HNC Electric HV480 Series, Руководство пользователя

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3 Modbus protocol description
3.1 protocol description
Protocol brief introduction
①
Modbus master slave protocol. Only one device can send commands in the network at any time.
②
The master station exchanges management information by polling the slave stations. No slave station can
send messages without the approval of the master station. If there is an error in data exchange, if the master station
does not receive a response, the slave station absent from the poll will be interrogated again.
③
If the slave station cannot recognize the information sent by the master station, it will send an abnormal
response to the master station.
④
If there is no direct communication between the slave station and the slave station, one slave station's data
must be read out through the master station's software and sent to another slave station Two types of dialogues can
be realized between the master station and the slave station:
The master station sends a request to the slave station and waits for the slave station to respond.
The master station sends requests to all slave stations without waiting for their response broadcast mode.
Transfer
The transmission mode is RTU (remote terminal unit) mode, and the frame does not contain any message
header bytes or message word segment terminators. The classic RTU frame format is shown in the following table:
Slave address Function code Data CRC
1 byte 1 byte 0 ... 252 bytes CRC low bit
①
Address 0 (reserved), used for broadcasting.
②
All slave nodes must identify the broadcast address of the write function but do not need to respond.
③
The master node does not have a specific address, only the slave node must configure the address.
RTU transmission method has four character formats, as follows:
①
1 - bit start bit, 8 - bit data bit, 2 - bit stop bit, no check
②
1 - bit start bit, 8 - bit data bit, 1 - bit stop bit, even check
③
1 - bit start bit, 8 - bit data bit, 1 - bit stop bit, odd check
④
1 - bit start bit, 8 - bit data bit, 1 - bit stop bit, no check
Characters or bytes are transmitted from left to right, as shown in the following table:
< - least significant bit ( LSB ) Most significant bit ( msb ) - >
Starting bit 1 2 3 4 5 6 7 8 Stop bit Stop bit
Starting bit 1 2 3 4 5 6 7 8 Even check Stop bit
Starting bit 1 2 3 4 5 6 7 8 Odd check Stop bit
Starting bit 1 2 3 4 5 6 7 8 Stop bit -
The above table shows that RTU transmission mode
Information frames are separated by at least at static interval of 3.5 bytes. The entire frame must be transmitted
in a continuous stream of bytes. If the interval between more than two frames is less than 3.5 bytes, the receiving
device will mistakenly assume that the slave station address of the second frame is the continuation of the previous
frame. CRC check failed due to frame disorder, resulting in communication failure. If the rest interval between two
bytes exceeds 1.5 bytes, the receiving device will consider the information frame incomplete and discard it.