Basics, concepts & data format
I do not want to reinvent the wheel and will refer to other sources if topics are already discussed
thoroughly elsewhere. Thus just very briefly:
System
The Märklin digital train control is based on the principle that a Control-Unit generates control
signals for every receiver (decoder in locos, wagons, turnouts etc.) which are amplified by a
Booster and ‘coined’ on the operating voltage. This voltage effectual on the track is continually
switched between +18..22V and -18..22V according to the control signal. The decoder built in
locos, wagons, turnouts, etc. react only to specific signal packets: every packet consists of an
address and a control section. Only if the address matches the selected decoder address the control
data are evaluated and executed.
Concretely the so-called Motorola protocol is used being implemented in the Motorola IC
MC145026, MC145027 and MC145029 (the latter is not available anymore). It is worth to
be noted that the address code does not correspond with the binary logic thus called
bits but on a ternary logic: the trits or t-bits can adopt the values 0, 1 or open. For the
transmitted pulse this is realised by two bits together representing a trit. Further comments
are omitted as they can nicely be read up at Dr. Froitzheim or in the respective issues of
Elektor. Meahnwhile even Märklin provides such informations incl. informations reg. decoder. But you should not hope for too much informations. Carsten Meyer provides a description although not allowing for the new data format.
It is furthermore significant that since 1994 a new or extended Motorola format - or rather
Märklin format - is in use if DIP switches of the Märklin Control Unit (6021) are set
accordingly. This new data format is excellently described by Dr. Andrea Scorzoni:
briefly, the binary correlation between each bit of the four control data bit streams is
dissolved thus every bit of the stream acts as a discrete bit. But EDITS as well as simple self-
made constructions for instance are based on the Motorola chip MC145026 which is not
capable of this new format. If the features of the new format are to be used the digital model railroader is confined to the Märklin Control Unit, a - immanently flexible - software solution or a complete new hardware construction around a microcontroller.
Maximally 80 locos can be controlled independently in direction, speed (15 levels) and a direction
dependent auxiliary function. The new protocol offers among others an improved redirection
control as well as four extra functions, however, not dependent on direction. Now loco decoder for H0 are available supporting the four extra functions as part of some locos. But it is not very difficult to add this feature to every decoder with a 701.17(b). Up to 256 - with EDITS 324 - turnouts and other devices can be switched. For further reading the interested is referred to the US
Märklin WWW site and the Märklin book on digital train control. A very general introduction can be found at Modeltreno. A very detailed Digital-FAQ - not restricted to the Märklin-system - offers the DER_MOBA - in German 
only.
Equipment
The original Märklin controller is the Control Unit (6021) pushed by the transformer (6001) with
the respective extensions for more locos or ‘loco operators’, turnouts, remote control, PC
interface, etc. (6036, 6040, 6070/71, 6050/51, etc.). Alternatively the less capable Delta Controller
(6604) can be used that can be extended with a function switch though. Other controllers are for
instance the Modeltreno substitute, the Intellibox from Uhlenbrock/Modeltreno, the EDITS control unit by Elektor, various self-made
developments based on the Motorola chip MC145026 or with own pulse generation as well as
software solutions where the computer generates the signals itself.
The signal amplification is done originally with the Märklin Booster (6017) in - the to be expected
- case that the output of the Control Unit is insufficient. The same is achieved with a higher output
and a stabilised voltage by the EDITS booster and other self-made developments. It is also worth a try to construct a booster from a hybrid amplifier module.
As loco decoder the Delta decoder (6603) is to be mentioned first: compared to the more
expensive Digital decoder c80 (6080) it has the drawback of only 15 possible addresses and the
absence of the auxiliary function. The 6090-decoder with load-regulation will be substituted with a new decoder which is able to use the new four extra-functions, too. Although the Delta has - at present - the advantage that the
latest decoder chip 701.17b is used which is consistent with the new format. Märklin offers for
wagons and other special cases various decoders that are not of major interest; similar third party
products are available. Even self-making is possible within the bounds of the EDITS project by
Elektor. But this is feasible only for soldering experts with nerves of steel and a steady hand and as
it is not capable of the mentioned new format it should be less attractive. Those fond of soldering
can convert the Delta decoder functionally to a Digital decoder and can add the extra functions. Other third party products (e.g. from Uhlenbrock or Modeltreno) are in my opinion no real alternative from the monetary point of view, although they are smaller and possibly less sensitive. And some user are developing her own loco-decoder and we can exspect usuable results in the future.
For turnouts or similar devices various switching decoders (6073, 6083, 6084, etc.) are available.
These can also be replaced by third party or self-made products. Particularly for turnouts it is
attractive to provide every turnout with a separate decoder since the layout can be rearranged
without changing the wiring. As the single decoder is priced higher than a pair of turnouts much
money can be spared by self-making the EDITS turnout decoder with a specially designed PCB.
Note that the turnout control did not change in the new data format, thus the EDITS decoder can further be used unchanged.
A detailed description of the Märklin digital equipment can be found on the US Märklin WWW
site. For those interested in the EDITS project and do not have access to the British version of
Elektor please contact e.g. Dipl.Ing. Manfred Röhrig for a copy of the component you need.
Problems
My choice for a loco decoder - if it should be original Märklin parts - is the new Delta decoder.
Elder Delta as well as Digital decoder are still equipped with the somewhat aged 701.13 chip. On
the one hand the new protocol can reasonably utilised in H0 only with decoder provided with the
701.17b decoding chip. In fact the 701.13 accepts the new or a rather mixed format adjustable on
the Control Unit; but the four extra functions as well as the permanent direction control are not
available. On the other hand I have noticed that at least my decoder with the 701.13 cause
problems if the redirection signal is not given by pushing the control knob over the left stop like
the Märklin controller but by turning the knob to the left/right of the medial zero position. If locos
are redirected fast with the latter it has to be ensured at all costs that the standstill signal is
transmitted prior to that of redirection. The elder chips seem to wait for this because these locos
very frequently go on unperturbed. This considerably complicates the circuits of self-made
controllers. However, the new features - as mentioned above - can only be accessed by controllers
that are capable of the new protocol. The reconstruction of elder controller seems difficult but depending on the construction possible. Such an upgrade of EDITS is in work, but will take still some time.
Translation
This translation was made by Dipl.Ing. Manfred Röhrig from Germany.
Many thanks to him for this grateful work. Some mistakes you probably find are my work as result of trying
to correct some misunderstandings.
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