Application Of The Vfd; Rs485; Rtu Mode - INVT Goodrive200A Series Operation Manual

GD200A series VFD

9.3 Application of the VFD

The MODBUS protocol of the VFD is RTU mode and the physical layer is 2-wire RS485.

9.3.1 RS485

The interface of 2-wire RS485 works on semiduplex and its data signal applies differential
transmission which is called balance transmission, too. It uses twisted pairs, one of which is defined
as A (+) and the other is defined as B (-). Generally, if the positive electrical level between sending
drive A and B is among +2–+6V, it is logic"1", if the electrical level is among -2V–-6V; it is logic"0".
485+ on the terminal board corresponds to A and 485- to B.
Communication baud rate means the binary bit number in one second. The unit is bit/s (bps). The
higher the baud rate is, the quicker the transmission speed is and the weaker the anti-interference is.
If the twisted pairs of 0.56mm (24AWG) is applied as the communication cables, the max.
transmission distance is as below:
Baud rate
2400BPS
4800BPS
It is recommended to use shield cables and make the shield layer as the grounding wires during
RS485 remote communication.
In the cases with less devices and shorter distance, it is recommended to use 120Ω terminal resistor
as the performance will be weakened if the distance increase even though the network can perform
well without load resistor.

9.3.2 RTU mode

9.3.2.1 RTU communication frame format
If the controller is set to communicate by RTU mode in MODBUS network every 8bit byte in the
message includes two 4Bit hex characters. Compared with ACSII mode, this mode can send more
data at the same baud rate.
Code system

1 start bit

7 or 8 digital bit, the minimum valid bit can be sent firstly. Every 8 bit frame includes two hex
characters (0...9, A...F)

1 even/odd check bit . If there is no checkout, the even/odd check bit is inexistent.

1 end bit (with checkout), 2 Bit(no checkout)
Error detection field
 CRC
Max. transmission
distance
1800m
1200m
-128-
Communication protocol
Baud rate
9600BPS
19200BPS
Max. transmission
distance
800m
600m