Made progress. Have tested a number of different configurations and now have something (renderings) to show for it. Thank you http://3dbrdviewer.cytec.bg/ 3D Board Viewer for the images.
Reminder – long post about where this came from. Now it’s about building it.
The main board is simple with 2 LEDs on it. By default it receives serial data, runs it through the LEDs (Neopixels), and then out the other side again as serial (rs485). There is the need to source the signal from a micro controller. It was desirable to not make a special board for this. To that end there are different traces on the board that can be cut to change the behaviour of the one board. Cut a single trace and a pin (header of three pins) can be used as ttl data IN for the first LED. Cut a different trace and you bypass the LEDs but get the benefit of converting the ttl to serial for the benefit of the next board and so on in the chain. To make it even more fun cut a pair of traces and the IN and OUT of the board are now capable of driving an arbitrary local set of LEDs. To demonstrate this feature a mini shield board was designed with up to 8 LEDs on it. Solder pads on the shield can be used to select for the number of LEDs you solder to it.
Raw parts and board costs for the main board are ~$10 currently. A run of many many boards and part selection optimization could bring this price point significantly lower. Price is noted here but not yet a critical goal.
Don’t those look just fantastic? Here is the board layout before it is rendered.
The shield was a fun bit to try to make. Again these boards are working only in theory today. Parts and boards are in the process of being made and shipped. Once they arrive soldering and testing of these final solutions will commence.
Here is a rendering of the shield with the accompanying board design image.