600
9
9 Communication9.4 Modbus Protocol
coils at a time. If the number of registers to be read or written exceeds the limit,
a parameter error will be returned.
4) Example 1: The PLC continuously reads
registers whose slave address is 100.
Data is stored in D10.
D0 = H0103 (Addr&Cmd): Indicates that the slave address is 01 and the Modbus instruction code is 03.
The instruction is to read registers.
D1 = H0064 (RegAddr): Indicates the slave register address.
D2 = H0001 (RegLen): Indicates the number of registers.
D10 (Buf): Indicates the data buffer. After the instruction is executed, data is stored in D10.
The ladder chart is as follows.
M8002
ǒMOV H64 D1Ǔ
ǒMOV H103 D0Ǔ
Station
number
and
command
Register
address
Number
of
registers
Communication
buffer
Station number and command
Register address
ǒMOV H1 D2Ǔ
Number of registers
M8000
ǒMODBUS D0 D1 D2 D10 Ǔ
Result: The PLC continuously reads registers whose slave address is 100, and sends the frame
(hexadecimal) 01 03 00 64 00 01 C5 D5 through COM1.
01 indicates the slave address, namely the upper eight bits of D0.
03 indicates the Modbus instruction code, namely the lower eight bits of D0. It means to read slave
registers.
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