RS-232 Isn't the TXD on Mark until the start bit?

Thread Starter

System/360

Joined Feb 1, 2020
2
My first post, please pardon me if this is not in the right group.

I have a USB-serial adapter using the FTDI FT232R chip. Specifically it is this item I bought on taobao.com. It is connected to a Windows 10 laptop.

I connected the TXD pin to a simple oscilloscope. When I send some data from the PC, the pin appears to be showing the data that is sent. I don't know the oscilloscope enough to freeze the display so can't exactly verify that the exact data bits are correctly on the pin.

However, the thing that puzzles me is that when nothing is happening, the TXD pin is showing 0V, ie same level as the Gnd pin. It's been more than 30 years since I last played with RS-232. I have the impression that the data line should be outputting the Mark voltage until a character is to be sent, whence it changes to Space for a duration equal to the start bit.

Can someone help explain the phenomena I am seeing? Does a USB-serial adapter behave exactly like a real RS-232 device?

Thank you.
 

ericgibbs

Joined Jan 29, 2010
10,797
hi 360,
Welcome to AAC.
Look at Section 1: 4 of this PDF.
E
Clip:
RS232 Level Shifter
The RS232 level shifter converts the signals provided by the FT232R into the voltage levels
required by RS232 devices.
 

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Analog Ground

Joined Apr 24, 2019
416
It looks like the only device on the circuit board is the FT232R chip. This device needs another IC to convert the TxD and RxD signals (and control signals) to RS-232 voltage levels. This board is not a full USB to RS-232 converter because it does not use correct voltage levels for Mark and Space.
 
Last edited:

sagor

Joined Mar 10, 2019
249
Analog is correct, what you have is a USB to TTL serial converter, not an actual RS-232 converter. The TTL signals are "normally" low for idle, and high for data (bits). Converters to RS232 like the MAX232 chip invert the signal from TTL to RS-232 voltage levels, giving an idle high voltage on the actual RS-232 wires.
 
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