pyserial: Unexpected Local Echo

John Nagle nagle at
Fri Jan 15 21:04:07 CET 2010

Grant Edwards wrote:
> On 2010-01-11, Steven Woody <narkewoody at> wrote:
>> I am using pyserial.  But I always get the local echo after I
>> write some characters onto serial port
> I really doubt you're getting a local echo.  Is the data coming
> out the serial port?  Do you get the echo if you disconnect the
> serial cable?
>> and I find no way to disable this behavior. When I say 'local
>> echo', I mean the next read operation will get characters that
>> was just write to the same port.
> The device to which you're connected is echoing them.  There's
> also a chance that your rxd line is floating and there's enough
> crosstalk in the cable to "echo" the data, but I'll bet money
> it's not being done locally (in the serial driver or port).
>> I run my program on cygwin (pyserial was also built on the
>> system from source code) and the serial port i am using is a
>> USB adapter that simulates a port (COM4 on my XP) because my
>> laptop don't have a real serial port.  But I checked my COM4
>> settings, there is no any think like 'local echo'.

     You're using what?  Some version of Python built on Cygwin
running on a Windows XP system?   What if you just run a stock
Python built for Windows on Windows XP?  Or run Linux?  That
half-and-half environment may not work right.  pyserial has
special cases in it for Windows and Linux, and it's not
clear what it will do on Cygwin.

     That said, if you're getting echo from output back to input,
I'd look at the USB to serial device.  I've used devices with
the Silicon Laboratories CP2102 part, and they work fine.

     Do you have something plugged into the serial port?  If
so, what?

				John Nagle

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