P4me - DCC - Jargon

DCC Jargon Buster

This page is intended to explain some of the jargon which often confuses new comers to DCC, and also to describe some of the terms used in the DCC Systems Comparison Table. I would like to thank the members of Model Rail Forum for their assistance in compiling this guide.

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Auto Shuffle Train/Signal Speed Control, is where a system can tell a train what to do automatically, without any input from the user. For instance if a signal is red, the system might tell any loco approaching that signal to slow down and stop, then when the signal turns green the system will tell that loco to gradually accelerate till it reaches it's original speed setting. Or it could tell a train that has reached the end of a branchline to stop, wait a few minutes then accelerate back in the opposite direction.

This system based automation should not be confused with decoder based automation such as Lenz ABC/Asymmetric Signal.

Bi-Directional Communication is a bit like CV readback and feedback combined, in that it means a system can read a decoders CV settings while the loco is running, not just while programming, and the decoder can report back its condition (i.e. its current speed and direction) to the command station. So with Bi-Directional Communication the system is talking to the decoder and the decoder is talking to the system, and all using just the rails - no extra wiring is required.

There are currently 2 forms of Bi-Directional Communication on the market, Digitrax Transponding, and Lenz Railcom. Lenz' system has recently become a NMRA DCC recomended practice, so a number of systems and decoders are being modified and introduced to support this feature.

A Booster is located between the Command Station and the track, the booster 'boosts' the commands from the command station up to a usable level, a bit like an audio amplifier. Most systems have the Command Staion and Booster built into a single unit, however additional boosters can be added for layouts that need more power than a single booster can provide.

A BUS (note capitalised) is any wire or multiple wires which has multiple connection points along its/their length. Think of a water pipe with several taps along it, that's a pretty fair representation of what a BUS is.

How many BUSes are needed? In DCC terms there are 2 BUSes, the track BUS, and the communications BUS.

What are the differences between BUSes? Why is a BUS needed - what is it used for?

The track BUS - this is the 2 wires that come from the digital system and connect to the track at various places. This BUS is made up of 2 wires, one for each rail, as there is only 2 rails on a length of railway track you only need 2 wires for this BUS.
The communications BUS (Com BUS)- this BUS is made up of 4-8 wires depending on the system, and is what the throttles are connected to. This BUS allows the throttles and other devices (such as accessory decoders and block detectors) to communicate with the Command Station so that they are not only receiving commands but can also send back messages to the rest of the system. You can even eradicate this BUS almost completely if you use a system with wireless throttles.

Unfortunately it's not quite that simple, XpressNET isn't capable of handling all of the devices on your layout, so any system which uses XpressNET (Hornby, Lenz, Roco, ZTC?) also has a 3rd BUS (the feedback BUS), this additional 2 wire BUS makes up for the missing functionality of XpressNET. There are also some systems which have the same problem as XpressNET systems, however instead of adding a 3rd BUS they simply don't have full functionality, have a look at the DCC systems Comparison Table and you will see these systems marked with a 'No' in the feedback column.

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ACab is 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). The cab or throttle is the controller, fixed or handheld. Each cab can control multiple locos - either by switching between locos using pre-assigned buttons or by calling-up the loco in the normal way.

The Command Station is the brains of any DCC system, and is much like the CPU of a computer (indeed that is exactly what it is). The throttles send infomation to the Command Station, which then sends commands out to the decoders, on some systems the decoders will then send feedback to the command station to confirm that the command has been executed.

Consist & MU/Multiple Unit are one and the same in DCC talk, basically couple several loco's together and you have a consist/multiple unit. Multitraction, or double heading as some systems call it, is not a standard type of MU or consisting, a system which supports this is quite limited in its capabilities, as typically it can only handle 4 loco's in a consist.
There are 3 types of consisting which are accepted as NMRA standards:-

Basic consisting; this is where any number of loco's are programmed to the same address, all systems support basic consisting. It is however not very practical, as it requires decoders to be reprogrammed before and after running them in a consist.

Command Station Assisted Consisting; with this method you enter the addresses of the loco's you want in a consist and the command station will remember them, and send commands to all loco's in the consist. The limiting factor here is the max loco's the system can handle, as if the system can handle 10 loco's and you have 9 in the consist only one other loco can be run at the same time.

Decoder Assisted Consisting; with this method you enter the addresses of the loco's you want in a consist and the decoders will remember they are in a consist and will respond only to the "consist address". There is no limit on the number of loco's you can include in a consist with this method, if you have 10000 loco's you can put them all in a consist, even if your system can only run 10 loco's you can still have a consist of any number of loco's and run 9 others seperately!

CV is short for Configuration Variable, decoders have over a thousand CV's which define how it is configured, or rather how it responds to a command given from the throttle, and can all be altered by the user to alter the performance of each decoder, and therefore each loco.

Most of these CV's the user will never need to change, some they will, some can be adjusted if you wish. I'd put CV's into 4 categories, of which only the first category must be changed, the rest can be done if you want to change how your loco runs:-

Identity CV's, these determine the loco's address, and therefore must be differant in each loco. All decoders are supplied by the manufacturer set to address 3, this can be changed on all systems, and usually quite easily.
Running CV's, these determine how the loco runs, adjustments such as maximum speed, starting voltage, and mid point voltage can be set with these CV's. As an example a tank loco could be set to have a low top speed so it is more contollable at shunting, while a loco that needs a lot of power to get started could have it's start voltage increased so that it starts as soon as the throttle knob is turned. You can also set up acceleration and deceleration rates so that a loco slowly speeds up or slows down.
Function CV's, these allow you to adjust how a function performs, for instance you might want a light to flash when you press a certain function button on your throttle, you can do that with these CV's.
Hidden CV's, these are simply CV's that will you will probably never need or want to adjust, or they could be inactive in certain decoders, or even read only CV's such as the manufacturer ID which all decoders have set in CV08.

