There are several examples of SmartApps sending messages to the Arduino Things shield and in response Arduino performs some action like opening a garage door or turning on a coffee pot.
How do you accomplish the reverse?
A device connected to the Arduino triggers an Arduino input. Arduino responds by sending a message to the cloud. A SmartApps receives and processes the message. Up to this point I am fairly clear.
This next step is where I need example SmartApp code.
In response to received message SmartApp sends command to SmartThings controlled device.