DCC Decoders: Types, Features and Applications

Types of DCC Decoders

Decoders are in charge of managing the vehicles and/or carrying out specific tasks around a layout after receiving orders from throttles or other devices. There are two types of decoders: the mobile decoders (also called the multifunction decoders); and, the stationary decoders (also called the accessory decoders).

Mobile Decoders

In locomotives, mobile decoders are mounted. Controls for locomotive speed, direction, illumination, and sound effects are a few typical features. Additionally, DCC decoders include capabilities like numerous lighting effects, configurable acceleration and deceleration speeds.

These are frequently referred to as mobile decoders, although the NMRA calls them multifunction decoders. Vehicles like locomotives and other rolling equipment that travel along rails are equipped with mobile decoders. It enables total control over all motors, lights, audio, and other features. Installation can take place at the production facility, be done by a third party, like a hobby store, or you can do it yourself.

A mobile decoder (sometimes commonly referred to as a “decoder”) is a compact circuit board containing at least four wires and frequently more. Both two and two link to the motor and track, respectively. A sound-equipped decoder will have two wires for connecting to speakers, and most decoders will have extra “function output” cables for controlling lights. With function wires, there is normally one “common” wire and one wire for each function. A light, for instance, is linked between the function wire and the common wire.

“Function-only” decoders are another type of mobile decoder. These would be used, for instance, to operate lights in a dining car or a flashing-rear-end (FRED) device on the end of a train, which are not in the engine. Additionally, they come in extremely useful on passenger trains with numerous units since there are two cars equipped with head and taillights, one or both of which will be independent of the motor car.

Mobile decoders can be further divided into two categories: OEM and retail. Retail decoders are full-featured decoders that you can purchase from a hobby store and install yourself or have the store do it for you. OEM decoders may not have all the features of a retail decoder because they are provided straight to the factory for installation in a vehicle while it is being made. OEM decoders can be programmed specifically to do various tasks unique to that model and to play the relevant sounds.

Decoders are delicate electronic devices that can be easily broken when handled. Keep them sealed up until use and handle them with proper electrostatic discharge (ESD) safety practices. Decoders can be permanently linked to a train or connected using a plug-and-socket system, allowing a non-technical train operator to install or replace the decoder. A socket is frequently given when a train is delivered without a decoder and is used to connect to a temporary circuit board that is removed when a decoder is fitted.

A decoder generally has a default set of actions and is preconfigured to respond to address “3” (the “default address”) when it leaves the factory. By adding data to what are referred to as Configuration Variables (abbreviated CV), these can be altered. The majority of DCC systems appear to have been created by and for computer programmers, which can make actually figuring out what you need to do very difficult for individuals who aren’t computer programmers, even if this isn’t extremely difficult.

Decoders that are pre-installed in trains could have their CVs pre-set to mirror the typical behavior of the train or they might need to be programmed. See the train’s owner’s manual.

Stationary Decoders

Turnouts and signals can be managed by stationary decoders. Other accessories such as structural lights, scene lighting and animation may all be managed by stationary decoders.

The same DCC power used for mobile decoders or through the throttle bus is used to operate the stationary decoder (also known as a stationary decoder). The distinction from a DCC standpoint is in the address partitioning and output kinds. A mobile decoder regulates a motor’s speed, momentum, braking, and other operations, whereas a stationary decoder just controls the on/off (or “forwards/backwards”) functions for accessories. It’s possible for stationary decoders designed for signaling purposes to have extra states.

Sound Decoders

A sound decoder is a computer on a chip that receives signals from the DCC base station, decodes them, and then outputs an analog signal to a speaker linked to it to generate the required sound. Modern model railroaders may improve their locomotives, rolling stock, and layouts with a variety of sounds using DCC sound decoders, which heightens the realism and impact of their layouts.

But DCC sound decoders do more than that with its two different categories: the multifunction decoders and sound-only decoders. A multifunction decoder not only generates synchronized sound effects but also manages the motor, lights, and other operations of a locomotive. Since the noises are connected to the motor’s activity, realistic effects like a prime mover loading or the rhythm of a steam locomotive’s exhaust may be achieved. Dynamic brakes, rod clanking, the opening of steam cocks, and, in the case of internal combustion, the starting or stopping of the prime mover are further consequences. In addition, there are additional related sounds such coupler noises, horn/whistles, bells, and other ambient effects. Nowadays, many locomotives have the option for a multifunction decoder or even come equipped with one when they leave the factory.

