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Why the ATV61/71 will not respond to the command given over Modbus TCP/IP?

Why the ATV61/71 will not respond to the command given over Modbus TCP/IP?
Issue: 
Why the ATV61/71 will not respond to the command given over Modbus TCP/IP?
 
Product Line:
Altivar 61/71 using Modbus TCP/IP communication protocol
 
Environment:
All serial numbers
 
Cause:
What will cause the VFD not to respond to the command and reference signal when the VFD is setup and reading correctly.
 
Resolution:
When using Modbus TCP/IP, there is a place for the Master IP address, if an IP address is assigned to the Master IP address, that is the only address the VFD will accept commands.   The Master IP address should be the IP address of the only PLC that is controlling the VFD.    To troubleshoot, unassign the Master IP.  Test by writing to the command word and the speed reference of the VFDs.                 
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