1 FRAME SPECIFICATIONS
1.1 MODBUS Protocol Frame Specifications
9
1
• Data (2 bytes): Start coil number specification (0000H), Number of read points (0008H)
Data is stored in ASCII code as follows:
The slave, which received the request message, recalculates the check code in the request message and determines
whether the message is correct or not.
Error check procedure LRC in request message transmission
1 Convert the ASCII codes within the error check range into the RTU format
(binary).
Address field (02H) 0000 0010
Function code (01H) 0000 0001
Start coil number specification (H) 0000 0000
Start coil number specification (L) 0000 0000
Read points (H) 0000 0000
Read points (L) 0000 1000
2 Add the figures. (Excluding carries during addition.) Addition result 0000 1011
3 Change the result to a 2's complement. (Reve rse the bits and add 01H.) Reversed bit value 1111 0100
01H 0000 0001
LRC va lue 1111 0101
4 Convert the result to an ASCII code. LRC value (ASCII code) F 5
46H 35H
Data
category
Address field Function code Start coil number
specification
Read points Error check
Value 02H 01H 0000H 0008H F5H
Character0 2 0 1 00000008F5
RTU (binary) 0000 0010 0000 0001 0000 0000 0000 0000 0000 0000 0000 1000 1111 0101
ASCII code 30H 32H 30H 31H 30H 30H 30H 30H 30H 30H 30H 38H 46H 35H
Error check procedure LRC check when receiving a request message
1 Convert the ASCII codes within the error check range into the RTU format
(binary).
Address field (02H) 0000 0010
Function code (01H) 0000 0001
Start coil number specification (H) 0000 0000
Start coil number specification (L) 0000 0000
Read points (H) 0000 0000
Read points (L) 0000 1000
2 Add the LRC value. (Excluding carries during addition.)
Normally, the addition result is zero.
LRC va lue 1111 0101
Addition result 0000 0000
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