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VFD-200G Modbus communication protocol

Jul 15, 2019

VFD-200G series frequency convertor provides RS232/RS485 communication interface and supports Modbus communication protocol. Users can realize centralized control through computer or PLC, set run command of frequency convertor through communication protocol, modify or read parameters of function code, read working condition and fault information of frequency convertor, etc.


Protocol content

The serial communication protocol defines transmissive information contents and using format of serial communication, including format for polling of host (or broadcast), encoding method of host such as function code of required action, transmission data and error verification, etc. The response of slave also adopts same structure and the contents include action confirmation, data return and error verification, etc. If any error of slave when receiving information or failure to finish the action required by host, the slave will organize a  fault message as response feedback for the host.

             Application mode: frequency convertor accesses to “single-host and multiple-slave” PC/PLC control network with RS232/RS485 bus.


Structure of bus

(1) Interface mode

RS232/RS485 hardware interface

(2) Transmission mode: asynchronous serial and half-duplex. For the host and slave at the same moment, one can only send data and another can only receive data. During serial asynchronous communication process, data is sent with the form of message frame by frame.

(3) Topological structure: single-host and multiple-slave system. The setting range of slave address is 1~247 and 0 is address of broadcast communication. Slave address in network should be unique.


Protocol description

Communication protocol of VFD-200G series frequency convertor is a kind of asynchronous serial master- slave Modbus communication protocol, and only one device (host) in the network can establish protocol (called as “query/command”). Other devices (slave) can only response the “query/command” of host by providing data or take corresponding actions based on “query/command” of host. Host refers to personal computer (PC), industrial control equipment or programmable logic controller (PLC), etc, and slave means VFD-200G series frequency convertor. The host can not only communicate with certain slave separately, but issue broadcast information to all inferior slaves. For separately accessed “query/command” of host, the slave needs to return a message (called as response). For the broadcast information issued by the host, the slave does not need to feedback response to host.

Structure of communication materials: communication data format of modbus protocol for VFD-200G series frequency convertor is as below:

              For RTU mode, message sending starts with pause time for at least 3.5 characters. Diverse character time under network Baud rate is easily realized (as shown in below T1-T2-T3-T4). The first domain of transmission   is equipment address.

The available transmission character is hexadecimal 0...9, A...F. Network equipment detects network bus constantly, including pausing interval time. When receiving first domain (address domain), each equipment will decode to judge if sending to own. After the last transmission character, the pause time of at least 3.5 characters marks the ending of message. A new message will start after the pause.

              The whole message frame should be continuous streaming transfer. If the dwell time exceeds  1.5  characters before frame finishes, receiving equipment will refresh incomplete message and assume that next byte is address domain of a new message. Similarly, if a new message starts within time of 3.5 characters following previous message, receiving equipment will consider it as the delay of previous message, and then error will be caused, as it’s impossible for the value of final CRC domain to be correct.