Put this (or similar script) on the exit event of the text entry field. This example is done in formCalc.
for i=0 upto 14 step 1 do
if(hasValue(TextField[i]))then
continue
else
TextField[i]=$
$=""
break
endif
endfor
All of your textfields have to have the same name.
All of your textfields have to be siblings
Good luck!
Stephen