Message Structure For Rtu Mode; Address; Function Code; Data Field - Fairford synergy User Manual

4. Communication (continued)

4.1.4 Message Structure For RTU Mode

The Modbus RTU structure uses a master-slave system for message exchange. In the case of the
synergy system, it allows up to 32 slaves, and one master. Every message begins with the master
making a request to a slave, which responds to the master in a defined structure. In both messages
(request and answer), the used structure is the same:
Address, Function Code, Data and CRC.

Master (request message):
Slave (response message):

4.1.5 Address

The master initiates the communication by sending a byte with the address of the destination slave.
When responding, the slave also initiates the message with its own address. Broadcast to address 0
(zero) is not supported.

4.1.6 Function Code

This field contains a single byte, where the master specifies the type of service or function requested to
the slave (reading, writing, etc.). According to the protocol, each function is used to access a specific type
of data.

4.1.7 Data Field

The format and contents of this field depend on the function used and the transmitted value.

4.1.8 CRC

The used method is the CRC-16 (Cyclic Redundancy Check). This field is formed by two bytes; where first
the least significant byte is transmitted (CRC-), and then the most significant (CRC+). The CRC calculation
form is described in the Modbus RTU protocol specification.

4.1.9 Supported Functions

Modbus RTU specification defines the functions used to access different types of data.
synergy parameters are defined as holding type registers.
•
For Modbus RTU/TCP Client devices that use Modicon style addressing, place a 4 as the
•
high digit followed by the Modbus address defined in the parameter mapping table.
Note that synergy Modbus addressing starts at zero; not 1 as some devices do.
synergy 32-bit parameters are High Word / Low Word in Modbus format.
•
Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
Address Function
(1 byte) (1 byte)
Address Function
(1 byte) (1 byte)
115
Request Data (n
CRC
bytes) (2 bytes)
Response Data
CRC
(n bytes)
(2 bytes)