A speaker is the sole attachment that a sound-only decoder, a type of accessory decoder, can manage. It lacks the ability to power other devices like motors and lights. To create sound effects in a freight vehicle or other fixed structure on the layout, a modeler might choose to use a sound-only decoder. Additionally, they may be used to add sound to a locomotive that already has a motor-only decoder.

When choosing a sound decoder for locomotive, modelers need to do their homework. HO scale sound decoders are typically larger than N scale sound decoders, and O scale or large-scale sound decoders are much bigger since their components must handle greater current. There are also sound decoders available that have been pre-programmed with various sound sets for various types of locomotive engines. Both the DCC sound decoder and the locomotive markets provide a wide range of alternatives that are appropriate for various models, periods, and scales.

Some sound decoders allow users to upload or download custom sounds from a desktop computer or the manufacturer’s website to program them with. Modelers who desire a sound decoder for a rare locomotive but can’t find one with pre-programmed sound effects have this alternative. To create a clickety-clack sound or radio chatter, programmable sound-only decoders may be put in cabooses. Similarly, they can be installed in cattle cars to provide sporadic mooing from the animals that ride within. 

Additionally, they can be employed in stationary applications to ring a bell at a grade crossing that has been indicated or to provide the background noise of a factory or farm. Some premium multifunction decoders allow for complete reprogramming with customized sound effects, but many multifunction decoders come pre-loaded with a specific sound set from the factory.

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4.1 Mobile DCC Decoders: Features and Applications

model railroad

Speed and momentum control

Digital processors working with the constant-voltage power provided by the track must manage the speed and direction of the motor when using DCC (the DCC on the track is alternating current, but in an approximately square wave, so when rectified to DC it generates a constant DC voltage). Pulse-wave modulation, where power is swiftly turned on and off to produce “pulses” of full-voltage power (the “pulse amplitude” being equal to the rectified DCC voltage), is the most effective method for accomplishing this. The “pulse cycle” is the regular repetition of these pulses; the longer the pulse, the higher the average voltage delivered. The average voltage also determines how quickly the motor turns.

These pulses are produced by the decoder using a group of four transistors that function as switches. The track voltage is rectified in the diagram below and sent to the left (positive) and right (negative) sides of a “H bridge” (turn it sideways to see the “H” form). The motor will revolve in one way when switches 3 and 2 are on (as indicated), in the opposite direction when switches 1 and 4 are on, and there will be no voltage given (the space between pulses) when all four switches are off.

But there is more going on than that. The voltage pulses really cause a current to flow through the motor. The energy required to move the train is produced by torque rather than speed by voltage. Additionally, once the current starts flowing, it doesn’t cease right away when the voltage is turned off since the motor’s wire is an electromagnet, which is a type of inductor. The circuit will feature “Flyback Diodes” that allow the current to bypass the open switches, allowing the current to continue flowing even while the switch (a transistor) is off; otherwise, the energy stored in the inductor would build to a dangerous level of voltage between pulses.

If the pulses are spaced far enough apart, the current never entirely ceases, which leads to more evenly distributed torque output and more sputter-free running of the train. Naturally, when the armature spins, the current finally reverses direction and changes direction, with the pulses now working in the opposite polarity. Because this occurs rather rapidly, there is less energy lost when the current is reversed. A “supersonic” decoder will generate at least ten and probably thousands of pulses at standard working rates before the armature can change polarity.

Thus, the DCC-operated train’s engine is a component of a larger system, the operation of which may be somewhat altered by the DCC decoder’s configuration. This has a variety of affects and is frequently used to accentuate one quality over another or select how the motor will act in certain situations.

Function mapping

Remapping functions enables changing the outputs that a function controls. It can also reduce the number of keystrokes required to complete a task. 

Functions that require many key presses or key combinations can be eliminated by remapping them. Thankfully, more decoder software makers now offer remapping. Others have used automation, whereby pressing one function key initiates a number of different actions.

Function mapping is the process of reassigning throttle buttons to do tasks other than those for which they were originally intended. As an illustration, F0 typically toggles the light on and off. Instead, you may remap it such that pressing F5 turns the light on and off. Manually doing it is a difficult task.

