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: Do not confuse MSS Crossover Elements with railroad track crossovers connecting 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!
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.