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clear the incomplete information, and mistake that the last byte is the address field part of the new frame.
Likewise, if the interval between t he start of a new frame and the previous frame is less than 3.5 bytes,
the receiving device will regard it as the subs equent part of the previous frame. Due to frame disorder,
the final CRC value is incorrect, which will l ead to communication failure.
Standard Structure of RTU Frame:
In ASCII mode, frame header is ":" "0x3A" ,frame tail is "CRLF" "0x0D""0x0A"). Except frame
header and frame tail, all other bytes are transmitted by ASCII coding system. It will transmit high 4 bits
first, then transmit low 4 bits . The data length is 8 bit. Capital ASCII is used to demonstrate 'A'~'F' and use
LRC check, cover the information from slave address to data.
(
)
(
START
Address Hi
Address Lo
Function Hi
Function Lo
DATA 0
( )
DATA N-1
(
)
LRC CHK Hi
LRC CHK Lo
END Hi
END Lo
: （0x3A）
Communication address:
2 ASCII combine 8-bit address
Function code:
2 ASCII combine 8-bit address
Data content:
nx8-bit 2n ASCII combine data content
n<=16,maximum 32 ASCII
LRC check:
2 ASCII combine 8-bit check code
End:
END Hi=CR(0x0D),END Lo=LF(0x0A)
6. Command Codes and Communication Data
6.1 Command Code: 03H (0000 0011), read N words (can ready a maximum of consecutive five words)
For example: for an inverter with the sla ve address of 01H, the start address of memory is 0004, ready
consecutive two words, the structu re of the frame is as follows:
RTU mode:
RTU Command Message of the Master
START
ADDR
CM D
Higher bits of start address
Lower bits of start address
Higher bits of data number
Lower bits of data number
CRC CHK lower bit
CRC CHK higher bit
END
T1-T2-T3-T4 (transmission time of 3.5 bytes)
01H
03H
00H
04H
00H
02H
85H
CAH
T1-T2-T3-T4 (transmission time of 3.5 bytes)
- 8 7 -
- 8 8 -
Frame header (START)
Slave address field (ADDR)
Function field
(CMD)
CRC CHK lower bit
CRC CHK higher bit
Frame tail END
T1-T2-T3-T4 (transmission time of 3.5 bytes)
Communication address:
0~247 (decimal) ("0" stands for the broadcast address)
03H: Read slave parameters;
06H: Write slave parameters
Data of 2*N bytes: this part is the main con tent of
communications, and is also the data exchange core in
communications.
Detection value: CRC value (16BIT).
T1-T2-T3-T4 (transmission time of 3.5 bytes)
Data field
DATA 0
( )
DATA N-1
(
)
START
"0x3A"
END
"0x0D""0x0A"
slave
address
data
CRC
check
word
ASCII mode Data Frame
operation
command
code
ASCII Response Message of the Slave
START
‘:'
RTU Response Message of the Slave
START
T1 T2 T3 T4 (t
i i
ti
f 3 5 b t
)
Appendix B Serial Communications
DZB Series
Appendix B Serial Communications
DZB Series
