Goodrive300-21 integrated machine for air compressor
Communication protocol
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Appendix C Communication protocol
C.1 Application of Modbus
The inverter uses MODBUS RTU mode and communicates through RS485 interfaces.
C.1.1 RS485
RS485 interfaces work in half-duplex mode and transmit data signals in the differential transmission
way, which is also referred to as balanced transmission. The two-wire RS485 interface uses a twisted
pair, in which one wire is defined as A (+), and the other B (-). Generally, if the positive electrical level
between the transmission drives A and B ranges from +2V to +6V, the logic is "1"; and if it ranges from
-2V to -6V, the logic is "0".
On the inverter terminal block, the 485+ terminal corresponds to A, and 485- corresponds to B.
The communication baud rate (P14.01) indicates the number of bits sent in a second, and the unit is
bit/s (bps). A higher baud rate indicates faster transmission and poorer anti-interference capability.
When a twisted pair of 0.56mm (24 AWG) is used, the maximum transmission distance varies
according to the baud rate, as described in the following table.
Baud rate
Max. transmission distance
Baud rate
Max. transmission distance
2400BPS
1800m
9600BPS
800m
4800BPS
1200m
19200BPS
600m
In long-distance RS485 communication, it is recommended that you use shielded cables, and use the
shielding layer as the ground wire.
When there are fewer devices and the transmission distance is short, the whole network works well
without terminal load resistors. The performance, however, degrades as the distance increases.
Therefore, it is recommended that you use a 120 Ω terminal resistor when the transmission distance
is long.
C.2 RTU command codes and communication data
C.2.1 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 inverter. 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
inverter.
C.2.2 Command code 06H, writing a word
This command is used by the master to write data to the inverter. One command can be used to write
