Disable DTR in pyserial from code


I'm trying to use pyserial to send data to an arduino. But when I open the COM port it sets DTR low and resets the board. However, I have my arduino code setup such that I have to put it into serial receive mode by holding two buttons for 1 second. I would rather not have to do the serial input on boot of the arduino, if possible.

Apparently you can modify serialWin32.py file, changing the line that reads:

self._dtrState = win32.DTR_CONTROL_ENABLE


self._dtrState = win32.DTR_CONTROL_DISABLE

But, is there a way to just disable this directly in my python script? I also need to do this for all systems. I would rather not force people to change their base serial config just to use this script.

The serial port is opened as follows:

 com = serial.Serial(port, baud, timeout=1);

Update: In the end I found a solution that works fine for my setup. Since I didn't need to do Serial data all the time, only when I put the device into serial receive mode I found a way to disable the reset on serial connect from the arduino itself.

Many posts have said you can disable DTR reset by placing a ~100 Ohm resistor between 5V and reset. But I didn't want this to be a permanent thing. So, instead I placed a resistor between PD5 and reset. Then, in software:

void setup() {
    DDRD &= ~(_BV(PD5)); //Set PD5 as input initially
    PORTD |= (_BV(PD5)); //Set high

inline void setResetDisable(bool state)
    DDRD |= (_BV(PD5)); //Set PD5 as output to put 5V on reset line
    DDRD &= ~(_BV(PD5)); //set back to input mode

So now, when I want to be in serial mode, I call setResetDisable(true) which throws 5V on that 100 ohm resistor and the reset pin, preventing DTR from pulling it low and resetting the chip :) I then just call setResetDisable(false) when I leave serial mode so the chip can be programmed as normal.

3/18/2013 8:07:53 PM

Accepted Answer

You ought to be able to disable DTR before opening the port, like this:

com = serial.Serial()
com.port = port
com.baudrate = baud
com.timeout = 1

However, doing so on the current release of pyserial (2.6) on Windows throws the following exception:

..., line 315, in setDTR
ValueError: Attempting to use a port that is already open

This seems to be a bug which is fixed in the latest version of the source, SVN revision 445 on 29th December 2011 (see http://pyserial.svn.sourceforge.net/viewvc/pyserial/trunk/pyserial/serial/serialwin32.py?view=log) with comment:

allow setRTS, setDTR before opening on Win32 (to set initial state), doc update

Which looks like it may have just missed the 2.6 release (uploaded on 2nd November 2011 see: https://pypi.python.org/pypi/pyserial).

Furthermore, looking at the current implementation of setDTR() for POSIX (in serialposix.py) it looks like this bug is not fixed and an exception is thrown if the port is not open, so a cross-platform solution looks unlikely.

3/18/2013 2:21:17 PM

Disabling DTR does not work for me:
ser.dtr = None
(Linux 4.4.0 x86_64 / Python 2.7.12 / PySerial 3.4)

But this works:

import serial
import termios

port = '/dev/ttyACM0'
f = open(port)
attrs = termios.tcgetattr(f)
attrs[2] = attrs[2] & ~termios.HUPCL
termios.tcsetattr(f, termios.TCSAFLUSH, attrs)
se = serial.Serial()
se.baudrate = 115200
se.port = port
print 'dtr =', se.dtr

I found it here.

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