Switch a Transistor ON/OFF with Txd Line From USB2SER Adapter (CH340)?[Resolved]
Almost certainly you are measuring your Tx line wrong. If you take a look at the data sheet http://www.seeedstudio.com/wiki/images/7/7c/CH340DS1_EN.PDF Table 6.2 says that you should be seeing ~3 volts. A break command does not normally send a DC level - it sends a logical level for 1 or more character transmit times. So your multimeter is looking at a fairly narrow pulse and averaging it.

As a quick check, try sending a continuous stream of 00 data. This should (assuming 1 start, 1 stop, no parity) give you a DMM reading of ~80% of the actual high voltage.