Can the ATV320 be set to maintain running speed if the Modbus communication is lost?

Yes this can be done.
Parameter SLL in the Com Fault Management menu and can be used to modify the behavior of the VFD after a communication interruption: including the option to run at the last commanded frequency or to run at a fixed (fallback) speed.

Care must be taken to ensure safe operation of the equipment...see the drive programming manual for details.

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