Modbus TCP Overview as it relates to the AKD drive

Modbus TCP is a communication protocol that is available on all models of the AKD drive ( except the AKD-P-NAPN or AKD-P-NBPN ( Profinet ).  The communication is supported on Ethernet physical hardware, the X11 Ethernet port on the top of the drive.

The Modbus protocol supports reading and writing integer values from/to AKD drive parameters and commands.  It is not considered to be a "fieldbus" protocol, since it does not have a priority channel or features such as a watchdog or heartbeat.  It is only used to read an integer number value from a memory register and write an integer number value to a memory register.  Each read/write is performed individually.  The AKD drive has a communication buffer and supports "block read/write".   But each request is processed individually.  Modbus  is not deterministic, which means the master has no control over the timing of when a request will be processed by the slave.  The data is transmitted via Ethernet packets.  The time is takes to receive a packet depends on the amount of data traffic on the network.  Packets are not guaranteed to arrive in the same order they are sent.

Each Modbus address corresponds to a parameter or command in the AKD.  Most of the parameters consist of two 16bit registers (total 32 bits).  Some of the parameters consist of four 16bit registers (total 64 bits).  The Modbus address refers to the upper register number.  The upper register is the register with the lowest value address.

Example: VL.FB (velocity feedback)

Modbus address 856

Upper register = 856

Lower register = 857

Bit 0 of the lower register is the least significant bit of the parameter.  Bit 15 of the upper register is the most significant bit of the parameter.

The registers can be read individually as 16 bits only, but it is most common to setup the master device to read/write as either 32bit or 64bit.