Signalling System Outline

1.0 Signal Plant
A signal system for a model railroad consists of a series of Signal Plants.  A Signal Plant consists of a series of components and, depending on the type of plant, may include:

• Occupancy Detectors which detect whether an electrically isolated section of track (a block) is occupied by a train.  The detection method can be infrared, light interruption, or current sensing.
• Turnout Direction Indicators - a system of switch contacts mounted on the turnout or on the switch machine that determine if a turnout is thrown for the "mainline" (straight-thru) or the "diverging" route.   
• Signal Control Boards (SCBs) which take information from the Occupancy Detectors, Turnout Direction Indicators, and other Signal Control Boards.  Using this information, the SCB determines whether the signal masts that it controls should display red, yellow, or green.  (The SCB may also turn off all of the LEDs in its control if there is no train occupying the track that it controls.)
• Lineside Signal Masts located alongside the track display lights (LEDs) "telling" the train whether to stop, slow down, proceed, take the main line route or the diverging route of a switch.  (In our example, the lights are simply for display and have no control over the train.)  These different signal aspects (red, yellow, or green) are determined by the controlling Signal Control Board connected with that Signal Plant.  
• Wiring System - cables or wires that physically connect the components together and transmits information to/from the Occupancy Detectors, Turnout Direction Indicators, and Signal Control Boards so as to illuminate the LEDs in Trackside Signal Masts in red, yellow or green. 

(Insert graphics of each of the above)


2.0  East-West, Mainline Route - Diverging Route
The following conventions will be used when looking at trackwork within a signal plant.

2.1  East End - West End
An insulated section of track has two ends - the East End, and the West End.  A train entering the block is either an Eastbound train heading East or a Westbound train heading West.

2.2  Turnout Routes
Trains can go through a turnout in two directions - the Mainline Route, and the Diverging Route.  The Mainline Route is that part of the turnout which obviously takes the train through the mainline mainline.  It has two parts - the mainline entering route (MAINENTER), and the mainline continuing route (MAINCONT).  The diverging route is that part of the turnout that curves away from the main line. 

Turnout Plant

3.0  Types of Signal Plant
There are two types of signal plants

• Block plant
• Turnout plant

3.1  Block Plant
The block plant is the simplest type of plant in the signaling system.  It consists of:

• An electrically-insulated section of track
• Occupancy Detectors that determine whether a train is on that section of track
• A Signal Control Board (SCB) that processes the occupancy information on that section of track and information received from adjacent signal plants.  It then lights up the signal masts under its control according to the information it has processed.  It also passes its information on to the adjacent Signal Plants. 
• Lineside Signal Masts located at the EAST and WEST end of the boundary for that section of track. 

Block Plant

3.2 Turnout Plant
The Turnout Plant consists of

• A Turnout that will route a train through the Mainline Continuing Route (MAINCONT) or onto the Diverging Route (DIVERGE)
• Turnout Direction Indicators that indicate whether the turnout is thrown for the Mainline Continuing (MAINCONT) Route or the Diverging (DIVERGE) Route
• Signal Control Boards
• Lineside Signal Masts

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Signal System Criteria

Signalling systems can range from very simple to very complex - from simple block occupancy detection which simply light up some signal LEDs or changes the signal from green to red, to full CTC dispatcher controlled. 

I'd like to start off with a simple block occupancy system which lights up some signal LEDs or changes the signal from green to red whenever a train is in various parts of my modules.

So, here is my "statement of requirements" for my signalling system. 

1.0    Statement of Requirements

1.1    Off-The-Shelf Components
The core electronic components of the signalling system should utilize "off the shelf components" from a variety of manufacturers - signal masts, detectors, logic circuits, connections between electronic components.  At a later date, as I gain more experience, this should not prevent me from building some of my own electronic components (eg block occupancy detectors, signal control boards), so long as they are compatible with the "off-the shelf" components I use in this signalling system. 

