Introduction For engineers working with industrial automation, Modbus Poll is an indispensable tool. It acts as a master/client, allowing you to query PLCs, drives, and RTUs. However, few errors are as frustrating as the dreaded “Port not available” (or “Unable to open port”) message. You click "Connect," and the software refuses to talk to your hardware.
This article dives deep into why this error occurs, ranging from simple Windows OS conflicts to obscure driver fragmentation, and provides a step-by-step guide to exorcising this ghost. In operating system terms, a serial port (COMx) or TCP port is a resource . When Modbus Poll attempts to connect, it asks Windows for exclusive read/write rights to that resource. The error means Windows responded: "No. That resource is already locked, doesn't exist, or is in an invalid state." modbus poll port not available