pyserial: Unexpected Local Echo

Steve Holden steve at
Mon Jan 18 07:09:41 CET 2010

Steven Woody wrote:
> 2010/1/16 John Nagle <nagle at>:
>> 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
>> --
> Now I tried run the same test code using pure Windows python +
> pyserial, the result is same.
> Actually, the USB device is a optic head, that read data from an
> electric energy meter:
> PC USB Port -> Optic Head -> Meter
> I get the echo even when the meter itself is disconnected, that mean
> the echo is not generated by firmware inside the meter. So, if the
> echo is also not generated from my PC, that much be from the optic
> head itself.  Is it possible?
> Regards,
> woody
Of course this echo is good, as it allows you to verify that your
transmission has been received ... somewhere ...


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