By following this investigative flow, you will spend 5 minutes debugging instead of 2 hours blaming the cable. Remember: Modbus Poll is just asking Windows for the keys to the car. If Windows says "keys not available," you know exactly where to look.
If you work in industrial automation, building management, or SCADA integration, you have likely encountered a frustrating error message: modbus poll port not available
A converter may draw power but fail enumeration. Alternatively, a virtual COM port (from a Bluetooth dongle or VPN software) may be stuck in "DTR/DSR" pending state. By following this investigative flow, you will spend
Sometimes the software reports "port not available" when the real issue is a causing a timeout that Windows interprets as a hung port. If you work in industrial automation, building management,
USB-to-serial adapters often change COM numbers when plugged into different USB ports. This causes confusion and leftover ghost ports.