Function Decoders

What is a ZIMO function decoder?

Function decoders are vehicle decoders for non-driven vehicles, i.e. without motor power amplifier, but with some special characteristics to be used in cars, which are mostly part of a train driven by a motor vehicle (locomotive).

A special feature of all ZIMO function decoders is ...
A special feature of all ZIMO function decoders is the programmable SECOND ADDRESS (CVs #64 to #68), via which a car equipped with a function decoder can be addressed on an alternate address, which ususally is the same as the address of the loco decoder. If the other cars of the train also are equipped accordingly, i.e. all are addressable via the same address (e.g. to turn on all lights with one single button), this is the simplest form of a (virtual) "train bus" ("TrainBus") which in future will certainly play a major role in model railroading.
  • DCC addresses 1 - 10239 (as individual or second address), consist addresses 1 - 127, MM-addresses 1 - 80
  • Programming CVs in "service mode" (on the programming track) and "operational mode" (PoM = Programming on the main)
  • Error-free operation (without decoder reset and restart) on contact interruptions of 1 or 2 sec (also without external energy storage)
  • Updatable software: new software versions are loaded without opening the loco, with the help of the ZIMO Decoder Update Device MXULFA, the central controller MX31ZL or the central command station MX10 (planned); from your computer or directly from the flash drive (via MX31ZL, MX10 or MXULFA).

  • Overcurrent protection for all outputs, overtemperature protection, see details in "continuous current"
  • Dielectric strength 40 V (MX681), 50 V (MX685)
  • Wired types covered with refractory, insulating, transparent shrinking tube.

  • Full NMRA function mapping for F0 - F12, and ZIMO expansions (direction dependent assignments, "Swiss Mapping", etc.)
  • Dimming, flashing, American (Mars, Ditch, strobe, ...) and other light effects (Soft start, brake lights, flickering, automatic timeout, ...)
  • High beam / low beam switching via a function key
  • Time-limited coupler control for Krois and Roco, and "automatic uncoupling".

  • RailCom CV: acknowledged programming and reading of CVs "on the main"
  • many more RailCom applications are planned for future software releases.

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where you can find background information and application notes on the meaning
of the data in the table and the properties of the individual decoders.

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Expand all table rows (useful to print the entire information)


MX671 ZIMO-MX675V ZIMO-MX676VD MX685P16 MX686D MX689N18



MX671R, MX671N






MX685, MX685R



MX689N18 *

Dimensions (mm) 10,5 x 8 x 2,2 25 x 15 x 4 26 x 15 x 3.5 20 x 11 x 3.5 20.5 x 15.5 x 3.5 14 x 9.5 x 2.1

The dimensions of decoders with standardized interfaces (PluX, MTC, NEM-651, Next18) correspond to the corresponding standards by VHDM and NMRA or are smaller. In case of the wired decoders, which are protected by a shrinking tube, the indicated dimensions are WITHOUT this tube, because it can be taken off, if there is not enough space.

Operating modes DCC, MM, DC, AC DCC, MM, DC, AC DCC, MM, DC, AC DCC, MM, DC, AC DCC, MM, DC, AC DCC, MM, DC, AC

ZIMO decoders and system devices are multi-protocol capable and support various digital and analogue operating modes. The line above indicates this for the respective decoder.

Possible operating modes:

digital   analogue  
DCC (Digital Command Control)
Standardised (NMRA) digital system
AC Alternating current,
on the three-wire track system (Märklin)
mfx  Märklin digital system DC Direct current, on two-wire track without centre conductor
MM  Motorola digital system    

NOTE: The Systrix (SX) digital system is not supported by ZIMO components.

Connections wires,
with NEM-652, or NEM-651(pins)
10 wires 21MTC PluX-16 /
7 wires /
" with NEM-652
21MTC Next18
Continuous Current 0. 7 A 1.8 A 1.8 A 1.0 A 1.2 A 0.7 A

The "continuous current" indicates the maximum output rate of the decoder (total), whereby average surrounding conditions are assumed. What limits the continuous current is the heat development; the integrated temperature sensor turns off the consumers when the PCB reaches about 100°C.

Displaying overload (= overcurrent): the decoder quickly flashes the headlights (about 5 Hz= 5 times per second); it restarts the decoder after the PCB cooled down about 20°C (hysteresis).

Function Outputs (total) 12 (9) 10 8 8 8

ZIMO decoders have different types of outputs. They vary between the families in number and resilience:

  • "Normal" (often called "amplified") function outputs as they come in decoders of all makes, these are (technically) "open collector" or "open drain" outputs to which headlights, other lamps, smoke generators, uncoupler coils and other devices can be connected. The second connection of the consumer can be connected to:
    • the "common positive pole" of the decoder (blue wire), or
    • the "low voltage output" (purple wire) if the decoder has one (MX632V, MX632W).
    • it is also possible to connect the second output of a consumer to the left or right rail (in some vehicles this must be done anyway, because the light bulbs are connected via the conducting chassis), whereby the consumer only receives current half the time (using symmetric DCC signal), i.e. light bulbs only have half the brightness (although it seems brighter than that for the human eye).
  • "Logic level" (often also called "unamplified") outputs, see below.
  • "LED outputs", see below.
  • Outputs for servo control, see below.
Current limiter Fu. Out. in total 0.7 A in total 0.8 A in total 0.8 A in total 1 A in total 1.2 A in total 0.7 A

For reasons of limited space, the output current of the function outputs is presented in total (all outputs together or in groups). The individual FO is strong enough, though, to cope with all the current, if needed.

