Solution:

Sorry that it is impossible to transmit null character by using RS-232 block because we will regard NULL as terminal character, that implies we can't receive anything behind NULL character. So please give up to use RS-232 block to send the message which includes null character. However, you can try to use VB script block to do that. Under VBS, you can call Window API about com port or "gencomm" provided by Advantech to achieve your goal.