Serial port shown in device manager but unable to access/found?

Question

I've installed the Arduino virtual USB serial port driver, it's appeared as COM4 on my device manager. I double checked by listing all available serial ports in powershell

> [System.IO.Ports.SerialPort]::getportnames()
COM1
COM4

However, when I try to do an echo to the serial port, it says it does not exist

> echo helloworld > COM4
The system cannot find the file specified.

I am trying to upload some firmware in to my arduino, but I kept getting denied by the port. Trying a different USB gives me the same result. I wrote a small program to test the serial port and I get the error:

The serial port "COM4" does not exist.

Answer 1

The device name of a serial port on Windows is \\.\COMn where n is the port number. A device driver can emulate the DOS device name, like "COM4" if it chooses to do so, very simple to do in the device driver code. But that's an increasingly rare thing to do, especially for port numbers larger than 2 and extra especially for port numbers larger than 4. So seeing your echo command fail is not unusual.

A serial port cannot be shared, it can only be opened once by a process and everybody else that tries to open it will get the fail-whale ERROR_ACCESS_DENIED error code. Necessarily so, serial ports sit at the very bottom layer of the OSI model, there is no protocol that arbitrates access to the port. Or to put it another way, the operating system has no guidance on who should get the data that the port receives. There can be only one candidate, the process that opened the port first. So the error message you got is not unusual, you have to make sure that nobody else is using the port. Including the Arduino serial port monitor.