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Protocol
7 Protocol
The MODBUS serial communication protocol is a Master/Slave-Protocol. With the bus only one master
is connected (at a time). One or several (max. 247) slave-nodes are connected with each other on the
same bus. A MODBUS communication is always initiated by the master. The slave nodes do not com-
municate with each other. The master only initiates one MODBUS communication at a time.
The master sends a MODBUS request to the slave-nodes in two different operation modes::
− In the Unicast operation mode the master addresses a single slave-node. After reception and
processing of the request, the slave-node sends a message (reply) to the master. In this operation
mode a MODBUS transfer consists of two messages: a request from the master and a reply from
the slave-node. Each slave-node must have a unique address (from 1 to 247), so that the slave-
nodes can receive requests independent of each other.
− In the Broadcast operation mode the master can send a request to all slave-nodes. The slave-
nodes do not send a reply to the request.. The request consists of write commands. All slaves must
accept the write commands. Address 0 is reserved for the recognition of a broadcast request.
7.1 Address Representation
Up to 247 frequency inverters can be operated on MODBUS. These are assigned addresses in the
range 1 … 247. With address 0, all clients connected to the bus can be addressed simultaneously.
Address 0 is also referred to as the Broadcast-Address.
7.2 Message Structure
A MODBUS message is made up of the following fields:
Address Function Code Data CRC (or LRC)
The Address field contains the slave-address in the Modbus message.
Valid slave node addresses are in the range of 0 – 247 (decimal). The individual slave devices are
assigned addresses in the range of 1 – 247. A master addresses a slave by placing the slave address
in the address field of the message. When the slave returns its response, it places its own address in
the response address field to let the master know which slave is responding.
The Function Code indicates to the inverter what kind of action to perform. The function code can
be followed by a
Data field that contains request parameters (or, in the case of the inverter's re-
sponse, the response parameters).
If no error occurs related to a correctly received Modbus request then the data field contains the data
requested. If an error occurs then the field contains an Exception Code to indicate to the master that
the request was unsuccessful. Exception Responses and Codes are described in detail in chapter 7.3.9
"Exception Codes".
If a request is not received correctly (communication error, checksum error) then no response is sent.
The inverter waits for the next request.
The Error Checking Field is the result of a "Redundancy Checking" calculation that is performed on
the message contents. Two kinds of calculation methods are used depending on the transmission
mode that is being used (RTU or ASCII). See Chapter 7.3.10 "Modbus Transmission Modes".
7.3 Supported Function Codes
Modbus definitions for reading/writing data in a device do not fit directly to parameter access of inver-
ters (independent of inverter manufacturer). Modbus is defined for reading/writing bits and registers
in a different way. Furthermore, data access is limited to 16 bit wide data.
To fulfill the Modbus requirements the data access to parameters in the devices (inverters) will use
the following defined function codes:
16 bits values:
− Function Code 3, Read ONE 16 bit wide data
(Read Holding Register)
−
Function Code 6, Write ONE 16 bit wide data
(Preset Single Register)
− Function Code 16, Write ONE 16 bit wide data
(Preset Multiple Register)
−
32 bits values:
For data access to 32 bit wide data two new inverter specific function codes are defined:
− Function Code 3, Read two 16 bit (=32 Bit) wide data (Read Holding Register)
− Function Code 16, Write two 16 bit (=32 Bit) wide data
(Preset Multiple Register)
− Function Code 100, Read ONE 32 bit wide data
− Function Code 101, Read ONE 32 bit wide data
Note: The Modbus specification doesn’t specify the handling of 32 bit values. The implemented han-
dlings and function codes to access 32 Bit values are however widely spread and commonly used.
These functions allow data access to 32-bit "long" variables/parameters in the inverter.
For diagnostic purposes the Modbus function code "8" (
Diagnostics) is also supported.
These function codes and their corresponding data fields are described in detail in the following chap-
ters.
Note:
In all data fields with more than one byte, the highest order byte is transmitted first.
Message examples for all supported function codes in both RTU and ASCII transmission modes can be
found in Chapter 9 "Modbus Message Examples".