Goodrive350 IP55 High-ingress Protection Series VFD
Communication protocol
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}
}
return(crc_value);
}
In the ladder logic, CKSM uses the table look-up method to calculate the CRC value according
to the content in the frame. The program of this method is simple, and the calculation is fast,
but the ROM space occupied is large. Use this program with caution in scenarios where there
are space occupation limits on programs.
9.4 RTU command code and communication data
9.4.1 Command code: 03H, reading N words
Continuously reading a maximum of 16 words
The command code 03H is used by the master to read data from the VFD. The quantity of data
to be read depends on the "data quantity" in the command. A maximum of 16 pieces of data
can be read. The addresses of the read parameters must be contiguous. Each piece of data
occupies 2 bytes, that is, one word. The command format is presented using the hexadecimal
system (a number followed by "H" indicates a hexadecimal value). One hexadecimal value
occupies one byte.
The 03H command is used to read information including the parameters and operation state of
the VFD.
For example, starting from the data address of 0004H, to read two contiguous pieces of data
(that is, to read content from the data addresses 0004H and 0005H), the structure of the frame
is described in the following table.
RTU master command (transmitted by the master to the VFD)
START
T1-T2-T3-T4 (transmission time of 3.5 bytes)
ADDR (address)
01H
CMD (command code)
03H
Most significant byte (MSB) of the start address
00H
Least significant byte (LSB) of the start address
04H
MSB of data quantity
00H
LSB of data quantity
02H
LSB of CRC
85H
MSB of CRC
CAH
END
T1-T2-T3-T4 (transmission time of 3.5 bytes)
The value in START and END is "T1-T2-T3-T4 (transmission time of 3.5 bytes)", indicating that
the RS485 needs to stay idle for at least the transmission time of 3.5 bytes. An idle time is
required to distinguish on message from another to ensure that the two messages are not
regarded as one.
The value of ADDR is 01H, indicating that the command is transmitted to the VFD whose
address is 01H. The ADDR information occupies one byte.
The value of CMD is 03H, indicating that the command is used to read data from the VFD. The
CMD information occupies one byte.
"Start address" indicates that data reading is started from this address. It occupies two bytes,
with the MSB on the left and LSB on the right.
