LARTET DC-24, User Manual

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CHAPTER 4 COMMUNICATION PROTOCOLS 4-4
DC-24 and DC-25 operation manual
4.2. PROTOCOL MODBUS RTU - ASCII for RS-485
This protocol allows the user to communicate with devices that dispose of ModBus RTU
protocol
It uses the function 10h and the data block is in ASCII mode. The values can be read or sent.
If the user wants to send the values to the display, the sensor must be disabled (see
paragraph 3.3.3)
To read the values from the display, the following command must be sent
Addr. Func.
Register
High
Register
Low
Words
High
Words
Low
Bytes
num
Data
Low
CRC
High
CRC
10h 01h 01h 00h 01h 02h P T
03h 10h 01h 01h 00h 01h 02h 50h 54h 93h DEh
The first byte is the address of the display inside the network. The three following bytes
must be 10h, 01h and 01h. The bytes 5 and 6 correspond to the number of words of the data
field. In the data field the characters “PT” must be placed, so the display know the user is
requesting data
In the previous example, the CRC is calculated for the display at the 03 address.
An example of the response of the display is the following message
Addr Func. Register
High
Register
Low
Words
High
Words
Low
Bytes
num
10h 01h 01h 0 5 0Ah
03h 10h 01h 01h 00h 05h 0Ah
Data
Low CRC High CRC
P T 23,7 51
50h 54h 32h 33h 2Eh 37h 20h 35h 31h 20h D1h 19h
In the response message, the received code is repeated and the temperature and
humidity values are added, separated by the space character (20h). After the humidity, there is
another space character (20h) so the byte number is even. If the display detects an error in the
received block, the error code with the following data code is sent
Address. Error Error code Low CRC High CRC
90h
The possible errors are:
02 = CRC error.