Hi,
I am not sure that I understand your question. Are you going to try to power a relay with a serail port? I wouldn't do this at all. The serial port can't supply enough current to keep the gate closed. You run the risk of blowing up your serial port. I would create a circuit to buffer the output (or better yet, go with a digital card of some sort). You might be able to use some sort of optical isolation circuit to accept the 5 volt input and output the correct voltage/current to power a relay.
Best Regards,
Aaron K.
Application Engineer
National Instruments
