pyserial: Unexpected Local Echo

[Steven Woody]

Hi,

I am using pyserial. But I always get the local echo after I write
some characters onto serial port 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.

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'.


Thanks in advance.

--
Life is the only flaw in an otherwise perfect nonexistence
-- Schopenhauer

narke
public key at http://subkeys.pgp.net:11371 ([email protected])

 

[Nobody]

I am using pyserial. But I always get the local echo after I write
some characters onto serial port 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.

That explains the "echo" part. What makes you think that it's local?

 

[Grant Edwards]

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).

 

[John Nagle]

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?


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).

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.
("http://www.aetherltd.com/connectingusb.html")

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

John Nagle

 

[Steven Woody]

   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.
("http://www.aetherltd.com/connectingusb.html")

   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

--
Life is the only flaw in an otherwise perfect nonexistence
-- Schopenhauer

narke
public key at http://subkeys.pgp.net:11371 ([email protected])

 

[Steve Holden]

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 ...

regards
Steve