Goodrive3000 series medium voltage VFD
Moudbus protocol
-197-
8.4 RTU command codes and communication data
8.4.1 Command code 03H, reading N words (continuously up to 16 words)
The command code 03H is used by the master to read data from the VFD. The count of data to be read depends on the
"data count" 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 running status of the VFD.
For example, to read two contiguous data content pieces from 0004H from the VFD with the address of 01H (that is, to
read content from data addresses 0004H and 0005H), the frame structure is as follows:
RTU master command (from the master to the VFD)
START
T1-T2-T3-T4 (transmission time of 3.5 bytes)
ADDR (address)
01H
CMD (command code)
03H
Start address MSB
00H
Start address LSB
04H
Data count MSB
00H
Data count LSB
02H
CRC LSB
85H
CRC MSB
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.
ADDR=01H means the command message is sent to the VFD with the address of 01H and ADDR occupies one byte.
CMD=03H means the command message is sent to read data from the VFD and CMD occupies one byte.
"Start address" means reading data from the address and it occupies two bytes with the MSB on the left and LSB on the
right.
"Data count" indicates the count of data to be read (unit: word). "Start address" is "0004H" and "Data count" is 0002H,
indicating that data is to be read from the data addresses of 0004H and 0005H.
CRC check occupies two bytes, with the LSB on the left, and MSB on the right.
RTU slave response (from the VFD to the master)
START
T1-T2-T3-T4 (transmission time of 3.5 bytes)
ADDR
01H
CMD
03H
Number of bytes
04H
MSB of data in 0004H
13H
LSB of data in 0004H
88H
MSB of data in 0005H
00H
LSB of data in 0005H
00H
CRC LSB
7EH
CRC MSB
9DH
END
T1-T2-T3-T4 (transmission time of 3.5 bytes)
The response information is described as follows:
"ADDR" is "01H", indicating that the message is sent by the VFD whose address is 01H. The ADDR information occupies
