Well, you've all heard that line before from me, but I have begun to make some serious planning progress on the computer control system that will one day power Burrafield Junction, so here goes.
Initial Requirements
The basic requirement to allow computer control of any model railway is to permit the system to examine the track-circuits and turnouts, and to form a 'memory map' of the locations of all the trains. If the computer can see where the trains are on the track, it can also clear the signals and sections when a train has passed, and using the wonderful Automatic Route Setting system from last month, an automatic timetable can be implemented to run the trains while you enjoy the layout by only driving.
Track Circuits
The most simple track circuits can be micro-switchs under the track that switch as the weight of the train pushes down on them, or reed-switches can be used if you don't mind fitting magnets into all your stock - these register when a train passes a particular point on the track and are fine if you don't have such complications as loco changes or trains splitting.
For the gizmo merchants out there, I would recommend some form of electrical track detection such as Roger Amos' EDOTIS whereby a detector looks for current flow in the track section and lights an indicator when it finds something like a motor or tail-lamps.
Turnout Feedback
This is an easy one - just have a SPDT switch under the turnout that switches with the solebar, and this will provide a signal based on the position of the turnout.
To get the first phase underway, it would be desirable to add visual feedback to the existing control panel.
My usual method is to have the control panel as a large line drawing (using 5mm wide painted lines) with all of the section switches midway between each section and push-buttons fitted around all of the turnouts.
Now if we were to add those section indicators and turnout indicators, then our panel would begin to look like the one below.
Now to get a train from 1 to 8 requires changing turnouts A and C to the curve position, and powering up sections 1, 5, 7 and 8, and all of our green lights will light up in this pattern - sections 2 and 4 will have to be blocked to ensure that no collisions occur.
Now lets add some track detection components and see what this can do for us.
With a detector checking section 1, the tail of the train leaving the section will release the track detector, and the 'interlock' release will permit use of sections 1 and 3 as long as turnout A changes to straight.
Section 1 needs changing to another circuit, turnout A needs changing, and section 3 needs connecting the the same circuit as section 1 ..... that's a lot of switches.
By now, our train has cleared section 5, and suddenly sections 2 and 4 will be released - disconnect section 5, new circuit connects for section 4, then 5, then 2 (as long as turnout B is set straight) ..... good chance of some mistakes if you do this often enough.
If the control panel is lit up with indicators (our little green lights) which change direction with the changing of the turnouts and the switching of sections, then life would be a whole lot easier and the chance of a pile-up may be reduced, but still a lot of button-pressing seems to be done.
Now there has got to be a better way.
In the Computer Control world, our train is noted leaving section 1 and the section status is set vacant - the Route Selection program notices this and is able to set a new route through 1 and 2 as long as turnout A is required to go straight, else it will wait until section 5 clears before attempting to engage section 1 (as our control system keeps a single section clearance between trains to cover against emergency stops !)
When section 5 clears, the next train can proceed forwards if required, but our system is smart and knows that the end-section is a dead-end and is already full of another train (because section 8 is showing as engaged), so proceeding into section 5 would block the path of 2 and 4 - the obvious thing to do is to hold the next train in section 1 before it reaches turnout A, unless user-override permits it (possibly to permit a following loco to take the train back out again !!)
Under Computer Control, you've probably only had to hit one button and that is to acknowledge a call-on into the next section, and because the system switches power on in sections as it selects the routes, then any ham-fisted driving through signals (known affectionately as SPADs) will only end up with a train in a dead-section and alarms flashing on the control panel ....... with members of the modelling Railway Inspectorate visiting to see why you jumped the red light !!
Before I roll in the PC and get out a bucket full of TTL chips, let's review another method ..... but that can wait until next month.
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