Safest way to limit current output of a signal generator
It might be helpful if you would describe the "load" in more detail. I would expect that an input in a piece of test equipment is rather high impedance. I guess that the current values stated in your question are properties of the device and not absolute maximum ratings, i.e. the input impedance is such that a current of 10 mA typical, and 30 mA maximum, when a TTL signal is applied. 

In any case, a 500 Ohms series resistor won't damage the device. However, if the input is an actual TTL input (i.e. it does not just require TTL-style voltage levels), a 500 Ohms series resistor will prevent the signal generator from pulling the input to Low level.