5 common errors with COM port operations on Windows
Common Errors When Working with COM Ports on Windows
COM port is busy. Access denied
The error COM port is busy or Access denied usually means the same thing.
In the Windows operating system, only one program can use a COM port. That is, the port is opened in exclusive mode. At that moment, no other program can open and use the same port in a standard way.
A possible solution to the problem is to review all open programs that may be using this port and close them.
Windows services running in the background. You can stop a service in the Control Panel → Services. Only stop services you are familiar with, as some of them may be crucial for OS operation.
Background tasks and hung processes. You can inspect them in Task Manager → Details.
Particularly hung and forcibly terminated processes may leave the port busy even after being stopped. In that case, the only solution is to restart the computer.
Rarely, drivers for certain devices may occupy the port (for example, a mouse driver that incorrectly detects the presence of a device on that port).
Right-click on the program icon and select "Run as Administrator."
Select the Manual mode in the Mode menu.
Select the COM port from the list and click Open.
The program will not be able to open the port, just like another application, but it will show the name of the process.
If the name appears as "System", then the port is used by some device driver, and it is not possible to determine more precisely.
Using the process name, you can find it in the detailed view in Task Manager, and stop or kill the process.
COM port opening error
A COM port opening error may occur if the program uses incorrect communication parameters when initializing the port.
Baud rate. Attempting to use a baud rate that is not supported by the device. Try opening the port at a standard rate of 9600.
Data bits. The serial port driver or the device itself does not support the selected combination of data bits, stop bits, and parity. Try opening the port with relatively standard parameters 9600 8N1.
Other communication parameters. The program sets an invalid buffer size for incoming data (if there is such a setting). Transmission control parameters (Xon/Xoff) may be set incorrectly too.
The device is physically turned off (no power). This is a common error for RS232-Ethernet converters.
Attempting to open the wrong port. You are trying to open a port number that does not exist in the system.
COM port not accessible error
This is a specific type of the more general "COM port error" mentioned above and means that there is no communication with the remote RS232-Ethernet converter over the network. You need to check ping with the device and packet transmission. Ensure that network parameters have not changed. The RS232-Ethernet converter is powered up.
COM port error during operation
In this section, we exclude problems with transmitted data, such as electromagnetic noise on the line, when a single byte can be damaged and parity check failed.
A COM port error during operation typically occurs for removable devices: RS232-Ethernet, RS232-USB converters. This happens when the physical communication line with the device is interrupted during operation. Often, to resume data transmission, you need to reconnect to the device (re-open the port).
This error frequently occurs for USB-COM converters if there is an unreliable connection in the socket or if power optimization options for the device are enabled (which can be turned off in Device Manager).
Program does not see COM port
If the program does not see the COM port, possible reasons include:
Driver issues. Open Device Manager and ensure there are no exclamation marks on the device. You may need to (re-)install the device driver.
Old program. Some older programs cannot work with ports numbered greater than or equal to COM10. Try assigning another port number in the device properties in Device Manager. Use Advanced Serial Port Monitor, which supports multiple methods for finding ports, to ensure the port exists in the system.
Hidden port. Unfortunately, access to the port may only be possible through a special identifier from certain programs.
