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Fun with servo based interlocked lever frame

Fen End Pit


Over the last few months I've been working on a cunning plan to add interlocking to my lever frame. The frame itself is from the Shropshire and Herefordshire Area Group sold through the Scalefour stores. The frame is connected to a MERG CANACE3 circuit board so that each level generates a different event on the MERG CBUS. This means that the only thing coming out of the frame to the layout is the 12v and 0v power lines and the CANH and CANL data wires of the CAN bus.




This has been fitted with a small modification in the form of a strip of nickel silver soldered into the bottom of each lever made from scrap from the etch. This 'tail' sticks out of the bottom of the frame.




The locking is provided by means of a small servo under each lever which is fitted with a brass collar. The collar rotates to either lock the tail of the lever on one side or the other or present a slot through which the tail can pass.




The servos are driven using MERG CANACC8 boards configured with the new CANServo8 code. Last in the box is a MERG CAN-USB board which allows the CBUS to talk to a PC.




The PC runs the PanelPro from the open source JMRI project. Without going into too much detail (unless people want to know more) the levers are each defined as a 'Sensor', the turnouts on the layout and the servos which control the locks are defined as 'Turnouts'. I have set the CBUS boards on the layout which drive the points and signals to respond to the events direct from the lever frame so that I don't need to connect the PC. However when the PC is connected the 'Logix' programming in JMRI is configured so that each locking servo is set depending on the state of the levers.


The great virtue of this system is that I can program all the locking on the PC and change it easily if (when?) I get it wrong. In practice so far I have got about half the locks working (there is some alignment work to do between the tails and locking rings) but have demonstrated I can make the distance signal only be released when the relevant home and starters are cleared. Also I can show that clearing the distance signal locks the homes and starters clear. In simple terms each lock has two conditional actions applied, one which unlocks the lever when A normal and B normal and C reversed etc. and a second which locks the lever when A reversed or B reversed or C normal etc.


I could even get silly and add some train detection by track circuiting and then interlocking with that, or even only allow the starter to be cleared when I have a line clear from the box in advance.... or may implement locking that allows me to only pull the starter once per line clear.... This way madness lies.



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I currently use the Scalefour Society's Shropshire and Herefordshire Area Group designed lever frame as designed with the microswitches connected to relays and ultimately Tortoise point motors but at Railex there was a layout using relays to lock point and signals. I am thinking of adding interlocking to my layout at some stage but don't want to go the whole hog and using DCC accessories decoders etc.


In your system how do you change your signals and points, are they still connected to the lever frame or what?

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  • RMweb Gold

The CBUS system as designed and sold by the MERG includes circuit boards to control 8 relays/servos, a board called a CANACC8. There is also a board which can connect to 64 switches for input and 64 LEDs for mimic panels on control panels. In simple terms each point motor or signal is connected to an 'output' board and the lever frame is connected to an 'input' board. The connection between the board is a 2 wire bus plus 12v and 0v power, i.e. 4 wires.


The model is one of 'producers' and 'consumers', the lever frame or any other switch is connected to a 'producer' board which then sends out an 'event' in the form Node number Event number. A consumer board is then configures so that it learns that Node X event Y means turn this output on or off. There is a simple method of teaching events which doesn't require a PC configure using dip switches on the board or a more flexible solution where you can set the whole lot up from a PC (though the PC is then not needed once you have set things up).


The design of the system is separate from DCC and just greatly simplifies the wiring, particularly of a big layout. The costs are also very reasonable. Most of the boards are ~ £12.00 a typical layout might need one or two input boards for control panels and then one or two output boards per baseboard depending on the amount of devices you need to control.


for more details take a look at the merg cbus site


My system is set up so that the signals and turnout respond directly to events produced by the lever frame. The PC only comes in to play if I want it to also understand the state of the levers and move the locks. With no PC connection the lever frame is all unlocked and just behaves like a conventional model railway control panel.



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  • RMweb Gold

David that is really nice! I will have one for my layout!


Thanks for posting,





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David, I like the method you have used for the servos to interlock the levers. I was thinking of using the same method to interlock my lever frames, but I'm thinking about using an industrial PLC to control the locking.





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