What data is displayed on civilian ATC radar screens? I am developping an ATC radar for a simulation.

I don't think there is any standard for radars to show aircraft data on radar screens.
But there should be general data to show on radar screens.

For example:

CALL SIGN
ALT ASSIGNED ALT
SPEED HEADING



Can the data shown be changed on the clients?
Is there a difference between ground radar and air radar displayed data?
Is there any standard or can you give me any examples?
There are a few different ATC Systems and Radar Screens being used throughout the world, so there will be no set standard which applies globally, as vendors and setup differ.

The ATC screen can be adjusted to show what information the controller needs, and some items are universal to all positions being served (Callsign, Speed, Altitude, Assigned Altitude), some items are not required on each position to be displayed on the radar tag (Squawk, Routing, Aircraft Type, Wake Turbulence Category).

Some examples:

Source: www.andreas-milde.de

Source: www.euroscope.hu / Air Traffic Simulation Client

Source: fluglotse.wordpress.com