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19 MODBUS RTU / MODBUS TCP TRANSMISSION PROTOCOL
19.1 Insights
The device utilises the following Modbus functions:
03 (0x03) Read Holding Registers,
04 (0x04) Read Input Registes,
In compliance with the Modbus RTU standard, the frame (information sent) is in the form
of:
Start
T1 ... T4
In compliance with the MODBUS standard, the frame (information sent) in TCP/IP mode
is in the form of:
Information sent from the master computer to the device is in the form of Queries,
whereas the device itself sends Responses.
19.2 Functions
Functions 03 (0x03) and 04 (0x04) are applied conversely to obtain the register data.
Command 03 or 04 is in the form of:
Function – 03 HEX or 04 HEX – register data reading.
Initial address - address of the register from which data will be sent.
Number of registers – number of 2 byte registers to read.
In response, the device will send a string of characters in the form of:
Function – return confirmation; in the case of an error, 80 HEX is added to the command
code value.
Number of bytes – n bytes sent in response (instead of the number of registers).
Data – n bytes of register content.
Address
Function
1 byte
1 byte
MBAP
Header
7 bytes
Function
Initial address
(1B)
Function
Number of
(1B)
bytes (1B)
Data
n bytes
Function
Data
1 byte
n bytes
Number of
(2B)
registers (2B)
Data sequence
70
DL7, DL7L
CRC
control
2 bytes
T1 ... T4
(nB)
End
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