7. Interfaces and Communication
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7
7.5 Modbus Communication Protocol
The drive provides RS485 communication interface and supports Modbus-RTU communication protocol
so that the user can implement centralized control, such as setting running commands and function
codes, and reading running status and fault information of the AC drive, by using a PC or PLC.
This protocol defines content and format of transmitted messages during serial communication,
including master polling (or broadcasting) format and master coding method (function code for the
action, transmission data, and error check). The slave uses the same structure in response, including
action confirmation, data returning and error check. If an error occurs when the slave receives a
message, or the slave cannot complete the action required by the master, the slave returns a fault
message as a response to the master.
7.5.1 Application
The AC drive is connected to a "single-master multi-slave" PC/PLC control network with RS485
bus.
7.5.2 Bus Structure
Ɣ
Interface mode
The RS485 extension card MD38TX1 must be inserted into the AC drive.
Ɣ Topological structure
The system consists of a single master and multiple slaves. In the network, each
communication device has a unique slave address. A device is the master (can be a PC, a
PLC or an HMI) and initiates communication to perform parameter read or write operations
on slaves. The other devices (slaves) provide data to respond to query or operations from
the master. At the same moment, either the master or the slave transmits data and the other
can only receives data.
The address range of the slaves is 1 to 247, and 0 is broadcast address. A slave address
must be unique in the network.
Ɣ
Transmission mode
The asynchronous serial and half-duplex transmission mode is used. During asynchronous
serial communication, data is sent frame by frame in the form of message. In Modbus-RTU
protocol, an interval of at least 3.5-byte time marks the end of the previous message. A new
message starts to be sent after this interval.
Master sending 1 Slave response 1 Master sending 2 Slave response 2
Interval of at
least 3.5-byte
time
Interval of at
least 3.5-byte
time
Data frame
Data frame
A
B
The communication protocol used by the drive is the Modbus-RTU slave communication
protocol, which allows the drive to provide data to respond to "query/command" from the
master or execute the action according to "query/command" from the master.
The master can be a PC, an industrial device, or a PLC. The master can communicate with a
single slave or send broadcast messages to all slaves. When the master communicates with
a single slave, the slave needs to return a message (response) to "query/command" from the
master. For a broadcast message sent by the master, the slaves need not return a response.
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