Inovance H3U Series - Page 603

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9 Communication9.4 Modbus Protocol
5) Example 2: The PLC executes three Modbus instructions to read registers whose slave
addresses are H0064, F001, and F805, respectively. Data is stored in D10, D20, and D30.
The ladder chart is as follows.
Station
number
and
command
Register
address
Number
of
registers
Communication
buffer
Station
number
and
command
Number
of
registers
Communication
buffer
Station
number
and
command
Number
of
registers
Communication
buffer
Station number and command
Register address
Number of registers
M8002
ǒMOV H64 D1Ǔ
ǒMOV H103 D0Ǔ
ǒMOV H1 D2Ǔ
M8000
ǒMODBUS D0 D1 D2 D10 Ǔ
M8000
ǒMODBUS D0 HF001 D2 D10 Ǔ
M8000
ǒMODBUS D0 HF805 D2 D10 Ǔ
Result: The PLC sends xx (hexadecimal) repeatedly and successively through COM1: 01 03 00 64 00 01
C5 D5, 01 03 F0 01 00 01 E6 CA, and 01 03 F8 05 00 01 A5 6B.
Response: If the slave is an H3U PLC, select the Modbus-RTU slave station protocol. The ladder chart is
as follows.
M8002
〔MOV H5100 D100〕
Slave response
① For the rst frame, the slave returns the frame (hexadecimal) 01 03 02 51 00 85 D4.
It means that the slave sends H5100, the value of D100 (the register address is H0064), to the master.
② For the second frame, the slave returns the frame (hexadecimal) 01 03 02 00 00 B8 44.
It means that the slave sends H0000, the value of T1 (the register address is F001), to the master. See
“2)
Variable addressing” on page 609
.
③ For the third frame, the slave returns the frame (hexadecimal) 01 83 02 C0 F1.
It means Read error.
01 indicates the slave address.
83 indicates Read error.
02 indicates the error code. The address is incorrect because the register whose address is HF805 does
not exist.
C0 F1 indicates the CRC code.
9.4.2 Modbus Slave Station Communication
In some scenarios, as a part of the industrial automation system, the PLC must be monitored over the
automatic control network. Typical host computers (for example, DCS and industrial PCs for operation
conguration software) serve as monitoring hosts and communicate with the PLC through a Modbus master
station protocol. In this case, the PLC needs to communicate with host computers through a Modbus slave
station protocol. An H3U PLC has a Modbus-RTU slave station protocol. The protocol can run on both
COM0 and COM1.