Remapping a function can be done for a variety of reasons:

  • Standardize features across various multifunction decoder brands:
  • Function remapping can maintain a consistent function key usage throughout a number of different decoders in the motive power fleet because OEM will change multifunction decoder vendors over time and may supply various decoders in different locomotives.
  • Make button presses simple.
  • The number of keys on the throttle to control them was increased with the development of decoders that supported more than ten or twelve functions. Many DCC manufacturers started equipping their throttles with a shift key feature to address this problem. Similar to how hitting Control or Control-Shift on a computer keyboard enables access to different characters.
  • Sadly, that frequently necessitates pressing many keys in order to get F11 or F28 which could be confusing. A key or key combination may occasionally be required to access the function. As a result, the bell and whistle is prioritized above the entire set of operations.

4.2 Stationary DCC Decoders: Features and Applications

Turnout control

Controlling turnout motors is a major use for stationary decoders, and many of them were created with this goal in mind.  A turnout’s alignment is electrically controlled by a mechanical device known as a turnout motor or switch machine. 

There are several turnout motor kinds, including solenoid, stall, and motor-driven screw types. Each type requires a certain sort of electrical drive.

Solenoid Type.

The ideal technique to operate a capacitive discharge device is to use the energy stored in a big capacitor to provide a brief, intense pulse to the electromagnet without running the danger of overheating it. This is necessary to quickly flip the output from one direction to the other. Solenoid types are 2-wire and 3-wire solenoids:

2-wire solenoid. A permanent magnet and one coil are both found at the heart of a two-wire solenoid. The coil is powered by DC current, which pulls the magnet in one direction. The magnet is drawn in the opposite direction by switching the DC polarity. The switch is linked to a mechanism that the magnet is attached to. The coil is very briefly electrified because if it is, it would overheat and burn off.

3-wire solenoid. Twin coils and an armature made of iron make up the core of a 3-wire solenoid. The armature is drawn into the coil by activating one of the coils. The slug is linked to a device that is linked to the switch by a mechanism. The coil is very briefly electrified because if it is, it would overheat and burn off. To transmit the motion, a rod is attached to the armature. This attaches to a device that enables the position to be kept (latched) until altered. This device, often a lever, enables the motion of the armature to be modified into a more practical shape. Contacts are frequently offered, and they may be used to manage the frog.

Stall Type. To move a geared motor slowly from one side to the other, a steady yet low current is needed. Because they will stall when they approach the mechanical limit and the stall current may be continuously maintained to hold the switch blades in place, they are known as stall type switches.

A stall motor with big ratio gearing is used in slow motion machinery. The motor is always powered on and uses relatively little current when it is stuck. The motor stalls and keeps the switch points in place when they get near together.

Motor Driven Screw Types. A can motor with gearing drives a push-pull actuator in a limit-switch DC motor. Regular gears or a screw (or threaded rod) attached to the drive shaft can be used as the gearing. A nut is screwed onto the screw for the screw-type. The nut travels along the screw as the motor rotates. The points are activated by a mechanism that is attached to the nut. When the needed motion is finished, inbuilt limit switches turn the motor off.

No matter the motor type, every turnout motor needs two decoder outputs—one to establish the primary path and the other to set the diverging route. There is just one DCC Accessory Address on this output pair.

Signal operation

Signaling, whether by electromechanically driven semaphore arms or color light signals, is another typical application for stationary decoders. Once more, common practice dictates that a single signal be driven by a pair of decoder outputs. When there are more than two lights involved, Multi Aspect Signaling becomes a little more challenging.

For genuine operations, signaling is essential, but not for model railroading. Your layout may become extremely sophisticated by adding signaling, which can be done in a variety of ways.

Before getting a signaling system for a layout, there are a few key factors to think about before purchasing any signaling equipment.

First identify the signaling system to be imitated. Additionally, the level of prototype functioning hoped to be achieved also needs to be distinguished.

Second know the kind of logic control being planned to be used to set the signals.

Third get your DCC layout’s block detection up and running.

A signaling system often has to be aware of the present path (turnout routing) and the availability of the track in front of it (block occupancy). You might also be interested in the power management of the next route.

Prototype Signaling

Timetable Operations. The system can be operated most simply by following a schedule, at least in terms of the equipment involved. Every train crew is aware of and follows a set timetable. Only one train may operate on a given length of track at a time, and only that train has “possession” of that portion during that time.

