"RS-232 VDD. The RS-232 output signals (Pin 1 ~ Pin3) are designed for 5V, 3.3V or 3Voperation. VDD_232 should be connected to the same power level of the RS-232 interface. (The RS-232input signals are always 5V~3V tolerant.) Note: This document version only provides 5V DC characteristic information. Refer to future revisions for updates. (Document Revision: 1.6Document Release: April 26, 2005) "
RS232 VDD. The power pins for the serial port signals . When the serial port is 3.3V, this should be 3.3V. When the serial port is 2.5V, this should be 2.5V