Serial User System Interface - SUSI

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Summary: SUSI is an interface between DCC Multifunction Decoders and additional devices.

Short Definition

SUSI is an interface between DCC Multifunction Decoders and additional devices.

SUSI - Serial User Standard Interface

Interface definition between decoders and external extension modules (“SUSI-modules”)


Developed by Dietz the SUSI interface allows a multifunction decoder to pass information on to further devices - up to three (3) SUSI modules.

SUSI Applications

At present there are two core uses for SUSI-modules.

Sound Modules
Modules to generate sound based on the messages from the decoder. For example this allows the sound to change based on speed changes, as the decoder is requested to reduce speed the SUSI sound module could play a brake squeal sound.
Additional functions
Modules to increase the number of outputs available. These are defined as Auxiallary outputs. The spec. allows for Aux. outputs A-P to be defined. These can then be mapped (as with on-decoder functions) to DCC functions e.g. F5 triggers AUX Output D.

Some SUSI modules can control external devices such as a smoke generator. By using the SUSI interface, the smoke unit can respond to outputs from the decoder for more realistic effects.

SUSI can also be used as an interface to a LISSY module, a communications technique similar to RailCom except it uses Infrared Light.

With the arrival of multifunction decoders with integrated sound, the need for SUSI declined. SUSI devices are still available, often used with large scale decoders.


SUSI and the SUSI-LOGO were developed by

DIETZ MODELLBAHNTECHNIK Hindenburgstraße 31 D-75339 Höfen Germany



The connections between the decoderr and the SUSI module are by 4 wires:

Pin Number Signal Wire Colour
1 Ground Black
2 Data Grey
3 Clock (CLK) Blue
4 Power Red

PLuX connectors also support SUSI.


A specific set of CVs has been assigned to SUSI modules to store their settings: 897-1024

As you can connect up to 3 SUSI modules to a host decoder the CVs are split into 3 ranges to prevent conflicts between SUSI modules. These are selected by setting CV 897

Name CV Range CV 897
SUSI-I 900 to 939 1 ( or Zero)
SUSI-II 940 to 979 2
SUSI-III 980 to 1019 3

Full details of the CVs can be found in the NMRA SUSI 1.3 Specification

Decoders with SUSI Support

A number European DCC manufacturers offer compatible decoders and SUSI devices.

Multifunction Decoders

SUSI Modules

Further Reading