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MSS OCCUPANCY BUS LAYER is the backbone of the system, carrying track occupancy status through the layout from train detectors (in the MSS Detection layer) to signal drivers (in the MSS Signaling layer). Each signaled track has its own dedicated MSS Occupancy Bus, such that multiple Occupancy Busses are run in parallel for multiple signaled tracks. The MSS Occupancy Bus utilizes CAT5 modular network cables and RJ45 jacks and couplers, commonly used for Ethernet communication networks and readily available at computer and electronics stores. The 8-conductor MSS Occupancy Bus has two primary wiring patterns, also known as MSS Elements:

1.
MSS Crossover Elements fill out the central portion of each signal block (i.e. between MSS Cascade Elements at the block ends). The wire pattern "crosses" two pairs of wires in the Occupancy Bus such that the MSS Crossover Element's two ends are symmetrical (in an MSS-equipped modular layout, this enables modules to be oriented either way around). This wire pattern is inherently present in off-the-shelf TIA/EIA 568A/568B CAT5 crossover network cables (note: other types of crossover cables *cannot* be used). Alternately, MSS-compliant products are available with the MSS Crossover wire pattern built-in.
Note: MSS Crossover Elements are not to be confused with railroad track crossovers that connect two parallel tracks.

2. MSS Cascade Elements define signal block boundaries, where one block ends and the next begins. The wire pattern "cascades" the track occupancy status from one block to the next, in both directions. This MSS Cascade wire pattern can be created easily by modifying a straight-through ("patch") CAT5 network cable. Alternately, MSS-compliant products are available with the Cascade wire pattern built-in. A variant of the MSS Cascade Element, called an MSS Complex Cascade Element, is used at junctions involving multiple signaled track routes and associated MSS Occupancy Busses.

These MSS Elements are linked together along the layout using off-the-shelf CAT5 crossover network cables. The result is an odd number of MSS Crossover wire patterns between each MSS Cascade wire pattern, regardless of the number of Crossover Elements between Cascade Elements.

Combining an MSS Occupancy Bus Element with elements of the MSS Detection and Signaling Layers (described next) forms an
MSS Node, essentially a complete functional "unit" of the MSS.


MSS DETECTION LAYER senses the status of a signaled track, both train presence and turnout position, and sends this information via the MSS Occupancy Bus to the MSS Signaling Layer. MSS-compatible model train detection products are commercially available. Modelers may choose whatever brand detectors they prefer (or may design and build their own), as long as one requirement is met: all MSS detectors *must* have an active-low, open-collector output.
Note: not all commercial detectors meet this requirement - verify this detail before purchasing!

The MSS eliminates the need for resistor-equipped wheel sets on every train car by using a combination of two train detection techniques:

1.
Current detectors sense trains that draw power from the track, such as locomotives and lighted cars. These pulse-transformer type electronic detectors activate when electrical current passes through the feeder wires of the signaled track(s), indicating the presence of a powered train.

2.
Optical detectors sense trains at the signal block boundaries where MSS Cascade Elements reside. An optical (or infrared) sensor placed on the signaled track activates an electronic detector circuit while any portion of a train (locomotives or cars) is located at the sensor, indicating the presence of a train at that particular location.

This arrangement of current and optical detectors ensures trackside signals hold their indications until the entire train (not just the locomotives) clears the signal block boundary, without the need for resistor-equipped wheelsets on cars. The MSS also monitors track turnout position so that signal indications are affected when a turnout is set against the signaled track, just like the prototype.


MSS SIGNALING LAYER consists of the electronic logic circuits, or "signal drivers", that control the indications of trackside signals mounted on the layout's viewable area. This layer receives status of the signaled track from the MSS Detection Layer, via the MSS Occupancy Bus. Signals are located only at signal block boundaries where MSS Cascade Elements reside (including junctions with MSS Complex Cascade Elements).

MSS-compatible signal driver products are commercially available. Modelers may have any style of trackside signal they desire (e.g. targets, tri-lights, position-light, semaphores, etc.) and may choose whatever signal driver type is needed to support their choice of signal (or may design and build their own), as long as one requirement is met: all MSS signal drivers *must* have active-low inputs with pull-up resistors to ensure valid logic highs when MSS Occupancy Bus wires are not driven by any detectors.
Note: not all commercial signal drivers meet this requirement - verify this detail before purchasing!

copyright 2005-2017
Modular Signal System - MSS
The Modular Signal System (MSS) is a standardized method for animating model railroad signals to react to trains moving along a signaled track without the need for expensive computers or complex software. Having trackside signals automatically change aspects in response to train movement adds significantly to the realism and enjoyment of operating sessions, not to mention enhancing visitors' viewing experience.

MSS simulates basic Absolute Block Signal (ABS) functionality in any model railroad scale and format, though it's especially well-suited for modular model railroads. Any number of signaled tracks can be supported, and any style of trackside signal can be used (e.g. target, tri-light, position-light, semaphores, etc.).

MSS uses low cost CAT5 crossover network cables for interconnections, and can utilize commercially-available model train detectors and signal logic drivers. This gives modelers the flexibility to select components that best fit their budget and specific signaling needs.

MSS is organized as three "layers" of hardware as depicted here.