Page 11 - November December 2009

complex, realistic operation of a signalling system in response to all of the normal events that happen on a railroad.
DTM30 board
The DTM30 is designed to automate user-provided layout control panels or CTC panels. This is to allow control and display of points, block occupancy sensors and other devices on a layout. All of this is controlled through the DCC and Loconet® buses without requiring any further wiring to the accessory devices themselves.
The board has 30 control “cells”. Each cell controls and/or displays a layout device; it can have one or two LEDs and one or two pushbuttons.
Digitrax BD4 – four-block occupancy detection board
Provides a direct output into the SIGM20 board regarding the occupancy state of up to four occupancy blocks – does not have any Loconet® messaging capabilities but outputs a logic state directly into the SIGM20 signal control board.
Signals
To date, the best scale signals available in N scale have been those produced by Sunrise Enterprises, and they represent the US-style “searchlight” target configuration favoured by many Western US railroads. They are based on the SP/ATSF prototype. Unfortunately Sunrise no longer make these signals, but Craig Martyn of BLMA Models will soon be releasing working proto- types in N scale. If you feel ambitious, you can purchase Craig’s non-operating signal heads or Train Cat Models signal kits to make up your own signals using bi-colour or tri-colour surface- mount LEDs (more on that later). Other manufacturers to con- sider are Atlas and Tomar Industries (both slightly over-scale in my view) and a Canadian company called Scale Models Division.
Design of the TCS/CTC interlocking plant
Defining the interlocking logic
• Note 1: Switches 501–507 connected to DAC10 accessory decoder, switch 508 connected to Digitrax Twin point accessory decoder with CDU.
• Note 2: Switches 503–507 operated via a local panel for switching purposes. The “state” of these locally controlled switches is reported over the “Loconet” to the SIGM20 Signal board and DTM30 CTC panel.
• Note 3: Switches 501, 502, 508 are operated by dispatcher from DTM30 CTC panel.
• Note 4: Signals operate automatically and are controlled by SIGM20 board – they can be set to red by a switch on DTM30 CTC panel.
Remote control of the interlocking plant
This is definitely an exercise in planning and will test your ability to document each switch, LED identities and locations, not forgetting your soldering techniques, to the maximum! I pur- chased a suitable control panel together with LEDs, momentary switches, plugs etc from Rapid Electronics Ltd. The front panel design was created using “Open Office” drawing package and was printed on heavyweight glossy photographic paper which was then laminated, and the holes for the switches and LEDs were punched through to match their approximate size. The panels were then glued into place on the aluminium panel incor- porating the switches and LEDs (photo of panel in operation is shown on page 13).
Note: this panel only has control of switches and running direction. Signal aspects are controlled automatically by the SIGM20. However, all signals can be set to Red with a “shunt” switch.
Wiring considerations
Good planning is essential here! We have two major wiring activities to consider.
The signalling side
The first step is to consider the wiring requirements between the signals, block detectors and SIGM20 board. For this step I used 40-way multi-coloured “ribbon cable” with “IDC” connectors – although this style of ribbon cable is more expensive than the
standard grey product, it really makes life easier when checking the wiring out prior to and after commissioning. I also used the Digitrax-manufactured ( TMSK) signal connector boards to maintain a standard IDC connection interface between the indi- vidual signals and the SIGM20 board. A schematic of the adopted design is shown on page 13 together with an actual photo of the completed installation.
The track side
Our modules consist of a main Rail A and Rail B DCC power bus, with feeds being taken off along the bus to feed into the track. To implement block detection the Rail A track is sectioned to create the required number of blocks for the signalling to work correctly. These sections need to be isolated from the main DCC Rail A bus and fed into the appropriate block detector. With modules this unfortunately requires a considerable amount of additional cable and connections between the modules.
Interface to other signalling systems
This is one of the great benefits of using a DCC-based system. All devices, be they switches or signals, are assigned a unique “accessory decoder” number. Our group assigns blocks of these numbers to each member to ensure that no conflicts can occur for different combinations of modules. I have been assigned a block of numbers between 500 and 600. Numbers 501–508 are used for my switches, whilst numbers 520–535 are used for my signals, direction control etc.
It is very straightforward to include a signal on an adjacent module to interact with my signals, switch states, traffic direction and vice versa. This can easily be achieved by simple re- programming of the SIGM20 board to include other signal numbers within the logic.
Working with the system
What’s good – in no particular order
1. It’s great to watch signals and switches that operate prototypi- cally.
2. This passing loop gives our dispatcher the ability to regulate train operations over what is, in principle, a single main line – just like in the prototype!
3. It greatly enhances the existing visual/diorama presentation of my modules.
4. On a personal level it was a challenging yet satisfying project to undertake.
What’s not so good – in no particular order
1. The additional bus wiring and connectors required to make the block detection work whilst maintaining compatibility with other members’ modules.
2. For the block detection to work prototypically, resistor wheel sets would need to be fitted to a significant proportion of the stock – in the short/medium term this is prohibitive in terms of cost to each member of the group.
3. The cost – at an approximate estimate to signal and control 13 ft and siding and main line I spent in excess of £550!
What I would change
At extra expense I would fit an additional SIGM20 interlocking board to control the “West End”. This would enable me to elimi- nate the 12 ft of 40-way ribbon cable that links my first and fourth modules and remove some of the additional track feeds used for block detection that are currently carried across each board.
Useful references/acknowledgements
This list is by no means exhaustive, but includes some of my favourite sites which will hopefully be useful for any of you considering taking the plunge into signalling to enhance your railroad operations.
Prototypical signalling information
www.lundsten.dk/us_signaling/index.html
Signal control logic
www.cmlelectronics.co.uk www.digitrax.com
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