Although it is easy enough to use a multimeter to determine whether or not the attached serial port is able to provide enough electricity, a better method is to use the built-in "LED Port Power Indicator" designed especially for the TCC-80I. To do this, connect the TCC-80I to the target RS-232 port, and then turn the SW4 switch to the right to Test mode. If the LED lights up, the TCC-80I is receiving enough power. If the LED does NOT light up, you will need to attach the external power cord to the TCC-80I.