Selectrix (sometimes referred to as SX) is a digital model train control system developed by German company Doehler and Haass for Trix in the early 1980s. Selectrix is based on a data communication protocol originally developed by Siemens for communications between mainframe computers. Döhler & Haas offered their system to several model railway manufacturers in the 1980s. The only interest in the system came from Trix GmbH and resulted in a licensing agreement between the two companies, giving Trix exclusive marketing rights.
In 1999 the two companies mutually agreed to terminate their licensing agreement. Since then, Selectrix has become a widely recognized open standard and is supported by several equipment manufacturers in Europe. Selectrix is especially popular in smaller N and Z scale model railways. European Model Train Association, MOROP has covered Selectrix data protocols partially in their NEM standards.
Selectrix first appeared in 1982, and later was rebranded in 1987 as Trix Selectrix.
Selectrix uses one data bus for controlling everything, including rolling stock, accessories, and includes feedback from various system components.
The data bus is bi-directional and synchronized. This makes it possible to integrate many types of automatic functions without a separate computer and software by using only a Selectrix central unit. Because the data packets are sent and received within a fixed timeframe, system speed and reaction times are not dependent of the number of decoders controlled, as is the case with other systems, most notably NMRA DCC. Selectrix is also very resistant against data communication errors.
Selectrix Signal Format
The Selectrix data format seems at first glance to be similar to the FMZ format. Positive or negative pulses pulses follow each other. The pulses are separated by a Pause Cycle. The pulses are 40 microseconds long, breaks are 10 microseconds.
In Selectrix, the problem of the voltage reversal is solved quite differently. It is not whether the voltage is positive or negative, but each pulse is compared with the previous ones. A pulse that has the same polarity as the previous one is a logical "0", a pulse which has a different polarity than the previous one is a logical "1". Unlike the digital packet used with DCC, where time determines the value of the pulse, and a complete period requires a cycle of positive and negative voltages.
Therefore the resulting voltage is not necessarily zero volts on average. There may be a residual voltage level, but that is not a problem as Trix never sought to mix digital and analog modes.
Details of the SX protocol can be found in the NEM 680 and 681 on the Morop Site.
The term SX is an abbreviation of SelecTriX
Manufacturers of Selectrix equipment are Rautenhaus, Müt-Digirail and MTTM. Selectrix components from various manufacturers are compatible at all levels and can be freely combined in real-world Selectrix control solutions.