A rule on many prototype railroads that specifies conditions for lighting and dimming the headlight. Rule 17 dimming requires locos waiting to be passed and in other circumstances to dim, but not extinguish their headlights.

Security element

The plant, including trackage, associated with any reporting, interlocking and/or signaling for that trackage. This is also simply called plant.

Service Mode Programming
This method is used when programming decoders on the programming track. It is characterized by using broadcast packets and a safe power level. It is programming information broadcast by the command station to all decoders on the rails. A programming track is used to isolate decoders for individual programming.

Slave Decoder
A special type of decoder that is intended to increase the power available from one conventional locomotive decoder. Slave decoders are quite inexpensive and are very useful in the larger scales. The output from each slave decoder then drives one motor. Slave decoders do not interpret digital packets from the command station, but simply repeat the output of a conventional decoder with additional power to the device being controlled.

Memory location in the command station which holds an active mobile decoder address.

Slot following
Mobile decoder under the control of two input devices simultaneously. This can be used for teaching operators or for a computer to override a throttle in a simulation of automatic train stop (ATS).

Slow motion (or stall motor)
Turnout motor that is operated by the stalling of a DC motor. Tortoise™ and Switchmaster turnout motors are examples of slow motion or stall motors.

Scale miles per hour, model speed converted into prototype terms. An HO loco traveling at about 1 foot per second is traveling at 60 smph. 60 mph is 88 feet per second and, HO scale is 1:87.2.

Soft switch
Memory location used to switch a feature or capability on or off.

Solenoid motor
Turnout motor that is operated by the magnetic effect of a coil. Atlas snap switches are one exam-ple of this.

Special Interest Group (NMRA DCC SIG)
The DCC SJG was established as a communications vehicle for exchanging DCC information amongst users. Membership is open to the public.

Speed stabilization
Use of back emf by the decoder to modify power to the motor to keep speed constant. Speed stabilization can be scaled to make this feature more useful.

Speed table
A list of 14 or 28 cus-tomized power settings for each speed step. The table also includes Kick start, forward trim and reverse trim values.

Speed Steps
Cab-controllable voltage increments which are used to control motor speed. With some decoders, the output power can be set for each speed step. A discrete power level provided by a decoder to the motor. The range from zero to full power is divided equally into 14, 28 or 128 speed steps.

Spring switch
Turnout which can be run through against the direction in which it is set. Afterwards a spring returns it to its original setting. These exist in the prototype as well as the model form.

Square wave
Wave form with vertical sides and a flat top.

Stall Current
The maximum current draw in amps that a locomotive is capable of when stalled. When a motor is prevented from rotating and its maximum rated voltage is applied, the current draw of the motor is known as its stall current. Typically, it is safest to insure that the stall current rating of a locomotive decoder exceeds the stall current of the motor being controlled. In case of a derailment or gear bind and subsequent motor stoppage, the decoder will not be damaged.

Standards (NMRA DCC S-9.x)
Referring to NMRA defined Standards which is to "provide the pri-mary basis upon which Interchange between equipment and various North American scale model railroads is Founded." The NMRA Standards cover many aspects of model railroading. NMRA Standards provide the primary basis upon which interchange between equipment and various North American scale model railroads is founded. Under this requirement NMRA Standards include only those factors that are considered vital to such interchange. All Standards must be complied with in order for a product to be awarded an NMRA Conformance Warrant. Over 90% of the NMRA membership voted in favor of adopting the DCC Standards in 1994.

Start voltage (or Vstart)
Defined in configuration variable (CV) 02. It controls the voltage sent by the decoder to the motor for the first speed step.

Stationary Decoder
See accessory decoder.

Stop packet
A digital packet that commands a locomotive decoder to stop.

Supersonic decoder
Decoder designed to power an ironless core motor at high pulse width modulation frequency (20 to 30 kHz) to avoid heat-ing problems.


What action a function cell performs when it receives a valid trigger.

An electronic device to switch power based on temperature.

Electronic input device, often hand-held, that is used to tell the command station what commands to send to the decoders. A DCC system may have many throttles and a single hand-held throttle unit may include more than one control knob and be able to control more than one train at once. Throttles are sometimes called Cabs.

The state of a turnout or the decoder which controls it where the routing is through the curved leg or set for the diverging route.

Track feed
The short sections of wire which connect the power bus with the track and supply power to that track.

Track Power Bus
The bus used for connecting power stations to track feeder wires.

Train order signal
Signal at a depot which lets the train crew know whether or not they must stop for train orders.

A device used to convert house current to appropriate voltage levels suitable for model railroad equipment. In the USA typical house supply is 20 - 30 amps at 110 Volts. One or more transformers may be required to operate a layout and provide power to the DCC system, switch machines, lighting etc.

Transponder (or transponding device)
An electronic device which can be installed in any rolling stock and programmed with a transponder address. A transponder detector can receive the transponder address and, in some cases, other information which the transponder broadcasts. Transponding can be used to locate locos and rolling stock on the layout. Transponding is not covered by the DCC Standards and RPs.

Transponder detector
An electronic device which receives the address broadcast from a transponder. It also functions as an occupancy detector.

Event brought to a function cell by one or more input leads and which can cause that function cell to execute a task.


Unit of measurement for electrical potential required or provided by an electric device. This is a gradient or "pressure" measurement.


Unit of measurement for power required or provided by a device. In electrical devices this is the product of the current and potential.

Whole layout common rail
Method of wiring layouts where power districts and their boosters are connected electrically by a common rail or common power bus return wire.

Computer term for a group of 2, 4 or 8 bytes.

Working Group (NMRA DCC)
A group of DCC manufacturers and NMRA members who volunteer their time and expertise to create the many Standards and Recommended Practices that constitute the defining documents of digital packet command control systems.

X section

Segment of track on a con-ventionally controlled layout which can be linked temporarily to other electrical blocks. It is used so that a turnout can be used for switching without reserving a whole mainline block.
XOR (or eXclusive OR)
Logic func-tion which compares two bits and generates a new value based on that com-parison. If the two bits are the same, a 0 is generated; if they are different, a 1 is generated. It is used in DCC for calculating the error detection byte in a packet.

Zero bit stretching

Process by which one half of the zero bit of the DCC signal, either the positive or neg-ative part of the wave, is made longer to provide power to a conventional motor running on a DCC layout.