Hacking ttyACM and ttyUSB

[Member sterretje]

Hello

I work with Arduinos (embedded computers) and (hence) I seem to be in need of hacking ttyACM and ttyUSB. The problem is that if a serial port is opened on one of those, the DTR line is asserted which will reset the Arduinos. I’ve done some research and came up with a python script that partially prevents this from happening.

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'
#port = '/dev/ttyACM0'
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()

lsusb

Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 008: ID 0403:6001 Future Technology Devices International, Ltd FT232 Serial (UART) IC
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 011: ID 2a03:0043 dog hunter AG Arduino Uno Rev3
Bus 002 Device 010: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

dmesg

[45908.623134] usb 2-1: new full-speed USB device number 10 using uhci_hcd
[45908.778166] usb 2-1: New USB device found, idVendor=1a86, idProduct=7523
[45908.778171] usb 2-1: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[45908.778174] usb 2-1: Product: USB2.0-Serial
[45908.780329] ch341 2-1:1.0: ch341-uart converter detected
[45908.793302] usb 2-1: ch341-uart converter now attached to ttyUSB0

[45912.847140] usb 4-1: new full-speed USB device number 8 using uhci_hcd
[45913.034166] usb 4-1: New USB device found, idVendor=0403, idProduct=6001
[45913.034172] usb 4-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[45913.034177] usb 4-1: Product: FT232R USB UART
[45913.034180] usb 4-1: Manufacturer: FTDI
[45913.034184] usb 4-1: SerialNumber: A5025WMS
[45913.041298] ftdi_sio 4-1:1.0: FTDI USB Serial Device converter detected
[45913.041412] usb 4-1: Detected FT232RL
[45913.043313] usb 4-1: FTDI USB Serial Device converter now attached to ttyUSB1

[45915.607101] usb 2-2: new full-speed USB device number 11 using uhci_hcd
[45915.795100] usb 2-2: New USB device found, idVendor=2a03, idProduct=0043
[45915.795107] usb 2-2: New USB device strings: Mfr=1, Product=2, SerialNumber=220
[45915.795111] usb 2-2: Product: Arduino Uno
[45915.795115] usb 2-2: Manufacturer: Arduino Srl
[45915.795118] usb 2-2: SerialNumber: 754393332353518081A1
[45915.798332] cdc_acm 2-2:1.0: ttyACM0: USB ACM device

With the earlier given python script, the FTDI and the dog hunter only assert DTR the very first time that the serial port is opened after the boards are connected; successive opening/closing does not result in assertion of DTR. The behaviour of the CH340 did not seem to change with the above script.

I found https://unix.stackexchange.com/questions/446088/how-to-prevent-dtr-on-open-for-cdc-acm which describes how to hack cdc-acm and have questions:
1)
Is https://github.com/torvalds/linux/tree/7876320f88802b22d4e2daf7eb027dd14175a0f8/drivers/usb/class still the correct source code for ttyACM? Or does Antix have it’s own version somewhere?
2)
I will follow https://tldp.org/LDP/lkmpg/2.6/html/x181.html to created the module. Do I need to do rmmod before running insmod?
3)
And lastly I’m looking for a similar hack for ttyUSB; where can I find the source code?

Sorry for the long story, tried to be as complete as possible but might have gotten carried away.
Thanks in advance.

• This topic was modified 2 years ago by sterretje.

[Member Xecure]

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antiX’ kernel is compiled directly from the official linux kernel, with some variations (but for your case it should be the same).
All sources are available on the antiX repo or any other mirror to the repo.
An example for all kernels in antiX 19: https://repo.antixlinux.com/buster/pool/main/l/
And more specifically for the latest 4.9.0-264 (64 bits), the source is inside the linux-4.9.0-264-antix.1-amd64-smp_4.9.0-264-antix.1-amd64-smp.orig.tar.gz file.

I have never created my own module, but I have compiled a custom kernel (with modifications) before, so it is doable. I download and uncompressed the source, edit the files, edit the config to create a new name for the kernel (to not confuse with antiX kernel name), build the .debs and then install the .debs for linux-image and linux-header. Probably creating and compiling a module as you have pointed out should be easier.

There seems to be a lot of discussion on the arduino forums, for example here: https://forum.arduino.cc/t/the-linux-ttyacm0-drama-more-details-after-a-lot-of-experimenting/126927
Where they also mention uninstalling modemmanager (installed in antiX).

antiX Live system enthusiast.
General Live Boot Parameters for antiX.

[Member sterretje]

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Thanks Xecure

I had forgotten about modemmanager; but that will result in blocked ports.

I’ve now upgraded the kernel as I could not straight away find the source code for the .235 kernel. Not really prepared to download the full kernel source; but it might be easier to find what I’m looking for if I do so.

Anyway, question not answered was if I had to do a rmmod before an insmod?

Thanks again.

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