Hi Dan, wow interesting it works to 3.2V. Yes RS485 should work well and it can go a long distance.
You do need a physical RS485 driver wing board that will fit with the UART pins. RS485 is a specific differential voltage protocol that can travel thousands of feet on two wires. The instrument requires 4 wires. See above for examples.
My observation is that enviroDIY is about a safer space to try things out, but you still have to figure out the process that works for you, and as ever the budget that can make it work.
EnviroDIY can be the place to share insights like I’m doing and Anthony is doing. As its your instrument you are the lead person, others with deeper knowledge of say the protocols can lend some insight, but implicitly you own the making it work.
Gosh it seems like you have your feet very wet already with the Mayfly. My experience is to plan the development process and manage the expectations of others of the risk. This is what we teach Engineering students.
This can be low budget, but only if you have the time and commitment to learn the Modbus protocol. Keller does publish where the extra 36XiW-CTD registers are in the Modbus map. Modbus is archaic but because it runs on RS485 it works over a long distance. For trying it out you only need one 36XiW-CTD and your commitment. For serious development we typically plan how to reduce the risk. The first part is to test that the protocol can be interfaced to. Then how to do the full integration test. That is one 36XiW-CTD for the lab bench to check the protocol with quick turns of the software. Then one for a realistic practice area otherwise known as stability testing, and one for the final stream. In the long run maybe the tests ones can be deployed to streams, but every time there is a new software release its better to have a safe place to check the new software. In the open source world, other people may offer to lend their experience, then typically you pay for their instruments.
Hope that helps. 😊