Note/Update: 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 ??

UPDATE : 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 !!!

  • \$\begingroup\$ If you can find a germanium transistor (0.2V turn on) - yes. \$\endgroup\$ – JIm Dearden Feb 4 '15 at 16:13
  • \$\begingroup\$ Thank you for the help.but do you have any name ? \$\endgroup\$ – user38218 Feb 4 '15 at 16:15
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    \$\begingroup\$ If TX is really only going as high as 420 mV, then something else is already wrong. \$\endgroup\$ – Olin Lathrop Feb 4 '15 at 16:17
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    \$\begingroup\$ Something seems really wrong with your logic HI voltage. Are you sure that it isn't driving the pin of a micro that is set to output and LO? Please try disconnecting the TxD pin from whatever it is connected to and measure again. \$\endgroup\$ – Dwayne Reid Feb 4 '15 at 16:17
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    \$\begingroup\$ If you are measuring the 0.42 v with a meter, the lead might be pulsing and you are seeing an average. Do you have a scope so you can observe the waveform? \$\endgroup\$ – tcrosley Feb 4 '15 at 19:22

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.

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  • \$\begingroup\$ When Trasmitting 00 on TXD, I got : 0.2V !! It's abnormal.So, To test My adapter I used AccessPort software with LoopBack mode (Txd to Rxd),and send Some Words, The test is Passed !!! What i send i'll get it on the Receive Window.it means the Adapter function well. Any Suggestions ! \$\endgroup\$ – user38218 Feb 5 '15 at 6:35

I'm Such an idiot Lazy Hobbyist, I didn't mention that I'm Using an "RS232 extension Cable"

Adapter (Male) ---> (Female) Null Modem Cable (Female) ----> Multimeter

And After Some help & tests I figured out that this cable is RS232 Null Modem without handshaking cable. and RxD, TxD are inverted. http://www.lammertbies.nl/comm/cable/RS-232.html#null

And As @WhatRoughBeast Said i got : ~3V in the TxD (Pin 2 in the Other Side) line when transmitting 00.

Thank you Very much Guys for The help.

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