My BAE Hawk T1A Simulator Architecture...

I intend using a RESTful API into Simconnect, with a master Arduino controlling multiple slaves over an I2C bus. Not the simplest option, but probably one of the lowest cost. Simconnect comes free with FSX (so long as you have the Deluxe edition) and Arduino clones can be picked up for a few pounds.

I2C control is included in the standard Arduino library and is the most sensible way to talk to multiple devices. A RESTful API is pretty much the norm for new software applications and I hope will give me flexibility in the future should I move from FSX.

So far I have the RESTful API communicating with Simconnect (and into FSX) and have tested it with the Chrome Browser Simple REST plug-in. At the other end I have successfully driven several instruments from an Arduino. Now I just need the bit in the middle!