Like many people here, I wanted to make an arduino physical display then quickly realized unity does not have the required serial IO name space. So the solution is to borrow someone else's code (http://sourceforge.net/projects/serialportnet/), which turns out is kind of incomplete but works. This plugin sends/receives data packets over the serial port, which should make it much easier for people wanting to build hardware for KSP but can't be bothered with making plugins as well.

Note: currently not fully working for windows 10 for some reason! You can receive data from KSP but not control anything.

Compatible with KSP 1.2. Versions 0.18.2 and older compatible with 1.1, 0.17.6 and older compatible with 1.0.5, 1.0, 0.90, versions 0.15 and older compatible with 0.23, 0.23.5, 0.24, 0.25

This plugin implements the following:

1. Packeting of serialized C# structure for efficient communications including checksums

2. Listing of available serial ports (only works on Windows PC, using registry hacks)

3. Automatic handshake and detection of display COM port at 38400 Baud

4. Bi-directional data transfer: control KSP from arduino [supported but not fully implemented yet, easy if you want to do it yourself] (added in 0.13)

5. Analogue axes control (added in 0.14)

Check forum link for details: http://forum.kerbalspaceprogram.com/index.php?/topic/60281