"Device or resource busy" in RTU server example

[gigantino]

Hello!

I would like to say in advance that this is one of my first times working with the modbus protocol and, mostly, with Rust.

Recently I've been working with a PLC (Xenje XC3-24RT-E) and wanted to build a small interface for it. I managed to make it work almost on the first attempt using the first Node.js module I could find, but since I wanted to experiment with Tauri I thought I might as well try to do the backend Rust.

So, I cloned the repo and ran the rtu-server example, but after building it gave me the following error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Unknown, description: "Device or resource busy" }', examples/rtu-server.rs:53:68

I then tried to check if the port was being used somewhere else, and I confirmed it wasn't by running the command lsof /dev/ttyUSB0 (lsof).

I would also like to note that in my Node.js code I'm using the following port settings, so maybe I'm doing something wrong by not changing the default example.

{
  "baudRate": 19200,
  "parity": "even",
  "dataBits": 8,
  "stopBits": 1
}

Thank you to anyone who's willing to help.

[thienandangthanh]

I find out that it's used again in this line.

tokio-modbus/examples/rtu-server.rs

Line 53 in 5fc09c4

let client_serial = tokio_serial::SerialStream::open(&builder).unwrap();

And I still don't know how to make it work.

I think the examples and docs should be more easy to understand. Their purpose is to guide library user, not to confuse us.

[thienandangthanh]

Should we use other device path, like "/dev/ttyUSB1" to create the builder for client_serial instead of using the same builder?

[wheregmis]

You can create a virtual serial ports using socat

and run
socat -d -d pty,raw,echo=0 pty,raw,echo=0
You will get two virtual serial ports for unix. (development purpose)
`
2023/11/28 15:10:19 socat[66531] N PTY is /dev/ttys031

2023/11/28 15:10:19 socat[66531] N PTY is /dev/ttys033

2023/11/28 15:10:19 socat[66531] N starting data transfer loop with FDs [5,5] and [7,7]

`

Use one for the server and another for the client
`

let server_builder = tokio_serial::new("/dev/ttys031", 19200);

let client_builder = tokio_serial::new("/dev/ttys033", 19200);

`

Use these builder for server and client. It will work.

[chadwagner1]

I had to run using sudo under Ubuntu to get arround this problem.

[gigantino]

I don't have means to test what @chadwagner1 said as of right now, if it's that simple and someone can confirm we can probably close this issue and open a PR to document it better