Appendix A: Modbus Communication Protocol; Protocol Contents; Communication Frame Description - Veichi JT550 Series Manual

JT550 Series Inverter Manual

Appendix A: MODBUS Communication Protocol

JT550 series inverters provides RS485 communication port and supports the Modbus-RTU slave
communication protocol. Central control may be realized via computers or PLC. Through this
communication protocol, it is possible to change the settings of inverter's operation commands, to modify
or read function parameters, and to read inverter's operation status and error information, etc.

1.1 Protocol Contents

This serial communication protocol defines the contents and use format that transmits in the serial
communication, which includes polling (or broadcasting) format; for encoding methods of host, it
includes the functions requiring actions, transmitting data and error calibration, etc. For the response of
slave, it has the same structure, including actions confirmed, returning data, and error calibration, etc. If
error occurs while slave is receiving information or it can not complete the actions host requires, it will
return an error information to host as feedback.
1.1.1 Application
A PC/PLC control network that has "single host and multiple slaves" of RS485 bus is connected to
inverter as a communication slave.
1.1.2 Bus Structure
1). Hardware Connection
RS485 needs connected to inverter.
2). Topology Structure
A system of single host and multiple slaves. Each communication device has only one slave address
in the network, and one device as the communication host (PC Host , PLC and HMI, etc) spontaneously
initiate the communication to read or write the parameters on slaves. Other devices as the communication
slaves respond to the access from the host or communication operations. Only one device can send out
data at one time, and other devices are receiving data.
The setting range of slave address is 1~247 of which 0 is the broadcasting communication address.
The slave address in the network has to be unique.
3). Communication Transmission Methods
Asynchronous serial communication and two-way alternating communication. While transmitting
data through asynchronous serial communication, the data is sent out as messages, one frame at a time.
According to MODBUS-RTU protocol, when the free time of no data on communication cables is larger
than the transmission time of 3.5 Byte, it indicates a new beginning of a communication frame.
JT550 series inverter's built-in communication protocol is Modbus-RTU slave communication
protocol, responding to host's "reading/commands" or doing the corresponding actions according to host's
"reading/commands" then responding with communication data.
Hosts can be referred to personal computer (PC), industrial control device or programmable logic
controller (PLC),etc. The hosts can either communicate with one slave alone or send out some
broadcasting information to all the slaves. For the individual "query/command" of the host, the accessed
slave must return a response frame. For the broadcasting information the host sends out, the slaves do not
have to return a feedback.

1.2 Communication Frame Description

1.2.1 Communication Frame Format
JT550 series inverter's Modbus communication data format is RTU mode as below:
A byte contains: initial bit, 8 bits of data, check bit and stop bit.
Initial Bit Bit1
In the RTU mode, messages have to sent out in between of the pause intervals of at least 3.5
characters time, which can be easily realized given the various character time in the network baud rates.
Bit2 Bit3 Bit4 Bit5 Bit6 Bit7
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Chapter 7 Maintenance and Troubleshooting
Bit8 Check Bit Stop Bit