Hi jtr!
Don't know which a/c you fly on, but I can tell you how it is on the F100. Basically you can select TA/RA all the time. RA only works above 900' AGL when taking off and below 1100' AGL when descending. Besides that, due to system logic, the system knows when the a/c is on ground (input of flight/ground switch) and therefor will only display traffic on ND and nothing else!
Greetz