Check list

Is the PLC connected to the power supply

Are the participants available in the TCP/IP network

Can the PLC be reached via the PING command

Can the PLC be reached via TELNET

Are the PLC and the PLC connected with the right cable

Did you select the right COM port

Do the communication parameters match (Baud rate, parity, start/stop bits,...)

Is the COM port blocked by another application?

Did you configure the net address correctly, both in the driver dialog and in the address properties of the variable

Did you use the right object type for the variable?

Does the offset addressing of the variable match the one in the PLC?

Use the DiagViewer for further analysis -> Which messages does it show?