CDE500 Vector Control AC Drive
Chapter 8 Communication Protocol
8.4 Protocol Format
The MODBUS protocol of the AC drive is RTU mode.
Figure 8-1 shows the frame format of RTU.
Figure 8- 1 RTU frame format
In RTU mode, each character format is as follows: 8 bits, 8 bits in each frame domain, containing
two of sixteen hexadecimal characters (0 ~ 9, A ~ F); in order to distinguish clearly, the following
sixteen hexadecimal data end with "H".
In RTU mode, the idle time between frames to follow the MODBUS internal agreement. MODBUS
internal least frame agreed between idle as follows:
(1) The frame head and frame end is defined through the bus idle time is equal to or greater than 3.5
bytes of time.
(2) The frame after the start, the interval between characters must be less than 1.5 bytes of time,
otherwise the new to receive characters will be considered as a new frame.
(3) Using CRC check mode, the high 8 bits and 8 bits low must swap before send.
(4) At least 3.5 bytes of free time to maintain between frames.
The standard structure of RTU frame:
Table 8- 1 RTU frame format
START
（
Frame head
）
T1-T2-T3-T4
（
3.5 chars space time
）
ADDR
（
slave address
）
1
～
247
（
0 is broadcast address
）
CMD
（
function code
）
03H: read register
06H: write register
（
DATA
）
DATA
（
0
）
…
DATA
（
N-1
）
2*N bytes data,this is the main comment of
frame.
CRC LOW
CRC
（
16bits
）
CRC HIGH
END
（
Frame end
）
T1-T2-T3-T4
（
3.5 chars space time
）
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