1. Most often, the port number on the Citect Project Editor | Communications menu | Ports form is invalid. Either the specified COM port does not exist (check the Windows Device Manager) or it is in use by another program. If the port is listed in the Windows Device Manager and it says it is working properly, then open the port in Windows HyperTerminal to confirm that it is available. Run HyperTerminal, enter anything for the connection name, then select the COM port to connect to (instead of a modem). If it says 'Unable to open COMx' then the port is probably already being used by another program. CitectSCADA will not be able to open the port until the other program closes it.

Note: This error will pop up even if you are not using that port in the project (setting the device to "Memory" does not resolve the issue).

2. An I/O device is configured but there is no corresponding Port (or Board) configured in the project, or the Port specified in ‘Port’ field of the I/O Devices form is invalid. Note that the Port specified in ‘Port’ field of the I/O Devices form must exist under the I/O Server with which the I/O Device is associated.

3. It is possible, although unlikely, that the driver DLL file or another DLL that it requires is missing or incorrect. See Citect Knowledge Base article Q3275 for instructions to troubleshoot this. Check for both the Board driver (normally COMX.DLL) and the I/O Device protocol driver.