SD1 Series Inverters Communication Protocol
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Chapter 7 Communication Protocol
7.1 Modbus Protocol Introduction
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 a
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 be identical.
Modbus network is a controlling network with single-master and multiple slaves, which means
that only one device performs as the master and the others are the slaves on one Modbus
network. The master means the device which has active talking right to send messages to
Modbus network for the controlling and inquiring to other devices. The slave means the
passive device which sends data message to the Modbus network only after receiving the
controlling or inquiring message (command) form the master (response). After the master
sends message, there is a period left for the controlled or inquired slaves to respond, which
ensures only one slave sends a message to the master at a time for the avoidance of
collisions.
Generally, the user can set PC, PLC, IPC, and HMI as the masters to realize central control.
Setting certain device as the master is undertaking other than setting by a bottom or a switch
or the device has a special message format. For example, when the upper monitor is running,
if the operator clicks sending command bottom, the upper monitor can send command
message actively even it cannot receive the message from other devices. In this case, the
upper monitor is the master. And if the designer makes the inverter send the data only after
receiving the command, then the inverter is the slave.
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.
7.2 Application
The Modbus protocol of the inverter is RTU mode, and the physical layer is 2-wire RS485.
7.2.1 Two-wire RS485
The interface of 2-wire RS485 works on half-duplex and its data signal applies differential
transmission, which is also called balance transmission. 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
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