15.Modbus TCP/RTU communication interface
15.1.Control unit mains and sensors readings
Introducing MODBUS protocol
The Modbus serial line protocol is a Master-Slaves protocol. This means that only one master and one or
more slave nodes (max. 247) can be connected to the same serial bus. A Modbus communication is always
initiated by the master and there is only one transaction at the same time.
For further information: www.modbus.org
If you intend to use Modbus, you should only use ASCII characters in the Web UI. Unicode characters will
not be displayed in Modbus.
Modbus frame description (RTU mode)
Address PDU Frame Error Check
Address Field Function Code Data CRC
1 byte 1 byte 0 - 252 bytes 2 bytes CRC
L
, CRC
H
ADU Application Data
PDU Protocol Data Unit
Stopbit 1
Address Field contains the slave address
Function Code: indicates what kind of action to perform
Data contains request and response parameters
CRC contains the value generated by the cyclic redundancy check
(standard CRC-16 defined by CCITT)
The maximum size for a Modbus RTU frame is 256 bytes.
In RTU mode, message frames are separated by a silent interval of at least 3.5 character times.
The entire message frame must be transmitted as a continuous string of characters.
If a silent interval of more than 1.5 character times occurs between two characters, the
message frame is declared as incomplete and should be discarded by the receiver.
Modbus Data Encoding
Modbus uses a big-endian allocation for addresses and data items. This means that, when a numerical
quantity larger than a single byte is transmitted, the most significant byte is sent first.
SCU100 INSITE PRO M COMPACT USER MANUAL 63
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