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A - 8
A.4 Communication Protocol RTU transmission mode
This section describes the messaging for a port that has been configured to support Modbus RTU
Transmission mode
An RTU transmission message Frame is shown below
START
No data input
≥
10 ms
Slave Address
Slave Address: the 8-bit binary Modbus address
Function code
Command Code: the 8-bit binary Modbus function code
DATA (n-1)
…….
DATA 0
Data Contents:
n × 8-bit BIN data, n
≦
202
CRC CHK Low
CRC CHK High
CRC Checksum:
The 16-bit CRC checksum is composed of 2 8-bit binary codes
END
No data input
≥
10 ms
Address (Slave Address)
The target address for the Modbus message. Valid addresses are in the range of 0~254. An
address of 0 indicates a broadcast to all devices.
CMD (Function code) and DATA
The content of access data depends on the function code. For descriptions of supported function
codes, please refer to
A.6
in this chapter.
For example, the following two messages demonstrate reading the 8 timer words T20~T27 from an
ELC at Modbus address 1
The values of T20~T27 are mapped to Modbus Registers: H0614~H061B
Command message:
“ 01 03 06 14 00 08 04 80”
Sent message:
Field Name
Example (Hex)
START
No data input
≥
10 ms
Slave Address
01
Function code
03
06
Starting Address
14
00
Number of Points
08
CRC CHK Low
04
CRC CHK High
80
END
No data input
≥
10 ms
Содержание ELC-PB
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