Communication Protocol; Brief Instruction To Modbus Protocol; Application Of The Inverter - Unitronics UMI-B1 UL Series Operation Manual

UMI-B1 UL Series Inverter

Communication protocol

7 Communication protocol

7.1 Brief instruction to Modbus protocol

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 convenient 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 means the device which has active talking
right to sent message 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) from the
master (response). After the master sends message, there is a period of time left for the
controlled or inquired slaves to response, which ensure there is only one slave sends
message to the master at a time for the avoidance of singles impact.
Generally, the user can set PC, PLC, IPC and HMI as the masters to realize central
control. Setting certain device as the master is a promise 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 can not 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 of the inverter

The Modbus protocol of the inverter is RTU mode and the physical layer is 2-wire
RS485.
7.2.1 2-wire RS485
The interface of 2-wire RS485 works on semiduplex and its data signal applies
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