Chapter 8 MODBUS Communications
ASDA-B2
8-8
Revision September 2013
11-bit character frame (For 8-bit character)
8N2
Start
bit
0
1
2
3
4
5
6
Stop
bit
8-data bits
11-bits character frame
Stop
bit
7
8E1
Start
bit
0
1
2
3
4
5
6
Stop
bit
8-data bits
11-bits character frame
7
Even
parity
8O1
Start
bit
0
1
2
3
4
5
6
Stop
bit
8-data bits
11-bits character frame
7
Odd
parity
Communication Protocol:
ASCII Mode:
Start
Start character’: ’ (3AH)
Slave address
Communication address: 1-byte consists of 2 ASCII codes
Function
Function code: 1-byte consists of 2 ASCII codes
DATA(n-1)
Contents of data: n word = n x 2-byte consists of n x 4 ASCII codes,
n
10
…….
DATA(0)
LRC
LRC check sum: 1-byte consists of 2 ASCII codes
End 1
End code 1: (0DH)(CR)
End 0
End code 0: (0AH)(LF)
ASCII Mode: ’:’ character
ADR (Communication Address) consists of 2 ASCII codes and it ends in CR (Carriage Return) and
LF (Line Feed)
CR (Carriage Return) is represented by ASCII number 13, and LF (Line Feed) is represented by
ASCII number 10.
There are communication address, function code, contents of data, LRC (Longitudinal Redundancy
Check) between start and end characters.
VARITEL INGENIERIA ELECTRONICA S.A.
[email protected] - www.varitel.com - Tel. (54) 11-4243-1171 / Fax: (54) 11-4292-7545
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