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CMS-660 SYSTEM
CIRCUIT MONITORING SYSTEM (CMS)
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Modbus
Introducing Modbus protocol
The Modbus serial line protocol is a Master-Slave 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.
Modbus frame description (RTU mode)
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
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Communication protocol
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ADU Frame
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)
Address
PDU Frame
Error Check
Address Field
Function Code
Data
CRC
1 byte
1 byte
0 - 252 bytes
2 bytes CRC
L
, CRC
H
