Modbus Protocol; Overview Of Modbus Protocol; Brief Introduction To Modbus; Application Of The Vfd - INVT Goodrive3000 Series Operation Manual

Goodrive3000 series medium voltage VFD

8 MODBUS protocol

8.1 Overview of MODBUS protocol

This chapter describes the communication protocol of Goodrive3000 series VFDs.
The Goodrive3000 series medium voltage VFDs provide RS485 communication interface. It adopts
international standard Modbus communication protocol to perform master-slave communication. The
user can realize centralized control through PC/PLC, upper control PC, etc. (set the control command,
running frequency of the VFD, modify relevant function codes, monitor and control the operating state
and fault information of the VFD and so on) to adapt specific application requirements.

8.2 Brief introduction to MODBUS

Modbus protocol is a software protocol and common language which is applied in the electrical controller.
With this protocol, the controller can communicate with other devices via network (the channel of signal
transmission or the physical layer, such as RS485). And with this industrial standard, the controlling
devices of different manufacturers can be connected to an industrial network for the convenience of
being monitored.
There are two transmission modes for Modbus protocol: ASCII mode and RTU (Remote Terminal Units)
mode. On one Modbus network, all devices should select same transmission mode and their basic
parameters, such as baud rate, digital bit, check bit, and stopping bit should have no difference.
Modbus network is a controlling network with single-master and multiple slaves, which means that there
is only one device performs as the master and the others are the slaves on one Modbus network. The
master can communicate with any single slave or with all slaves. For the single-visiting command, the
slave should feedback a response message; for the broadcasting message from the master, the slave
does not need to feedback the response message.

8.3 Application of the VFD

The Modbus protocol of the VFD is RTU mode and the physical layer is RS485.

8.3.1 RS485

The interface of 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
+2V–+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 follows:
Baud rate Max. transmission distance
2400BPS
4800BPS
It is recommended to use shield cables and make the shield layer as the grounding wires during RS485
remote communication.
1800m
1200m
-246-
Baud rate Max. transmission distance
9600BPS
19200BPS
MODBUS protocol
800m
600m