124ACS 400 User’s ManualCommunicationThis chapter describes the Modbus communication on ACS 400 drives.Introduction to ModbusModbus is a serial, asynchronous protocol. The Modbus protocol does not specify the physicalinterface. Typical physical interface is RS485.Modbus is designed for integration with Modicon PLCs or other automation devices, and theservices closely correspond to the PLC architecture. The ACS 400 drive ‘looks like’ a Modicon PLCon the network.If detailed information regarding the Modicon Modbus protocol is required, contact your ABBsupplier for a copy of Modbus Protocol Guide.Register Read and WriteThe ACS 400 has all drive parameter, control and status information mapped into a 4xxxx registerarea. This holding register area can be read from an external device, and an external device canmodify the register values by writing to them.There are no setup parameters for mapping the data to the 4xxxx register. The mapping ispredefined and corresponds directly to the ACS 400 parameter grouping.All parameters are available for both reading and writing. The parameter writes are verified forcorrect value, and for valid register addresses. Some parameters never allow writes (includingGroup 1 actual values), some allow only zero write (including Group 1 fault memories), someparameters allow write only when the drive is stopped (including Group 99 setup variables), andsome can be modified at any time (including e.g. Group 22 acceleration and deceleration ramptimes).Note! Parameter writes through Channel 1 (Standard Modbus) are always volatile i.e. modifiedvalues are not automatically stored to permanent memory. Parameter 1607 PARAM. SAVE can beused to save all altered values.
