The Problem was The Use of an extension cable (RS232 Null Modem Without Handshaking).So, The RxD & TxD are inverted.it means if you want to measure TxD voltage You should Get it from The Pin 2.
I have an USB2Ser Adapter (CH340), and want to use the TxD line to control an 13V voltage On/Off. (The pins RTS,DTR already occupied.So, really stuck only with TxD).
After some tests i found that the Txd Pin change state between (0V & 0.42V) (logical 1 & logical 0). So, We have Two states 0V & 0.42V.
0V ------> Transistor is OFF. is OK.
0.42V ---> Transistor is ON. is it possible ??
DataSheet : http://www.seeedstudio.com/wiki/images/7/7c/CH340DS1_EN.PDF (from @WhatRoughBeast)
TTL levels of My Adapter are : 0V & 3.5V
RTS, DTR lines : logical 0 = 3.5V , logical 1 = 0V . So, are normal. it's OK.
To test the TxD line i used a BREAK command to make it at logical_zero (3.5V), but when sending a BREAK (Pyserial.sendBreak(Time)) i got a 0.42V on TxD, and as the guys said is abnormal.
i think that My adapter doesn't support the BREAK fucntion.
And When Transmitting 00 on TXD Line I got : 0.2V not normal ... WTF !!!.
I used AccessPort software http://www.usconverters.com/downloads/tools/AccessPort136.zip to test the Adapter in loopback mode (TxD to RxD), And Transmitting Some Words, the Test is passed !! what i send, i get it in the Receive window. it means that My Adapter is O.K !!!