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4 MODBUS PROTOCOL SPECIFICATION
4.1 INTRODUCTION
MODBUS is an application layer messaging protocol, positioned on level 7 of the OSI model, which
provides client/server communication between devices connected on different types of buses or
networks. As an industry standard since 1979, MODBUS continues to enable millions of automation
devices to communicate.
MODBUS is a request/reply protocol and offers services specified by function codes, which are elements
of request/reply PDUs. The objective of this document is to describe the function codes and Protocol
Data Unit (PDU) used within the framework of MODBUS transactions.
For more information see www.modbus.org.
4.2 MODBUS RTU
Atlante Evo supports the communication with Modbus RTU protocol over RS485 bus with a 115200
default baudrate and a default slave ID of 0x18.
Function code 03 is used to read out data, while function code 16 is used to set the parameters.
In a normal data exchange between a master and a slave, Atlante Evo must be considered as a modbus
slave as reported in the following figure.
Initiate request
Function code
Receive th response
4.2.1 FUNCTION CODE 03 - READ HOLDING REGISTERS
This function code is used to read the contents of a contiguous block of holding registers; the request
PDU has to specify the starting register address and the number of registers. Registers are addressed
starting at zero, then registers numbered e.g. 1-10 are addressed as 0-9. The register data in the response
message are packed as two bytes per register, with the binary contents right justified within each byte.
For each register, the first byte contains the high order bits and the second one contains the low order
bits.
4.2.2 FUNCTION CODE 16 - WRITE MULTIPLE REGISTERS
This function code is used to write a block of contiguous registers in a remote device. The requested
written values are specified in the "request data" field. Data is packed as two bytes per register. The
normal response returns the function code, starting address, and quantity of registers written.
Master
Data Request
Fig. 1 - Request/Response example
Slave
Perform the action
Initiate the response
Function code
Data Response
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