Service manual

www.enc.net.cn/en Tel/Fax:[phone] Modbus communication protocol
129
11.4 RTU Communication Mode：
11.4.1 Data frame format
Using RTU mode, messages are sent at least 3.5 character time interval pause.
The first transmitted field is device address, the character you can transfer is
hexadecimal 0x00 ~ 0xFF. Network equipment Continuously monitor the bus,
including pauses. 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
complete, at least a 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 flow. If a new
message start transmitting in less than 3.5 character times after a message and
then receiving device will consider it a continuation of the previous message. This
will cause an error, because in the final CRC field value can not be right.
RTU frame format as the talbe below：
Frame Header 3.5 characters time pause
Slave address Slave value：1~127
Communication command code
03H：read slave parameter
06H：write slave parameter
Data content DATA
The contents of 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
Regarding generation method of CRC check value, please refer to part 11.8
for check method.
11.4.2 Host read slave parameter
Command code 03H. Host can read or 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 whoes address is 01, the contents of host command ：
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
CRC check value low byte Be calculated
CRC check value high byte Be calculated