Modbus Communication Protocol
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7.4 RTU Communication Mode
7.4.1 Data frame format
Using RTU mode, messages are start to send in no less than 3.5 character time
interval pause. The first field transmitted is device address, the character you can
transfer is hexadecimal 0x00 ~ 0xFF. Network equipment Continuously monitor the
bus, including pauses time. When the address field is received, all equipment
determine whether it is sent to their own. when the last character of the packet
transfer is completed, a no less than 3.5 character times pause mean the end of the
message. A new message can begin after this pause.
The entire message frame must be transmitted as a continuous steaming transfer.
If a new message start transmitting in less than 3.5 character times after previous one
message, then the receiving device will consider it as a continuation of the previous
one message. This will cause an error, because the final CRC field value cannot be
right.
RTU frame format as the table below
：
Frame Header
3.5 Characters time pause
Slave Address
Slave address: 1~127
Communication command code
03H:
Read slave parameter
06H:
Write slave parameter
Data content DATA
contents of message packet;
Parameter address (16bit) ;
Number of parameter or bytes of
parameter value
parameter value (16bit)
Data content DATA
……
……
CRC check value low byte
16bit Unsigned check value
CRC check value high byte
Closing Flag
3.5
characters time pause
Generation method of CRC check value, please refer to part 7.8 for verification
mode.
7.4.2 Host read slave parameter
Command code 03H. Host can read one or more parameter ( up to ten) by
initiating a communication transaction.
E.g., read 2 contiguous inverter parameter values from the address 0000H of
inverter whose address is 01, the contents of host command as belows:
ADR 01H
CMD 03H
Parameters initial address high byte
00H
Parameters initial address low byte
00H
Number of parameter high byte
00H
Number of parameter low byte
02H
ENC
