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LM P RS485 Data Logger
M
ODBUS
RTU
C
OMMUNICATION
8.1
M
ODBUS
RTU
P
ROTOCOL
The Modbus RTU protocol uses a Master/Slave technique to communicate between
devices. Any application that utilizes the Modbus RTU protocol will have a Modbus Master
and at least one Modbus Slave. A Modbus Master is typically a host supervisory computer
running software that will communicate with one or more Modbus Slave devices.
The security of standard MODBUS Serial Line is based on two kinds of error checking:
•
Parity checking
(even or odd) should be applied to each character.
•
Frame checking
(LRC or CRC) must be applied to the entire message.
Both the character checking and message frame checking are generated in the device
(master or slave) that emits and applied to the message contents before transmission.
The device (slave or master) checks each character and the entire message frame during
receipt.
When devices communicate on a MODBUS serial line using the RTU (Remote Terminal
Unit) mode, each 8
–bit byte in a message contains two 4–bit hexadecimal characters.
The main advantage of this mode is that its greater character density allows better data
throughput than ASCII mode for the same baud rate. Each message must be transmitted
in a continuous stream of characters.
8.2
M
ODBUS
RTU
M
ESSAGE
F
RAMING
A MODBUS message is placed by the transmitting device into a frame that has a known
beginning device that receive a new frame to begin at the start of the message, and to
know when the message is completed. Partial messages must be detected, and errors
must be set as a result. In RTU mode, message frames are separated by a silent interval
of at least 3.5-character times.
Start
Address
Function
Data
CRC Check End
≥ 3.5
character
8 bits
8 bits
N x 8 bits
16 bits
≥ 3.5
character
The entire message frame must be transmitted as a continuous stream of characters.
Summary of Contents for LM Pro 63 Series
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