Hi,

yes, you have to but a license for s7-400 - please check this FAQs for your topic:

How many licenses are required for the Modbus/TCP block in the S7-300/S7-400?

Which technical data applies for the SIMATIC Modbus/TCP blocks and for which CPUs and CPs is it released?

Regards,

Towome

Dear Forum user,

If the Modbus blocks are not licensed, the INTF LED or SF LED on the CPU will flash.

In addition, messages are cyclically entered into the CPU's diagnostic buffer, and the status value A090 (hexadecimal) is output to the Modbus block in the user program.

Could you please confirm whether the above information regarding licensing and feasibility is correct?

Furthermore, if the CPU needs to establish multiple Modbus connections with different devices, is a single license sufficient?

 To add extra information:

1- You can test the Modbus TCP blocks without a valid license for test only .This is not recommended for a commercial machine.

2- If the block is not licensed ,  it tries to access a non-existing memory and this error will log into diag buffer with the scan rate of the Modbus TCP block. It will bury all critical or important plant/system reported bugs in a pile of irrelevant messages.

3-In order to activate the license to the block , read the manual carefully and provide needed information to the SIEMENS support to provide you the license (a STRING as far as I remember). 

Kind regards,

hdhosseini