The Numbers Game

For the newcomer it is too easy to look at the technical specifications of DCC systems and be impressed by the numbers that are thrown at you without really understanding them.  It is generally true that the larger the numbers the more powerful and capable the system is likely to be but it is not necessarily a guide to how well it meets your requirements or how easy it is to use. 

The following are the main specification features to look at.


The maximum power that a system can provide is expressed in Amps and refers to the power available from the equipment in the original system.  There are commercial systems available with outputs from 1 to 10 Amps.  All systems can be upgraded by the addition of boosters which, in most cases, add power but not functionality.

Power determines the number of locos, lights and other accessories that the system can run at the same time but loco requirements are normally the first calculation.   No two locos are the same and there can be a significant difference even between two supposedly identical models.  In an ideal world users would test every loco individually.

Large gauge models, O and beyond, vary widely in their needs and there is no safe general formula you can use.  For modellers in the smaller gauges, N to OO, there is a widely accepted guide that you allow ½ Amp per loco in motion and thus, simplistically, a 2 Amp DCC controller can run 4 trains.  There are plenty of users who will tell you they have run more but an average ½ Amp each is a fairly safe assumption.


There are 2 totally separate address ranges for DCC decoders:

For loco decoders:

  • Short addresses 1 to 127.
  • Long addresses 128 to 9999.  There are also some systems that can use 0001 to 0127 as long addresses too and thus claim 10126 addresses but no DCC address can exceed 4 figures.
  • With one recent exception the fewest addresses offered by any DCC system is 10, addresses 1 to 9 plus one DC loco running on address 0.  A couple of other systems use short addresses only and not all the 127 available.  Long addressing should be the expected standard for all but the most basic train set controllers.
  • Address 0 for analogue locos is not available on all systems and even if it is we always advise against it; it is not good for locos and protracted use will cause damage.

For accessory decoders the same long and short address numbers are theoretically available but in practise systems use less.  It is difficult to conceive the layout that could use them all.  The maximum offered by any system is 2048 and the fewest is none.


Functions are the capabilities of loco decoders in addition to motor drive and the function range of a DCC system tells you the number of functions that the system is capable of turning on.  The maximum number of functions available with DCC is 29; F0 to F28.  There are 2 types of application of functions and the only function number which is use specific is Function 0 which, by convention, controls directional lighting.  The other functions work in one of two ways depending on the capabilities of the decoder.

  • They turn on and off a source of power which the user can connect and configure.  The commonest application is for additional lighting but they can also power other items such as smoke units, uncouplers or small motors subject to power requirements.  All commercial loco decoders manufactured now have a minimum of two functions which are normally configured as lights forward and lights reverse.
  • They turn on and off a specific capability of the decoder, such as a sound, which the user cannot change.


Number of Locos

As well as the limit on the number of locos that a system’s power output can sustain there are also limits imposed by the systems command station (its brain).  All DCC systems can maintain more than one train in motion but the maximum number varies considerably and has no direct relationship to power output.   Remember:

  • All DCC systems can operate more locos than most people could keep track of unaided.
  • The number of ‘Recalls’ is the maximum number of addresses that the user can access without having to input a new address; for most systems six is a sensible maximum because after that it is often quicker to input a new address.
  • The term ‘Roster’ is sometimes used to indicate the maximum number of loco details that the system can store.  It is not usually the same as the number of Recalls available.


Number of Cabs

Also called handhelds or throttles this is the number of input devices that can be used simultaneously.  It varies widely from just one to more than most railway clubs total membership.

Speed Steps

Speed steps are the graduations between stationary and maximum speed with low numbers producing marked jumps as speed increases.  The smallest number of speed steps allowed for by the NMRA Standard is 14 but all manufacturers, of systems and decoders, now have 28 as the start point.  The majority of systems and decoders now support 128 steps.

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