PySerial write() instant timeout

Question

EDIT

I found out what the problem was and have answered my own question.

Original question below this line

I have a serial bridge between COM4 and COM5 implemented in software (Specifically, HDD's Free Virtual Serial Configuration Utility)

I have two different python scripts starting up in two different instances of Powershell, receive first:

import serial
receive = serial.Serial(port = 'COM5', baudrate = 9600)
text = receive.read(100)
receive.close()
print text

And then the sender:

import serial
send = serial.Serial(port = 'COM4', baudrate = 9600, timeout = 0)
send.write("Hello")
send.close()

When starting the sender script, the receiver script gets the sent message (So communication is clearly established) but the sender script immediately ends with an error:

Traceback (most recent call last):
  File ".\sending.py", line 3, in <module>
    send.writelines("Hello")
  File "C:\Python27\lib\site-packages\serial\serialwin32.py", line 270, in write
    raise writeTimeoutError
serial.serialutil.SerialTimeoutException: Write timeout

I get the same error when I change the sender script to

send = serial.Serial(port = 'COM4', baudrate = 9600)

So my question is: What exactly is timing out? How do I prevent that from happening? I mean, the data IS being sent so I could probably just put the whole thing in a try/except(and do nothing) block but that seems like a bad solution in the long run.

Maybe bug in software bridge, i have not seen this exception on hardware ports COM or USB-COM. Passing(do nothing) on exception is bad because you have timeouts and it decrease transmition speed. — eri
– eri, Commented Aug 19, 2013 at 11:58
Actually, between me posting that and now, I think I've found the issue - pyserial appears to only work w/ 32bit architecture (At least on windows platforms) - the file serialwin32.py was a hint, can somebody confirm? — shieldfoss
– shieldfoss, Commented Aug 19, 2013 at 12:02
I just had this problem with an FTDI USB/RS-232 bridge. The driver seems to prevent the process from being killed (script halts at exception, python can't be killed, CMD not closed) only pulling the device (beware BSOD) gets things going again. — handle
– handle, Commented Nov 20, 2014 at 15:18
That might be a different thing - a recent FTDI driver-update bricked a lot of USB/RS-232 chips. I recommend going with a different vendor in the future. — shieldfoss
– shieldfoss, Commented Dec 16, 2014 at 9:14

shieldfoss · Accepted Answer · 2013-08-19 13:11:48Z

The clue is in the error message[1]

File "C:\Python27\lib\site-packages\serial\serialwin32.py", line 270, in write
raise writeTimeoutError

so we open that file and find:

if self._writeTimeout != 0: # if blocking (None) or w/ write timeout (>0)
            # Wait for the write to complete.
            #~ win32.WaitForSingleObject(self._overlappedWrite.hEvent, win32.INFINITE)
            err = win32.GetOverlappedResult(self.hComPort, self._overlappedWrite, ctypes.byref(n), True)
            if n.value != len(data):
                raise writeTimeoutError

Read that first conditional again:

if self._writeTimeout != 0:

so let us rewrite our code from before

send = serial.Serial(port = 'COM4', baudrate = 9600, timeout = 0)

becomes

send = serial.Serial(port = 'COM4', baudrate = 9600, writeTimeout = 0)

and Et Voila: No exception.

[1] Well Designed Error Messages? That's new!

JardaG · Accepted Answer · 2020-10-31 15:47:12Z

The problem may be that the interface tries to comply with RTS, CTS, DSR, or DTS signals. It is possible that if they are not properly virtually connected, they can mysteriously affect communication through a timeout.

I would also recommend looking at the configuration of the used virtual serial bridge.

One solution may be to ignore influence using rtscts=False and/or dsrdtr=False when opening the serial port in Python.

I could use an alternative solution for sniffing communication using hub4com, where I used the parameter --octs = off, for example in this way, but the virtual ports had to be created correctly before. hub4com --baud=115200 --route=0:All --route=1:All --route=2:All --no-default-fc-route=All:All --octs=off \\.\COM1 \\.\CNCB0 \\.\CNCB1

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