On a single-track railroad, meeting locations (also known as “meets”) are arranged where one train must wait for the other at a passing spot. Each train must wait for the other to arrive before moving. A second train is following the first in the US, and the waiting train must wait until the second train passes before continuing. This is indicated by the display of two green flags (green lights at night). In addition, when it draws near, the train pulling the flags blows its whistle eight times. Before the train carrying the flag may move on, the waiting train must fire eight blasts back.

Railway lines are split into portions known as blocks to prevent train collisions by prohibiting several trains from using the same segment of track at once. Normally, each block is only allowed to have one train in it at a time. The majority of railway safety measures are built on this premise. Blocks can either be moving blocks (block ends specified relative to moving trains) or fixed blocks (block limits are fixed throughout the route).

Permissive and Absolute Blocks. Trains are only allowed to pass signals indicating the track ahead is occupied under a permissive block system if they are moving at a pace that will allow them to stop safely in case an obstruction appears. This is mostly employed in the USA and in some circumstances allows for increased efficiency. In most nations, it is only permitted on freight trains, although it could be dependent on how visible the area is.

In an emergency, permissive block working may also be used if the signalman is unable to reach the next signal box to confirm that the previous train has passed, for example if the telegraph wires are down. However, this is only permitted when the signal does not protect any conflicting moves after a driver has been stopped at a danger signal for a specific amount of time. Trains must move very slowly in these situations (usually 32 km/h (20 mph) or less) in order to stop before any obstacles. In most circumstances, this is prohibited when visibility is bad (such as when there is fog or snow falling).

Several trains may enter a block with permission even under an absolute block system. This could be required to divide or connect trains, as well as to rescue derailed trains. When providing permission, the signalman also makes sure the motorist is fully aware of what is coming up. Considering this knowledge, the driver must run the train safely.

Typically, a yellow flag is used to verbally authorize the driver to pass a signal that is still in danger and to explain why the train in front is there. A subsidiary signal, commonly referred to as a “calling on” signal, is provided for these movements when trains frequently enter occupied blocks, such as stations where coupling occurs; otherwise, they are carried out through train orders.

Signal along the main line

Automatic Block Signals. Under automated block signaling, signals determine whether a block is clear so that a train may enter based on automatic train detection. The signals may also be directed by a signalman such that they only issue a go signal when the signalman directs the signal in that manner and the block is clear.

Fixed Block. The track between two fixed places is included in the majority of blocks, which are fixed. Blocks often begin and stop at specific stations on timetable, train order, and token-based systems. Blocks begin and stop at signals in systems that rely on signaling.

Block lengths are created to enable trains to run as frequently as required. A line with little traffic may have blocks that are many kilometers long, yet a line with many commuters may have blocks that are only a few hundred meters long.

Until a signal show that a train may move forward, a dispatcher or signalman gives the driver the go-ahead, or the driver obtains possession of the necessary token, the train is not allowed to enter a block. Most of the time, a train cannot enter a block unless both the block itself and the area beyond it—at least the length necessary to halt the train—are clear of other trains. This overlap might extend as far as the signal after the one at the end of the segment in signaling-based systems with closely spaced signals, thereby mandating a spacing between trains of two blocks.

The following factors must be taken into consideration while determining the size of the blocks and, consequently, the distance between the signals:

  • Line speed (the top speed allowed over a line segment)
  • Train speed (the top speed for various kinds of traffic)
  • (To account for greater or shorter braking distances) gradient
  • Train braking characteristics (various train classes, such as freight and high-speed passenger, have varied inertial numbers)
  • Sighting (the distance at which a motorist can see a signal)
  • Quickness of the driver’s response
  • In the past, certain large or fast trains were signaled under distinct criteria and only granted the right of way if there were no obstructions two blocks in advance of the train.

Moving Block. Faster trains require longer stopping intervals, resulting in longer blocks and a reduction in the line’s capacity. This is one drawback of fixed blocks. Regardless of the actual speed of the trains, fixed blocks must be designed for the longest stopping distance. Computers determine a “safe zone” surrounding each moving train in a moving block system that no other train is permitted to enter.

The system depends on accurate information on the location, speed, and direction of each train, which is gathered from a variety of sensors including active and passive markers along the track and speedometers mounted on trains (GPS systems are unreliable since they do not function in tunnels). Lineside signals are not required in a moving block configuration since instructions are sent directly to the trains. Due to the ability to operate trains closer together while still maintaining the necessary safety buffers, this offers the benefit of boosting track capacity.