Overview
FS X and Prepar3D provide a software interface to allow hardware to interact with them This software interface is known as Simconnect and it provides a large number of Events and Variables that can be used by the hardware.
What are Events?
Simconnect Events should be seen as actions in the flight simulator, i.e. do something now. An example of an Event is KEY_PARKING_BRAKES which toggles an aircrafts parking brakes on or off. Assigning an Event to some flight simulator hardware such as a button, switch, rotary or selector triggers the event to be sent when the hardware is used e.g. pushing a button would toggle the parking brakes.
Some Events required additional data to be sent with them. For example the Event KEY_THROTTLE_SET requires a value between 0 and 16383 to be sent with it, the value determing the position of the aircrafts throttle.
Sending Events will almost certainly change the value of at least one Simconnect Variable (see below) and sometimes it may change the value of many variables.
What are Variables?
Simconnect Variables should be seen as a way for the flight simulator to maintain knowledge of all things in the flight simulator. E.g. a real aircraft has actual moving flaps with sensors and hydralic pressure readings etc. A flight simulator needs a way to mimic these things and record their state. An example of a Variable is TRAILING EDGE FLAPS LEFT ANGLE which maintains the current aircrafts angle of the rear left flaps in Radians.
All Variables can be read via Simconnect i.e. their values retrieved and processed in some way. Some variables can also be written i.e. their value forced to be a specific value.
Variables are ideal for flight simulator hardware that uses LED lights because they can be used in logical expressions that result in true or false (light on or light off). For example if the Simconnect Variable AUTOPILOT AVAILABLE was assigned to an LED light and the result of the logical expression returned true, the LED light would be lit and you would know that an autopilot was available on the aircraft.
Variables are also used for the LED displays of flight simulator hardware. E.g. the Variable AUTOPILOT HEADING LOCK DIR displays the desired heading programmed into the aircrafts Autopilot. Linking this variable to an LED display would show the value in the display e.g. 020 degrees.
Assigning Simconnect Events & Variables
Goflight hardware can be assigned Simconnect Events and Variables using GoFlights own software called GFConfig or by using Pete Dowsons FSUIPC or by using the Goflight Interface Tool (GIT) found at this website.
GFConfig is simple to use, but is limited to a small number of Simconnect Events.
FSUIPC does a vast amount of things but is complex for casual users as handling LED's requires programming skills in LUA. FSUIPC is also payware.
GIT is specifically designed for GoFlight hardware, does not require complex programming skills and is donationware.