com2tty 0.1.2

A Windows COM port to WSL ttyUSB forwarder

com2tty

com2tty is a Python package designed to forward serial communications from a Windows COM port into a WSL (Windows Subsystem for Linux) instance, presenting it as a virtual ttyUSB (or similar) serial device in WSL.

It does this using a low-latency, firewall-resilient process-pipe bridge over standard input/output redirection. It requires no network or firewall configuration.

Architecture Overview

1. The Windows host process opens the physical COM port (using pyserial).
2. It spawns the WSL Python bridge helper in the background, redirecting its stdin/stdout.
3. Inside WSL, the bridge helper opens a pseudo-terminal (PTY) and creates a symbolic link to the PTY's slave file.
4. Data is piped bidirectionally between the physical Windows COM port and the WSL virtual device.

[Windows COM Port] <--> [com2tty host] <--> (stdin/stdout pipe) <--> [wsl bridge] <--> [PTY Slave /tmp/ttyUSB0]

Requirements

• Windows Host: Python 3.8+ and pyserial (installed automatically).
• WSL Guest: Python 3.x (uses standard library modules only, no dependencies required).

Installation

Install the package on the Windows host by running the following command in the project root:

pip install .

For development, you can install it in editable mode:

pip install -e .

Usage

Run com2tty from any Windows terminal (PowerShell or Command Prompt).

com2tty <COM_PORT> [options]

Examples

Bridge COM3 to the default WSL path /tmp/ttyUSB0 at 115200 baud:

com2tty COM3 --baud 115200

Bridge COM5 to a custom WSL device path /tmp/my_device:

com2tty COM5 --baud 9600 -w /tmp/my_device

Options

positional arguments:
  port                  Windows COM port to connect to (e.g. COM3).

options:
  -h, --help            show this help message and exit
  -b BAUD, --baud BAUD  Baud rate for the serial port (default: 9600).
  -w WSL_TTY, --wsl-tty WSL_TTY
                        Target symlink path inside WSL (default: /tmp/ttyUSB0).
  --bytesize {5,6,7,8}  Serial byte size (default: 8).
  --parity {N,E,O,S,M}  Serial parity: None, Even, Odd, Space, Mark (default: N).
  --stopbits {1,1.5,2}  Serial stop bits: 1, 1.5, or 2 (default: 1).
  --xonxoff             Enable software flow control (XON/XOFF).
  --rtscts              Enable hardware flow control (RTS/CTS).
  --dsrdtr              Enable hardware flow control (DSR/DTR).
  -d, --debug           Enable debug logging output.

---

Configuring /dev/ttyUSB0 in WSL (Highly Recommended)

In Linux, the /dev directory is owned by root. Running com2tty as a normal Windows user means the WSL subprocess cannot write directly to /dev/ttyUSB0.

To work around this cleanly without requiring root permissions or passwordless sudo at runtime:

1. Run com2tty with the default path (or any /tmp/ path):

   com2tty COM3 --wsl-tty /tmp/ttyUSB0
   

2. In your WSL terminal, run the following command once to create a permanent symlink pointing from /dev/ to the stable /tmp/ path:

   sudo ln -sf /tmp/ttyUSB0 /dev/ttyUSB0
   

3. Now, any WSL application (such as minicom, screen, esp-idf, or Python scripts) can read and write to /dev/ttyUSB0.

Whenever com2tty starts up, it updates /tmp/ttyUSB0 to point to the active pseudo-terminal (/dev/pts/N), and /dev/ttyUSB0 resolves to the correct endpoint automatically.

---

Troubleshooting

WSL Bridge warns about Permission Denied

If you specify -w /dev/ttyUSB0 directly and see: [WSL] Warning: Permission denied creating symlink at /dev/ttyUSB0. This is expected behavior. The script will automatically fall back to /tmp/ttyUSB0 and output instructions on how to link them.

Serial Port Busy / Access Denied

Ensure that no other application on the Windows host (like PuTTY, Serial Monitor, or another instance of com2tty) is currently holding the COM port open.

Debugging

Run com2tty with the -d or --debug flag to view detailed logs and data transfer statistics:

com2tty COM3 --debug

License

MIT