If you like the simplicity of the editor support but would prefer a simple C# API and code examples, instead of using PlayMaker, please let me know in comments.
This video demonstrates how to use this extension that provides Arduino compatibility for PlayMaker to detect the board, configure the pins and then, send physical events to PlayMaker as well as send digital outputs to the Arduino board.
(Shows Kreation.Arduino.Playmaker v1.0.5)
Version 1.0.8 now available on the Unity Asset Store https://assetstore.unity.com/packages...
This package is built independently from Huton Games or from Arduino.cc and has no endorsement from either party.