In case of overcurrent, the output is not turned off immediately, but after a time in tenths of a second or milliseconds, depending on the amount exceeding the defined threshold. This, inter alia, makes a cold start of light bulbs possible. In case this is not enough, the function "soft start" was developed.

Logic Level Outputs - 2, alt. to SUSI 2, alt. to SUSI 2, alt. to SUSI 2, alt. to SUSI 4, 2 alt. to SUSI

"Logic level" (also called "unamplified") outputs provide a voltage level (0V for off, 5V for on) via an internal 10K resistor on an output (i.e. max. 5mA output current), which can be used via an external amplifier. This is usually done by the user, but the M4000Z by ZIMO is also useful; see instruction manual of the decoders.

WARNING: "logic level" outputs are NOT active by default, because they use the same contacts as the SUSI-interface (Data & Clock) and as the servo control wires. The user defines, how the shared contacts are used, by setting:

CV #124:     Bit 7 = 0  ->  SUSI      Bit 7 = 1  -> Logic Level

and CVs #181, #182 (if those are set, the shared contacts are used as servo control wires).

Servo Outputs - 2, alt. to SUSI 2, alt. to SUSI 2, alt. to SUSI 2, alt. to SUSI 2

Outputs for servo control wires; with these, standard servos (Graupner, Robbe, etc.) can be controlled, whereby different operating modes as well as end positions, cycle times, etc. can be defined in CVs #161 to #182. Decoders with a 5V low voltage output (MX687W) also provide the operating voltage for the servos.

The servo outputs use the same outputs as SUSI (Clock & Data) and are activated with

CVs #181, #182; SUSI is then deactivated.

SUSI - yes yes yes yes yes

The "SUSI" interface originally was designed, because it was difficult to place sound production in the decoder (regarding space and processor performance), and therefore individual sound modules were created which were controlled via SUSI data lines (Clock and Data) by the decoder. Further function outputs could also be realized that way. Those additional modules are partly still on the market, although the solution technically is not up-to-date anymore and related to functional limitations.

Most of the ZIMO decoders have "SUSI" connections, because these lines feature a fast transmission protocol for the communication between train (locomotive decoder) and attached cars (function decoders) via conductive couplings. Another use of the "SUSI" interface would be the connection to environmental sensors (tilt, lateral acceleration, GPS, ...) and train radio modules or balise readers in the vehicle. Last, but not least, it made more function outputs possible (see above).

Direct Energy Storage yes (25 V) yes (16 V) yes (16 V) - yes -

ZIMO always recommended the use of stay-alive capacitors (for all decoders); nevertheless, until 2009 the user had to interconnect additional components (throttle, diode, ... available in a set as SPEIKOMP or SPEIGOMP) around the capacitor. This was to prevent disturbing effects while programming, during loco number recognition and because of the in-rush current (high amount of power after start-up, which can overload the central command station).

For function decoders, capacities of up to a few 1000 µF are practical, values greater than about 10.000 µF are NOT recommended (possible overload of the charging circuit).

DO NOT connect Goldcap modules, because they can cause problems!

The energy storage unit on a function decoder is designed to prevent light flickering.

Low Voltage Outputs - adjustable adjustable - - -

Some decoder types contain a built-in low voltage supply, fixed with 1.5V resp. adjustable (type V), or 5V (type W), which can be used as an alternate positive pole ("blue wire"). The low voltage source is based on a switching regulator which is efficient and produces just little heat loss. Such a low voltage source is used to:

  • operate low voltage lights (usually 1.2 or 1.5V) like headlights or other light bulbs
  • operate LEDs, whereby the 5V version is used and the LEDs are operated via a 330 Ohm resistor - this way, less heat loss is produced than when operating it with full voltage via a 1K5 resistor
  • supply standard servos (5V), which's control wires use the servo outputs of the decoder

The use of low voltage supply is additionally advantageous, because it provides stable voltage regardless of fluctuations in the track voltage, and by connecting an energy storage capacitor it works without contact interruptions.

*) Info: The name of the Next18 function decoder MX689N18 was chosen for organisational reasons. This decoder is to be distinguished from the MX689 shown here before,
                    a project of a sound function decoder, the development of which is only being considered if there is a corresponding need or demand.

DOCS and INFO Connection diagrams can be found in the user manuals
User Manual Manual for all MX function decoders