Com port access denied. This is assuming everything was setup correctly. Follow this guide to fix it fast. Feb 25, 2020 · 2 Since you tagged this asp. An alternative is that either your device or the computer is not configured correctly. How to determine what is using serial port resources causing access denied? Ask Question Asked 9 years, 2 months ago Modified 3 years, 6 months ago To fix this we change the com port the arduino was on to an open one. Close () code in your catch block to free that port. Jul 9, 2021 · Further troubleshooting: using Arduino IDE (and with PI shut down) I had no issues uploading code to the M5, but after closing that and trying to upload with PI, same “access denied” issue. Jan 18, 2026 · In this blog, we’ll demystify the "Access Denied" error by breaking down its root causes and providing step-by-step troubleshooting techniques. Feb 16, 2021 · Answer accepted by question author Anonymous Feb 16, 2021, 6:46 PM Hi, This error indicates "Access is denied". 1, suddenly cannot open COM ports, "access denied". Anyone know what would cause this and how I might resolve? Support from the CC Processor isn’t very helpful. I have tried entering the right baud rate of 9600 from the port setting of my device manager. another if you don't like that you could always close the application the that is using the com port but that's not always a Oct 16, 2020 · Tip #2: Check your port in the Arduino IDE Open Arduino IDE and check in the Tools –> Port menu that you are trying to upload to the port you have seen in the Device Manager. Nov 10, 2013 · A port stays open all the time, so be sure to close ports that you don't need open anymore. Explore common COM port issues on Windows, diagnose with Serial Port Monitor, and find effective solutions quickly! Feb 25, 2020 · If Com1 is not listed, you will need to enable it in the BIOS. This is easy to do. net, are you trying to access the port with multiple threads? (as in on a request, accessing the port, creating the connection, sending data, closing connection, closing the port? If you are, this is also a problem. serial. Open Windows Firewall by swiping in from the right edge of the screen, tapping Search (or if you're using a mouse, pointing to the upper-right corner of the screen, moving the mouse pointer down, and then clicking Search), entering firewall in the search May 26, 2017 · I just connect my Telemetry Radio via USB and it gets assigned with COM4 Port access. HyperTrm. Monitor works though. ') Here are what I did: 1) I check device manager, COM 4 can be seen, but cannot open 2) In my code, I do close COM port after using it 3) Here is COM config in my code: ser = serial. The Arduino IDE is quite good to detect connected devices, but in case you have multiple devices connected at the same time this is definitely worth to check: Sep 30, 2012 · I get the following error message Access to the port 'COM5' is denied. 4: Make object of Serial port before try catch block to make it universal. By the end, you’ll be equipped to diagnose and resolve COM port access issues in C# with confidence. When we use the CC processor to initialize the device, if fails, saying Device cannot be initialized. NET 4. If you have not already, be sure to include as much information about your issue that you can, including any error messages, error codes, what steps it takes to create the issue, and what you have done to troubleshoot. When I executed the application, I could see the com port to access. You may see this error when when attempting to use an already open COM port, it's been used by other program. You could try close other software that may be using the port in the Task manager, and try again. . We can see this COM port in Windows Device Manager. If I am not mistaken, only one thread can access a com port at a time. Fax software or a modem could have it open, or you could have left it open yourself. I hate win 10 with a passion because connecting anything other than a g-d cellphone is usually an enormous exercise in frustration. SerialException: could not open port 'COM4': WindowsError(5, 'Access is denied. I just cant get win 10 to let me use the com port to upload a program. Serial( "COM4", 9600, timeout=0. 3: If any exception (Access is denied) occurs, write the Port. serialutil. Aug 9, 2019 · The UNO powers up, is recognized on the com port, and returns the board info when requested from under the "tools" heading. 05) 4) I reboot computer, the problem is gone, but it happens again after a while 5 Apr 27, 2019 · The device plugs in to a USB port, which is set up to be a virtual COM port. If COM1 is there, then another program has left it open. I can't remember which version it was, but sometime around Vista or 7 they changed how Windows allows access to the COM ports and you need to run some legacy apps in compatibility mode to restore access. Aug 8, 2025 · If you get an "Unable to open a connection to com1" message, you won't be able to use your serial port. Apr 27, 2019 · We can see this COM port in Windows Device Manager. Dec 19, 2020 · Two existing EXEs, one written in C++ and Windows SDK, one written in C# and . When I look at their log files, there a message that says “Access to COM 2 is Denied”. (which we can change later for any other port from System Management) I created an application to connect to the radio with the serial port under the com port access given. when running the method below from my form. exe has no problem opening the same ports that both EXE cannot open. Access to COM1 is exclusive; only one program can have it open at a time. 5. ndgkzt qtub kui lmgget aawsjm mon abnca mskqi mpdmb mhv