Python: suppressing DTR when opening serial port

Wim Sturkenboom · 05-04-2021, 02:38 PM

Good day

I'm using Arduino for embedded applications and communicate over serial ports (e.g. ttyUSB0). With a number of Arduinos, the problem is that they reset when the serial port is opened because DTR is asserted; this is by design.

First attempts to suppress DTR were not quite successful but with some googling I got to below code; the only problem is that the very first time that the serial port is opened, DTR is still asserted.

How do I modify the below code that this does not happen? A link to some web page will be fine, modified code as well

Thanks in advance

Code:

import sys
import serial
import termios

# print some python info for fun
print("Python version")
print (sys.version)
print("Version info.")
print (sys.version_info)

# define the port
port = '/dev/ttyUSB0'
resetPort = False

if resetPort == False:
# to be able to suppress DTR, we need this
  f = open(port)
  attrs = termios.tcgetattr(f)
  attrs[2] = attrs[2] & ~termios.HUPCL
  termios.tcsetattr(f, termios.TCSAFLUSH, attrs)
  f.close()
else:
  f = open(port)
  attrs = termios.tcgetattr(f)
  attrs[2] = attrs[2] | termios.HUPCL
  termios.tcsetattr(f, termios.TCSAFLUSH, attrs)
  f.close()


with serial.Serial() as ser:
# setup serial port
  ser.baudrate = 57600
  ser.port = port
  ser.rtscts = False            # not setting to false prevents communication
  ser.dsrdtr = resetPort        # determines if Arduino resets or not
  ser.timeout = 2

# print settings
  print (ser.name)
  print (ser)

# open serial port
  ser.open()

# read initial Arduino message
  s = ser.read(20)
  print(s)

# send some data to be echoed
# b is needed for python3
  ser.write(b'hello world')
  s = ser.read(20)
  print(s)

ser.close
print('done')

Notes:

If I understand man pages correctly, the same problem will occur in C/C++ programs.
I'm absolutely green when it comes to Python
I've written similar code in C# (Windows) which successfully suppresses DTR when the serial port is opened

teckk · 05-04-2021, 04:15 PM

https://raspberrypi.stackexchange.co...b0/31298#31298

NevemTeve · 05-04-2021, 11:59 PM

You mean you want a computer-computer connection, using serial line? Check this link: http://www.cadxservices.com/guides/rs232.htm
The images at the bottom shows how to wire to cable.

Wim Sturkenboom · 05-05-2021, 01:21 AM

Quote:

Originally Posted by teckk

https://raspberrypi.stackexchange.co...b0/31298#31298

Thanks, but that is basically what the code that I provided is

And the first time that you open the connection, it will do assertion of DTR; after that, it works as expected.

Wim Sturkenboom · 05-05-2021, 01:42 AM

Quote:

Originally Posted by NevemTeve

You mean you want a computer-computer connection, using serial line? Check this link: http://www.cadxservices.com/guides/rs232.htm
The images at the bottom shows how to wire to cable.

Thanks, the cable is a USB cable

Maybe ttyUSB0 should have given it away

The PC (USB host) is connected to an embedded 'computer' (USB slave) via a standard USB cable; the embedded 'computer' has a USB-to-TTL chip which converts the USB to TTL level TX, RX and DTR for communication; the latter forces a reset of the microcontroller on the embedded 'computer' when asserted.

teckk · 05-05-2021, 08:06 AM

I don't have termios installed. I don't have the hardware that you have. That is going to be really hard/impossible for someone else to test. There are threads on stackexchange from people that say they solved it. That is probably the best anyone can point you to.

If you solve this let the forum know how.

Ser Olmy · 05-05-2021, 08:29 AM

Quote:

Originally Posted by Wim Sturkenboom

The PC (USB host) is connected to an embedded 'computer' (USB slave) via a standard USB cable; the embedded 'computer' has a USB-to-TTL chip

Which chip? What does lsusb report?

Wim Sturkenboom · 05-05-2021, 01:23 PM

Thanks for the follow ups / additional replies

The chip will probably not matter; testing when I started the thread was with

Code:

Bus 002 Device 002: ID 0403:6001 Future Technology Devices International, Ltd FT232 Serial (UART) IC

Not tested yet:

Code:

Bus 004 Device 002: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
Bus 004 Device 003: ID 2a03:0043 dog hunter AG Arduino Uno Rev3

The solution should work with all of those and any other USB-to-TTL converter (e.g. Prolific CP2102 is another one but I don't have it at hand).

I'm currently distro-shopping and testing different distros so yesterday's setup is gone; need to go through the pip installs again

mimorek · 05-06-2021, 05:53 AM

I don't understand your problem, but
this:

Quote:

Originally Posted by Wim Sturkenboom

Code:

ser.close

needs to be this:

Code:

ser.close()

Wim Sturkenboom · 05-06-2021, 12:14 PM

Thanks minorek, I've fixed it.

Some further observations/research:

With the given code, the CH340 will always assert DTR, the others in reply #8 only once.

For others that encounter this, a possible 'fix' is described at https://unix.stackexchange.com/quest...en-for-cdc-acm but it requires a compilation of the module(s). Maybe I will have the guts one day