I have read through the link above and everything is making a lot more sense. I think your effort in designing a new modbus wingboard is amazing!
However, I am in two minds given my current situation which requires deployment of 10 Mayfly stations within the next 3 months. Obviously waiting for your wingboard is the more reliable option, but I only need ten functioning boards and the adapter modules are cheap. We also have a heap of printed PCBs ready to use. I’m thinking I might ‘give it a crack’ with cheap modules from multiple sources and see if I can get them to work, perhaps with a 120 ohm resistor attached. It might be a frustrating process but I’m sure I can get some working.
Saying that, I fully support what you are doing and would definitely be interested in your boards if my attempts fail.