Modbus Tcp Master Communication (Recv); Read Data From An External Device - