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CMS-700 SYSTEM CIRCUIT MONITORING SYSTEM (CMS)
—
Modbus
—
Communication protocol
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)
—
ADU Frame
Address
Address Field
1 byte
ADU
PDU
Stopbit
Address Field
Function Code:
Data
CRC
The maximum size for a Modbus RTU frame is 256 bytes.
NOTE:
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.
Example: 1234h → first 12h then 34h
PDU Frame
Function Code
Data
1 byte
0 - 252 bytes
Application Data
Protocol Data Unit
1
contains the slave address
indicates what kind of action to perform
contains request and response parameters
contains the value generated by the cyclic redundancy check (standard
CRC-16 defined by CCITT)
51
Error Check
CRC
2 bytes CRC
, CRC
L
H
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