To the extent possible, electronic components should be capable of being used for more complex signalling systems with minor changes so that I don't lose my investment in the signalling system.  However, I recognize I may lose some of this investment if I ever convert to more complex signalling systems such as CTC dispatched which are controlled via a dispatcher and a computer. 

1.2    Basic Components
There are  basic components to a signalling system (the operative word here is "basic")

• Block detection circuits (eg NCE BD20, Digitrax BD4, optical/ infrared detection)
• Logic circuits or "Signal Control Boards" (SCBs) on printed circuit boards that control the LEDs on the signal masts (turn on, turn off, red, green, yellow, etc) (eg Atlas Signal Control Board, Digitrax SE8C, etc)
• Signal Masts that display the LEDs on the signals (red, green, yellow, off).  These Signal Masts can display a single light, two lights, three lights and have one or more targets.  LEDs can be cathode (negative) common or anode (positive) common.  Some SCBs can control either cathode common or anode common LEDs simply by using a "shorting plug" across pins on the printed circuit board.  Other SCBs can only control one type of common return.  See 3.0 below for further details. 
• Switch contacts on turnouts which electrically indicate whether the turnout is set for the mainline or the diverging route. 

To the extent possible, these basic components should be "off-the-shelf" items from various manufacturers. 

1.3    Component Connectivity
Connecting these 3 basic components all together is some physical wiring. 

• Wiring from the rails to the block detection circuit (if detection is through current flow);
• Wiring from the block detection circuits to the SCBs (PC boards that contain the logic circuitry);
• Wiring from the SCBs to the signal masts;
• Wiring from one SCB to other SCBs.  

To the extent possible, the wiring and connectivity should be "modular" between the various signal components (eg RJ12 cables/JST connectors between SCBs, signal masts, and between modules. 

For example, connections between the SCBs and the signal masts could be 6-wire RJ12 or 8-wire RJ45 cable if this provides the right number of wires for the system.  Or, it could be 10-wire ribbon cable provided that the ribbon cable is rigourous enough to withstand the handling that our modules are subjected to. 

To the extent possible, connecting/ soldering individual wires between components should be kept to a minimum. Where soldering is required, it should be done at the workbench and not underneath the module.  

1.4    Module Independence
As a first step, I want a simple block occupancy system which lights up some LEDs in a signal mast and changes the LEDs from green to yellow to red whenever a train moves through the various parts of my modules.  For example, as the train moves through my modules, I would expect the signal masts in the forward direction would show green or yellow - "Proceed", and the signal masts behind the train would show "stop" (red). 

Each module should be wired so that it's minimally dependent on other modules for signal operation.  Specifically, block detectors should be contained within the module where it is detecting traffic and should be located within or as close as possible to the track section it detects.  SCBs should be located within the module containing the signal masts and block detectors that are feeding the SCBs.  SCBs should also be located close to their respective signal masts.  

Signal wiring between block detectors, SCBs and between other modules should be modular with a minimum amount of wires as possible. Wherever possible wiring should be "plug-and-play" using wiring systems such as RJ12/45 cable, ribbon cable, etc.  Because my modules are subject to transportation, these wiring systems should be rigorous and as "sturdy" as possible. 

1.5    Signal System Expansion
The system should be readily expandable using the same "plug-and-play" principles and with minimal of change. 

2.0    Block Detection
There are two types of detection systems

• Position occupancy (eg passing a specific spot such as a signal mast)
• Block occupancy (eg being in a specific section of track) 

The signalling system should be capable of using both types of occupancy detection systems.  For example, signals could be triggered by block occupancy detectors (such as current-sensing detectors) as the locomotive enters the block but position detectors (such as infra-red detectors) located at the signal boundaries could hold the signal red until the entire train has left the block. 

Because freight and passenger cars come from a wide variety of members, block detection should not depend on resistant wheel-sets to detect block occupancy.

