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Signal Technology



The Friends' signal system is an accurate model of prototypical CTC signal systems. It is designed to ensure the safety of trains travelling in opposite directions on the same stretch of track.

Current of Traffic
Like prototypical systems, the circuitry maintains a "current of traffic". This means that at any given moment, there is an established direction in which trains are authorized to travel.

Having a current of traffic allows multiple trains to be allowed into a section of track. Trains can be allowed into an already-occupied section of bidirectional track if they are going with the current of traffic. In other words, if they are following the other train. Trains are not allowed to enter the occupied track against the current of traffic because that would result in a head-on collision.

Track Circuits
The system uses the rails for communication between the signals. Electrical pulses are sent in the rails to form a coded track circuit. The pulses are sent in both directions once every second. Fourteen different coded values are possible creating a digital communications link between signals. A continuous series of pulses is required before the system assumes that the track is vacant. The presence of an axle across the rails interrupts the communication between signals. This interruption is interpretted as an occupied section of track.

The current of traffic can only be changed to the opposition direction if the section of track if completely vacant. Changing the current of traffic requires a negotiation between the two endpoints of a bidirectional section of track. Naturally, this communication can only occur through the rails if the entire segment is vacant.

Control Points (CP)
Control Points are locations that are involved in the negotiation of the current of traffic. These locations have signals that allow or hold trains to/from a bidirectional section of track. Examples include queuing track locations such as Farmersville, wyes and stopping sidings such as Witcombe Wye, and meet tracks such as Coyote. These locations contain a proprietary circuit board, containing 32 inputs and 32 outputs, and a 8051 family of microprocessor. The boards operate from a 6v 5Ah lead-acid batery that is charged by a solar cell. These boards also contain an XBee radio module operating in the 900MHz range for communication back to the Central Station computer. Currently there are three types of control points implemented.

Intermediate Signals
Intermediate Signals are found between control points. These signals act as repeaters in the negotiation and communication process that occurs through the rails between control points. These signals contain a proprietary circuit board that fits inside the head of the signal. The board serves to hold the signal's LEDs in proper alignment against the signal lenses. These boards operate off of three NiCD AA batteries and are kept charged with a 1/4 watt solar panel.

Signal heads are built from 3/4" conduit bodies. The face is cast resin.

Fraen lenses are glued to the back of the resin face.

The circuit board contains all the circuitry for the intermediate signals and fits inside the conduit body. The board also aligns the Luxeon I LED against the lenses.

A newer design of board uses Luxeon Rebel LED and Carclo lenses.

The control circuit at sidings is more sophisticated. These boards have 32 inputs and 32 outputs. They also contain an XBEE radio.

Copyright 2007 and succeeding years -- Friends of Train Mountain

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