So what is CV Readback? Well quite simply it means a system can not only set a CV value, it can also read what that CV is already set to in a decoder when the system is in programming mode.

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Decoders are a small device which 'decodes' the infomation sent out from the command station so that it can be used to control a motor, lights, or any other device according to the actions of the throttle/cab.

There are 2 types of decoder;

Mobile decoders, these are the ones fitted in a loco.
Stationary decoders, these are generally fitted under the baseboard and are used to operate points/switches, signals, or other effects such as level crossing gates and lights or any other device you want to control from your throttle.

Feedback allows devices such as a stationary decoder to report back to the command station, this infomation can then be displayed on throttles or a computer screen.
For instance a stationary decoder might report back which route a point/switch is set to.
Feedback is also used for block detectors, a block detector can sense when a loco enters it's section of track, using feedback it can then tell the command station that a train has entered it's block.
Used simply like this feedback allows the operator to see what is happening on a layout without actually looking at the layout! For instance you could change a point/switch which is hidden in a tunnel or at the other end of the layout, and you can be certain that it has changed because it has told you so.

However feedback can also be used to affect other devices on a layout automaticaly, some examples are;

When a loco enters a block the block detector can tell a signal that it needs to change to red, or it could tell the level crossing lights to flash or the gates to close.
When a point is set to the right hand track it can tell the signals to show a red aspect for the left hand track.
You could use a block detector to sense when a loco is passing through a set of points, it can then tell the points not to change while the loco is passing through them.
Anything else you can think of.

Functions can be what ever you want them to be, that is the idea of them. Functions can be used to control lights, sound effects, or even mechanical devices such as uncoupling, and doors on carriages.

Most systems currently have 13 functions, F0-F12, however the recomended practive for functions has recently been extended to 19 functions F0-F18.

The only standard function is F0, this is for forward and reverse headlights, all decoders and systems must support this function. With this function you can turn the headlights on or off by pressing the F0 button on a throttle, also whether the forward or reverse headlight is lit is determined by the direction the loco is travelling in, even when the loco is stopped the lights will remain lit depending on which direction the loco is set to.

The recomended practices for systems has recently been upgraded from functions F0-F12 to F0-F18, systems don't have to support these functions, but it is recomended that they do. Some systems support all of these functions, some support only the old recomended practice of 13, others only a few of them, while some basic sets only support the standard F0.

What you do with functions is entirely up to you, for some loco's you might not want any functions, just speed and direction, or you may want some special effects to make your model more realistic. You can also buy function only decoders, these don't have controls for a motor, and so are mostly used in carriages and wagons for lighting, opening doors and such.

Some possible uses for functions are

individually controlled marker lamps
uncoupling
sound effects such as whistle/horn, couplings clanking, brake squeal, etc.
opening doors on carriages or multiple units
smoke units
working fans
anything else you'd like to add to you models

Also sound decoders which fit inside a loco use these functions to control specific sounds, as well as the chuffing or motor noise which is synchronised to the loco's speed, and because power is supplied even when the loco isn't moving you can activate these sounds at any time, such as sounding the whistle before departing from a station.

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Withspeed control knob and dial are one and the same. A pot which is short for potentiometer is what the knob actually turns, this gives an arc of around 270 degrees from stop to full speed.

The better throttles don't use a pot, instead they use a rotary encoder, but still with a knob. A rotary encoder has no physical end to it's travel, and so it can take several rotations from stop to fullspeed, this gives much finer speed control.

My system uses rotary encoders, and I have one loco that takes around 30 sec to move from one sleeper(tie) to the next at the slowest speed setting. Control like this is next to impossible with a pot, but it can also be achieved with buttons.

Some systems use both knobs and buttons to control speed, depending on the system you may be able to use one or the other, or you may be able to use both.

Speed Steps are literally the number of steps between stop and fullspeed, there are 3 modes of speed steps 14, 28, and 128.
14 steps is the original standard and was all we had when DCC first came out, this was later increased to 28 to allow better control, this has now been extended to 128 steps to give the ultimate in fine control.
14 steps is the baseline standard, all systems and decoders must support this to be compatible with each other, 28 & 128 steps is a recomended practice, systems and decoders are not required to support 28 or 128 steps, but it is recomended that they do.

By ensuring that all systems and decoders are compatible with 14 steps loco's fitted with an old decoder can be run on any system, while an older system can be used to run loco's with modern decoders.
This is "backwards compatibility," and ensures that all DCC systems can work with all DCC decoders, regardless of make or age.

Software upgrade, generally the system manufacturer will provide a chip which replaces the one currently in the command station. Fitting the new chip can either be done by the user, or by a dealer/service agent. The new chip is already programmed with the new software, so you just unplug the old chip, plug in the new one, and go back to running trains. Some systems can be upgraded be using a computer interface, the software upgrade can be downloaded from the internet then uploaded to the command station.

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