How to open serial port in linux without changing any pin?

Question

Posix requires changing RTS pin on port opening. I want a way to avoid it.

Answer 1

A change in the DTR pin can be (eventually) avoided using the command line

stty -F /dev/ttyUSB0 -hupcl

This has the effect of making DTR turn on; and subsequently when the port is opened and closed, DTR is not affected.

Source: https://raspberrypi.stackexchange.com/questions/9695/disable-dtr-on-ttyusb0/27706#27706

And there is code there to do the same thing from python via termios, this can be done before opening the port via pyserial:

import termios

path = '/dev/ttyACM0'

# Disable reset after hangup
with open(path) as f:
    attrs = termios.tcgetattr(f)
    attrs[2] = attrs[2] & ~termios.HUPCL
    termios.tcsetattr(f, termios.TCSAFLUSH, attrs)

The OP was running this on a Raspberry Pi, but I just tried it on Linux Mint on x86_64, it worked. I don't know how RTS is affected.

The reason I find this useful, is for communication with an Arduino Nano - which has a USB-> serial chip on board - and normally the Arduino gets reset every time you open the serial port from linux (rising edge of DTR causes reset). For some applications, this is not a problem, but it's clearly useful to avoid this for other applications, and it's not so easy to remove that tiny capacitor from the Arduino which connects DTR to reset.

You will still get a single reset when the stty command is executed (after plugging in the USB cable). But at least you can then keep opening and closing the serial port after that without further resets.