2.1    Position Occupancy Detection
There are two types of position occupancy detectors -
Optical detectors usually require adjustment according to the lighting in the room and the light around the detector.  This usually requires adjusting the sensitivity of the detector through a small potentiometer.  Since the detection unit is usually located underneath the module, this is not a recommended position occupancy detector. 

Infrared detectors have their own infrared light source in addition to the detection unit.  IR detection circuits are of two types -

• Beam-Blocking - turn the signal to red when the IR detector can't detect the IR beam from the IR emitter (ie the IR beam is broken).  With beam-blocking detectors, the IR emitter is located on one side of the track and the IR detector is located on the other side of the track.  A passing train breaks the beam thereby triggering the detection circuit.  
• Reflecting - turn the signal red when the IR detector detects the IR beam from the IR emitter (ie unbroken beam).  With reflecting detectors, the IR emitter and IR receiver are usually located side-by-side either between the ties in the track or on one side of the track.  As the train passes the IR emitter, the IR beam is reflected from the underside of the train back to the IR detector, thereby triggering the detection circuit.    

The Optek Optoelectronic Reflective Arrow types of IR emitters/ detectors such as the OPB704W detector have both units angled so that the IR emitter will reflect the light back to the IR detector when the IR beam is intersected by a freight car going over the top of the emitter/ detector.  The Arrow emitter/ detector is in one package that can be installed between the ties in the track. 

2.2    Current Detection
There are two types of current detection circuits -

• diode detection
• transformer detection. 

Only transformer detection can be used with DCC.  A current-sensing transformer detector is readily identified by one or two large black squares (about 1"x ¾"x¼) or ovals with a 3/16" hole in the middle that is mounted on a PC board. 

With transformer detection, current flowing through one side of the transformer (the primary winding) induces a current to flow through the other side of the transformer (the secondary winding).  Current flow through the secondary winding activates the circuitry in the block detector. 

This requires that track feeder wires are connected to the detection unit.  This can be done in two different ways:

• "Direct connect" requires cutting one of the track feed wires and screwing the wires under a screw-down terminal.  The detector must be located close to the track feeder wires.  Or track feed wires have to be run "longer" distances from the block detector to the rails. 
• "Indirect connect" where a track feed wire is looped through the "hole" in the middle of a detection transformer.  The transformer and detector can be located amongst the track feeds with wires running to the SCBs.  Some transformers can be unsoldered from the detector and located right within the track feeds.  Wires then run from the transformer back to the detector.  Some detection transformers that are soldered to the PC board can be unsoldered and placed next to the track power feeds with wires running back to to the PC board. 

Wherever possible, transformer-type block detectors with track feeder wires looped through the centre of the transformer should be used as this allows maximum flexibility in the placement, fastening, and wiring of the block detector.  This type of detector reduces the amount of wiring required and can be placed in close proximity to the signal logic circuits. 

2.3    Commercial Transformer/ Current Block Detection
There are a number of off-the-shelf transformer-type block detection circuits that we will consider. 

• Digitrax BD4 is a "direct-connect" transformer-type detection circuit that contains 4 independent block detectors on one PC board.  Connection to the track power feeds is via a 5-wire terminal block.  Pins 1 to 4 are connected to their respective block.  Pin 5 is the return side.  This implies that track feeder wires will have a longer run where the blocks are far apart.  In addition, connection to signal logic circuits such as the Digitrax SE8C is via a 10-pin ribbon cable.  The BD4 also has a 10-pin ribbon cable plug for "block monitor" LEDs.  The Digitrax BD4 appears to be limited in its connectivity to signal logic circuits other than the Digitrax SE8C.  The SE8C appears to focus almost solely on CTC, rather than automatic block lighting.  Cost of the BD4 is about $32 or $8 per block.  While the cost is reasonable, reject the Digitrax BD4 as a block detection circuit because of the absence of connectivity between different signal logic circuits, the fragility of the connection to the SE8C, and the length of runs for connection to the 4 differently located track power busses. 
• Digitrax BDL16 is the 16 detector version of the BD4 (ie 16 detectors on one PC board).    This one has not been considered due to it being very cumbersome. 
• NCE BD20 is an "indirect connect" transformer-type detector that contains one block detector on one PC board.  For connecting the block to the detector, one track feed wire is passed through the donut hole.  Detection is very sensitive with the track feed wire being simply threaded through the donut hole.  The BD20 transformer can be unsoldered from the PC board and located next to the track feeder wire with two wires running from the transformer back to the BD20 circuit board. 

