Block detection or, more specifically, the ability of the software to detect the presence of a train in any given section of track, is critical to any computer-controlled layout. The reason is simple: knowing where trains are is a fundamental requirement if a computer is to make decisions about what to tell a train to do. If it doesn't know where a train is it can't tell it to move somewhere else.
We solve this problem by splitting our layout into a number of sections, or blocks. Each is longer than the length of our longest train, so every train can move into every block and stop there if necessary without impeding a train in any other block.
The Train Controller Block layout also reflects the physical block layout
Each block's wiring passes through a device (in our case the Digitrax BDL 168) which then sends out an occupancy message across the system bus (LocoNet) to alert other devices that a train is present. This includes the control software, which can then issue other commands based upon this occupancy knowledge.
Occupancy, or block detection, is the eyes and ears of computer control.
There are over 40 blocks on the original layout which are capable of holding a full length train of eight carriages or its freight equivalent. When TrainController sends a train around the layout it expects the train to enter the next allocated block. When the train does this the BDL168 broadcasts an occupancy message over the LocoNet bus which will be received instantaneously by every other connected device.
We have put occupancy detection on all sections of track including points and track sections too small to be a block. The computer software has a safety feature that will not release a path through a junction until it has become unoccupied. To take advantage of this, we have glued small 10K Ohm resistors (surface mount technology) bridging one axle of every wagon and unlit carriage. The small flow of current across each axle ensures that the occupancy sensors indicate to the computer when a complete train has left a block, and not just the location of the engine at the front of the train.
Axle with resistor attached
We decided at the outset of planning the original McKinley layout that if computer-control was the aim then block detection had to be rolled out in a logical and complete way. No track is left uncovered by this effort, which has been a significant project in its own right. But it has been well worth the time, effort and cost, and the equipment we chose has proven to be very reliable over a long period of time which, in turn, has given us years of largely trouble-free computer-controlled operation.
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