DCC glossary

A 21-pin decoder interface (NEM 660 / RCN-121) with two rows at 1.27 mm spacing and one position left empty as a key.

See also: PluX, Next18

A decoder that controls things other than locos — points, signals, lights. It takes accessory commands from the DCC signal; its address is held in CV513 and CV521, and CV29 bit 7 is set to mark it as an accessory decoder.

See also: Point motor, CV29

A device that feeds a reversing section (such as a reverse loop) and swaps that section’s polarity automatically when a train bridges the gaps, so there is no lasting short. The reversing section should be longer than the longest train.

See also: Reverse loop, Power district

Also called a dummy plug: fitted in a loco’s decoder socket when no decoder is installed, so the loco runs on analogue DC. Keep it when you fit a decoder.

See also: NEM 651, NEM 652

Dividing the track into detection sections, each fed through a detector that senses the current drawn by a loco or by rolling stock fitted with resistor wheelsets, so a computer or command station knows which sections are occupied.

See also: Resistor wheelset, RailCom

The unit that supplies the track power for a section of layout, following the commands from the command station. Larger layouts use several boosters, each feeding its own power district.

See also: Power district, Command station

The heart of a DCC system: it turns what you ask for on a throttle or computer into DCC commands for the track. Many include a booster.

See also: Booster, DCC-EX

A numbered setting stored in a decoder, such as the address (CV1), start voltage (CV2) or acceleration (CV3). The NMRA standard S-9.2.2 defines the common ones; makers add their own.

See also: CV29, Programming track

The decoder’s main configuration CV. Each bit switches a feature: direction (1), 28/128 speed steps (2), analogue running (4), RailCom (8), speed table (16), long address (32) and accessory decoder (128). Its value is the sum of the bits that are on.

See also: CV (configuration variable), Long address

A system where the track carries constant power and digital commands together. Each loco has a decoder that obeys only commands sent to its own address. Standardised by the NMRA in the US and by MOROP (NEM) and the RailCommunity (RCN) in Europe.

See also: Decoder, Command station

A free, open-source DCC command station you build from an Arduino-type board and a motor shield. Its EX-CommandStation software is licensed under GNU GPL v3, and its EXRAIL language runs automation scripts on the command station.

See also: EXRAIL, Command station

The small circuit board in a loco (or accessory) that reads DCC commands addressed to it and drives the motor, lights and other functions.

See also: NEM 652, 21MTC, PluX

The part of JMRI used to read and write decoder CVs through friendly screens, and to keep a roster of your locos.

See also: JMRI

Also called a feeder: a short wire that joins a rail to the track bus. Digitrax suggests feeders roughly every 6–10 feet, to both rails.

See also: Track bus

A newer, very small decoder interface (NMRA S-9.1.1.6, February 2026) for N, TT and small HO models. It is physically symmetrical and not keyed.

See also: Next18

DCC-EX’s automation language. Scripts in a file called myAutomation.h use commands such as AT (wait for a sensor), FWD, REV, DELAY, THROW and CLOSE to run trains, points and signals.

See also: DCC-EX

A sensor that detects a magnet, usually fitted under rolling stock. Cheap, but it only detects where the magnet is and DCC-EX notes it can be unreliable at speed.

See also: IR sensor, Block detection

An infrared sensor — reflective or beam-break — used to detect trains. Cheap, but can be affected by room lighting; beam-break types are harder to hide.

See also: Hall-effect sensor

Free, open-source Java software for model railways, including DecoderPro (decoder programming) and PanelPro (panels and signals). It works with a very wide range of DCC systems.

See also: DecoderPro

A set of NMRA standards, developed by the OpenLCB group, for a CAN-based network that links layout devices such as signals and points.

See also: LocoNet

Digitrax’s layout network: a peer-to-peer bus using six-conductor flat cable with RJ12 plugs, connecting throttles, boosters and detectors.

See also: LCC (Layout Command Control), s88

A four-digit-style loco address stored in CV17 and CV18, used when CV29 bit 5 is on. CV17 is always 192–231, so the highest long address is 10239.

See also: Short address, CV29

The Model Electronic Railway Group: a UK-based society for electronics and computing in railway modelling, whose members design kits. Its layout control bus is called CBUS.

The 6-pin “small” decoder interface, 1 × 6 pins at 0.050 in spacing. Common in N and small OO/HO models; the NMRA marks it not recommended for new designs.

See also: NEM 652, Decoder

The 8-pin “medium” decoder interface, 2 × 4 pins at 0.100 in spacing, very common in older OO and HO models. The NMRA marks it not recommended for new designs.

See also: NEM 651, 21MTC

A small 18-contact decoder interface (NEM 662 / RCN-118) for N, TT and small HO installs. Next18-S is the sound version with speaker pins.

See also: PluX, E24

A family of decoder interfaces (PluX16 and PluX22 in current use) under NEM 658 / RCN-122, with speaker and capacitor pins for sound decoders.

See also: 21MTC, Next18

The motor or solenoid that moves a point (turnout). Solenoid motors need a short pulse; accessory decoders limit the pulse time to avoid overheating.

See also: Accessory decoder

Changing a decoder’s CVs while it is on the running layout, by sending commands to its address. Some decoders don’t allow every CV to be changed this way.

See also: Programming track, Service mode

An electrically isolated section of layout with its own booster (or circuit breaker), separated by gaps in both rails, so a short in one area doesn’t stop the whole layout.

See also: Booster, Auto-reverser

A separate, isolated length of track used for service-mode programming. Commands there reach every decoder on it, so only the loco you want to program should be on it.

See also: Service mode, POM (programming on the main)

A way for a decoder to send information (such as its address or CV values) back to the system. It needs a short “cutout” in the track signal from the booster, and is switched on with CV29 bit 3.

See also: CV29, Block detection

A wheelset with a resistor across it, so a carriage or wagon draws a tiny current and shows up on current-sensing block detection.

See also: Block detection

Track that lets a train turn back on itself, such as a balloon loop. It creates a short circuit unless the reversing section is gapped in both rails and fed through an auto-reverser.

See also: Auto-reverser

A feedback bus used by Märklin and others to report sensor inputs, with chainable modules of 16 inputs. Voltages vary between systems, so check before connecting modules.

See also: LocoNet

Programming a decoder on a programming track. Most systems can read CVs back in this mode.

See also: Programming track, POM (programming on the main)

A loco address from 1 to 127, stored in CV1 and used when CV29 bit 5 is off. The factory default is usually 3.

See also: Long address

How finely a throttle controls speed: 14, 28 or 128 steps. CV29 bit 1 selects 14 or 28/128-step mode.

See also: CV29, Speed table

28 values in CV67–CV94 that set the motor drive for each speed step, used when CV29 bit 4 is on. The alternative is the three-point curve in CV2, CV6 and CV5.

See also: Speed steps, CV29

Heavier wires running under the layout from the booster, with droppers to the rails. Digitrax recommends at least 16 AWG on an average layout and twisting the pair.

See also: Dropper, Booster

CV2, CV6 and CV5: the motor drive at the first, middle and top speed steps (255 = full voltage). ESU advises keeping start < middle < top.

See also: Speed table