3.0 Compatibility Issues
Not all components (block detection, signal control boards, signal masts) are compatible with each other.  This occurs in two major areas -

• Is the system computer-controlled?
• What is the common return for the unit - cathode (negative) common or anode (positive) common)?  This is particularly important for Signal Control Boards and Signal Masts using LEDs. 

3.1 Computer Controlled Versus Detection Controlled
Signal systems such as the Digitrax SE8C Signal Control Board require programming through JMRI software installed on a computer and the Locobuffer USB.  Locobuffer USB physically connects the LocoNet to a computer.  Once the SE8C is programmed, the computer must still remain connected to the LocoNet as it is the programming in the computer, along with the signals received from the SE8C that determine which lights in the signal masts will be turned on, off, etc. 

As we want our signal system to be independent of LocoNet and other modules, we have rejected this form of signal control.  Setting up a signalling system using the Digitrax signal system is very complex and well beyond the capabilities of all but an experienced electronics guru. We have therefore opted for a signalling system that is not dependent on a computer whereby signals from detectors in the blocks before and after the signals passing through a Signal Control Board (SCB) determine the colour aspects of the signal masts.  This type of system therefore emulates an Absolute Block Signal (ABS) system on the prototype. 

3.2  Common Return - Cathode Or Anode?
Another issue to be considered is the polarity of the components in the signal system. The majority of LEDs in Signal Masts are wired with the anode (positive) as the common return.  This means that the Signal Control Board must also be wired with the anode (positive) as the common return.  

Signals on prototype roads are either searchlight signals or D-type signals whereby each target on the signal mast can show red, yellow, or green.  The only way to accomplish this 3-colours-in-one-LED is to use 3-wire bi-polar LEDs which has a cathode common return.  Therefore, any system using the anode common return can't be used.  

We will restrict our signal system to cathode (negative) common components. 

(Note - Some Signal Control Boards can accept both anode common and cathode common returns.  These SCBs, however, have been very expensive.)

4.0  Connectivity Between Modules
Connectivity between modules might done with readily available RJ12 or RJ45 cables.  In any event, we want

• Plug and Play connectivity that accommodates module reversing and relocation anywhere in the layout
• Does not need to be reprogrammed
• No computer required to drive signal aspects
• Designed around prototypical ABS and accommodates Red, Green, Yellow, Flashing Yellow
• and off for approach lit
• Allows for expansion to CTC with DCC integration

We discard connectivity with RJ45 cables as they are too bulky, are not readily known to the hobby, and are relatively inflexible when compared to RJ-12 6-wire cables.  

The RJ-12 6-wire system has advantages over RJ45 in that:

• The hobby already knows about RJ12 and its cousins RJ11 4-wire as these systems are used in Digitrax, NCE, and Lenz DCC systems.  In addition, they are used by Atlas and Custom Signals in their signalling systems.  
• RJ12 and RJ11 are readily available in most building supply stores and electrical supply houses
• Relatively inexpensive to buy components as compared ot RJ45.  
• Relatively easy to fabricate cables (a crimper is the only tool that is needed) as compared to RJ45.  Most hobbyists already have the experience from their DCC systems.  

On the other hand, if information can be transmitted along one or two wires from one plant to the next, this would be the simplest of all connectivity systems.