Oh. Oops, apparently I didn’t refresh and see Beth’s latest response. She has much better detail than me. If you use the default “src” directory, you don’t need the src_dir = in your platformio.ini file. But you can only have a single ino in that folder, so if you want multiple programs for testing, you’ll need to specify the directories.
The two serial libraries are listed separately in the platformio.ini file because ModularSensors actually *doesn’t* require them, but most of the examples do. If you’re working with RS485 sensors and you’re also using some sort of remote communication, you’re almost certainly going to want to be using AltSoftSerial for the Serial communication with the sensors. AltSoftSerial is the best non-hardware serial port I’ve found. The SoftwareSerial_externalInts is pretty awful at best. NeoSWSerial is another workable alternative. There’s more detail in the ReadMe for ModularSensors about the serial port problems: https://github.com/EnviroDIY/ModularSensors#notes-on-arduino-streams-and-software-serial