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A. C o m m u n i c a t i o n s
A - 5
A.3 Communication Protocol ASCII transmission mode
This section describes the messaging for a port that has been configured to support Modbus ASCII
Transmission mode.
An ASCII transmission message Frame is shown below
Field name
Content
Explanation
Start bit
STX
Start bit ‘:’ (3AH)
ADR 1
Slave address
ADR 0
Address consists of 2 ASCII codes
CMD 1
Function code
CMD 0
Command code consists of 2 ASCII
codes
DATA (0)
DATA (1)
……….
Data
DATA (n-1)
Data content consist of 2n ASCII
codes, n
≤
205
LRC CHK 1
LRC checksum
LRC CHK 0
LRC checksum consists of 2 ASCII
codes
END1
Stop bit
END0
Stop bit consists of 2 ASCII codes
END1 = CR (0DH),
END0 = LF (0AH)
Corresponding table for Hexadecimal value and ASCII codes
ASCII
“0“ “1“ “2“ “3“ “4“ “5“ “6“ “7“
Hex
30H 31H 32H 33H 34H 35H 36H 37H
ASCII
“8“ “9“ “A“ “B“ “C“ “D“ “E“ “F“
Hex
38H 39H 41H 42H 43H 44H 45H 46H
ADR
(
Modbus Address
)
The target address for the Modbus message. Valid addresses are in the range of 0…31. An
address of 0 indicates a broadcast to all devices.
For Example, if the target address is 16 Decimal (10 Hex) the ADR values are:
(ADR 1, ADR 0)=’1’,’0’
Ö
’1’=31H, ‘0’ = 30H
CMD (Function code) and DATA (data characters)
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
Содержание ELC-PB
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