Fail on 2 digit com #'s in Windows

[pstlabdev]

COM1 - 9 work as expected, however COM10, 11, etc. give an open fail. I haven't had a chance to search the code for the reason. My work around was to change COM10 to COM1 and COM11 to COM2 in Device Manager port settings/advanced for those two ports.

[simonvetter]

Hi there,

thanks for the report. I'll try to see if I can reproduce this, but I'm not using windows that much, so don't hold your breath :) In any case, the client would be passing COM10 in about the same way as it passes COM1, so I'm wondering what might be causing the issue. Maybe something with the serial library I'm using ? I'll take a look there as well.

[Serhioromano]

How would you path it? I use windows but cannot get how to set a port?

URL: "rtu:///COM7"

Like this?

[simonvetter]

Try with two forward slashes instead of three? i.e. rtu://COM7

[Serhioromano]

Thank you. It works fine.

[lhopcroft123]

Just ran in to this same issue FYI, it seems that ports larger than COM9 are indeed special and need to be prefixed with a \\.\ string to work with the windows.CreateFile API. See https://support.microsoft.com/en-us/topic/howto-specify-serial-ports-larger-than-com9-db9078a5-b7b6-bf00-240f-f749ebfd913e.

The underlying serial library doesn't do this automatically (https://github.com/goburrow/serial/blob/bfb69110f8dd407add15aa6de3a7b6b31c9aa4a7/serial_windows.go#L160) whereas other better maintained libraries do (https://github.com/bugst/go-serial/blob/f0e4a45720a879803364f829e5e633e6f8f1be9b/serial_windows.go#L356)

A workaround for me when creating the RTU client:

const windowsPortPrefix = `\\.\`

serialPortName := os.Getenv("SERIAL_PORT")

if runtime.GOOS == "windows" && !strings.HasPrefix(serialPortName, windowsPortPrefix) {
    serialPortName = windowsPortPrefix + serialPortName
}

client, err := mb.NewClient(&mb.ClientConfiguration{
	URL:      "rtu://" + serialPortName,
	// etc...
})
// handle err, use client etc.

[simonvetter]

Huh, interesting. So it's all down to windows.CreateFile wonkiness. The MS article you linked to says

This syntax also works for ports COM1 through COM9. Certain boards will let you choose the port names yourself. This syntax works for those names as well.

so I may implement a fix in the same vein as yours in the client. I have a test setup I sometimes have access to with more than 10 Windows COM ports, so once I get around to play with it again I'll try and push a fix.

Thanks for taking the time to investigate!