Change of mind...
By webmaster

The basic principles of digital operation requires a change of mind from thinking "voltage-driven" to think "command-driven" instead. This is not always so easy to grasp if you are an experienced "non-digital" operator who knows all the wiring tricks you have to do on a conventional layout. Do not despair, those tricks are still useful when doing more advanced stuff. For the beginner, who is maybe not electrically educated, the digital way is really a lot easier to understand.

Different philosophy...
The basic "logic" difference between digital and analog operation is that instead of directly controlling stuff like locos and signals with voltage and electrical wiring tricks, you control the devices with commands to devices which in turn are connected to the real voltage driven units. This actually means that you need less wiring too since you basically have only one "logical current circuit" that is the same for the whole layout.. 

How is it achieved ? 
We can say that we introduce an additional "layer" between the control devices (transformers, pushbuttons etc) that isolates the control devices from the current consuming units on the layout (locos, signals, turnouts etc). 

This layer consists of 2 basic active components and a distributor in a simple setup 
  • The Command station
  • The Decoder(s)
  • The distributor

Or put in other words: 

  • The Command station generates track voltage and also send coded commands to the track.
  • The Decoder interprets the commands and obeys them.
  • The track has "constant" voltage, and also distributes the commands.

On the tracks
Instead of having the voltage level from a transformer controlling the speed of a loco, you have a "constant" voltage on the track. The Command station generates commands which it addresses to a certain address, via the track. Individual decoders are set to listen to a certain individual address to avoid confusion. 
Devices in locos (decoders) interpret the commands and decide if the command is given to that particular loco, and if it is, reacts to the command given. This means that e.g. locos are now more "intelligent" and do not react to voltage, but only to commands if they are addressed. Since the voltage on the track is "constant", this means that things like lights, smoke units etc can be actived also when the loco stands still. It means that e.g. the command "lights on, loco 37" is received by the loco and executed, since there is "constant" voltage on the track even if the loco is not running. 
Off the tracks
Instead of connecting turnouts/switches and/or signals electrically to contact tracks, pushbuttons and such, you can connect them to accessory decoders instead and give commands to them instead of having them totally dependent of electrical wiring.. 
The principle for accessory decoders is the same as for the loco decoders, "listen and obey if you are addressed"... 
So what does this mean ?
This means we can control stuff independently from the actual wiring. Suddenly we have a new angle of freedom on control mechanisms. We are not totally dependent on how we have wired everything... 
"Hey there, what about AC-digital and DC-digital ?" 
What's the difference ? Do the AC-digital systems need AC-motors in the locos ?
There is actually no such beast called "AC-digital"... All digital systems work on "digital current", which is basically DC that varies between 2 fixed voltages with the overlaid digital command signal. AC current is sinusoidal, DC is a "constant" level of voltage depending on the transformer knob setting, "digital current" is not any of these. All digital systems are basically the same electrically in this aspect. Of course there are minor differences but the basic principle is the same. 
The differences in digital...
The big difference is that the various systems give the digital commands in different "languages"... 
There are the Märklin Digital, Lenz (NMRA DCC), Fleischmann FMZ and Trix Selectrix languages that are the most common today. 

It should be noted that the Lenz "language" is supported by the US NMRA (NMRA = National Model Railroad Association) organization as "NMRA DCC" and can be called an "open standard". Of course there are also a few differences on how, and to which extent, the standard is implemented by different manufacturers. Once upon a time, Märklin also had a "DC" variant of their digital system, developed by Lenz, which can be seen as the "forefather" to the NMRA DCC system. These old "DC" units can still be used with the Lenz (NMRA DCC) protocol ("language"). 
But we run Märklin, don't we ? ... 
Since this is a site focused on Märklin, the Märklin Digital system will be the one we'll focus on in the rest of the chapters, but we will not totally forget the Lenz (NMRA DCC) standard. In fact, we will learn how to get the best of the two worlds later on when some products will be discussed....
However, we will "forget" the minor proprietary systems, e.g. Fleischmann FMZ and Trix Selectrix, since I think these are of a minor interest today for the model railroader who is going to choose a digital system for his/hers layout... It's either the proprietary Märklin or the "open" DCC protocol ("language") that is the main question for the moment. However, the Selectrix system has some following in the N-scale.
Please do not be confused by this, since it's all basically the same thing. We will explain more as we progress in our little overview.... 
Motors in digital...
When it comes to motors, the loco decoders generally give PWM pulses (PWM = Pulse Width Modulation) to the motor at a certain frequency. This means that the voltage to the motor is "pulsed" DC with different pulse length to control the speed of the motor. Short pulses mean slow speed and longer pulses mean higher speed. Please note that the good old Märklin motor can also be driven this way (as a DC motor) by a special arrangement with diodes on the decoder board as on e.g. the Delta decoder. However, the PWM motor drive can make the good old Märklin motor even more noisier than it is originally... 
"I run Märklin trains, so I have to get the Märklin system, right ?"
As a Märklineer, it seems of course the obvious thing to get the Märklin system. But it's not necessary to do this... You can actually also do very well with the NMRA DCC system too, since many manufacturers have loco decoders for the old Märklin motor too...  The important thing is to have a Command station and decoders that understand each other - i.e. speak the same language.

How about turnouts and other electromagnetic devices ? 
Well, since these devices need a current pulse to switch, the digital system chosen doesn't really matter since both Märklin and NMRA DCC have accessory decoders which give the needed current to switch turnouts, signals etc. The difference is what "language" the decoders listen for. 

So, what did we learn here ? 

Basically it's like this.

  • A transformer is reduced to act as a mere power supply to the Command station. 
  • Locos and accessories do not react on transformer voltage but rather on commands from the Command station which gives "orders" to the decoders that control the voltage to the locos & accessories.
  • All decoders need to have a specific address so they will only react to commands given to them, sent to that specific address. 
  • All loco motors are driven by pulsed DC generated by the decoder, not by the voltage present on the track.
  • Decoders for electromagnetic accessories give the necessary pulse to throw these when they are addressed.

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