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Maximum RS-485 ADU = 256 bytes, limit the length of Modbus PDU, so for serial link communication, Modbus PDU =
256-additional address (1 byte)-CRC(2 bytes) = 253 bytes. Thus RS-485 ADU = (253 bytes) + additional address (1 byte) +
CRC (2 bytes) = 256 bytes.
For example: taking the communication address control bit 0x9C8E(40078) of the first indoor unit Mapping Table as
example, refer to the Mapping Table for other indoor units as follows:
Modbus RTU code: power on the first indoor unit, write command format.
Additional address
Function code
Data
CRC CHECK
32
10
9C 8E 00 01 02 00 01
65 86
Modbus device
continuous write
command
Communication protocol start address Write hold register
number Byte Write value
Modbus RTU code: Modbus adapter write response code.
Additional address
Function code
Data
CRC CHECK
32
10
9C 8E 00 01
4A 71
Modbus device
continuous
write command
Communication protocol start address Number of
successful write hold register
Modbus RTU code: Modbus adapter write error response code.
Additional address
Function code
Data
CRC CHECK
32 90
01
7D
CF
Modbus device
Function
code+0x80
Modbus RTU code: the first indoor unit system Data0x9C40 (40000), read command format.
Additional address
Function code
Data
CRC CHECK
32
03
9C 40 00 45
AE 7E
Modbus device
continuous
read command
Communication protocol start address Number of
read hold register (69)
Modbus RTU code: Modbus adapter read response code.
Additional address
Function code
Data
CRC CHECK
32 03
8A 00 52 00 41 00 53 00 2D 00 32 00 30 00 30 00
46 00 53 00 44 00 4E 00 59 00 32 00 51 00 20 00 08 00
EF …
9E D3
Modbus device
continuous
read command
Number of read bytes Successfully read Data (69
register Data, 138 bytes)
Modbus RTU code: Modbus adapter read error response code.
Additional address
Function code
Data
CRC CHECK
32 83
01
70
FF
Modbus device
Function
code+0x80
Note
●
When using Modbus RTU, need to place terminal resistance at both ends of the RS-485 communication line, this
Modbus adapter is defaulted as supplied with terminal resistance, bit2 of DSW2 is defaulted as ON
。
4 Air Conditioner Monitoring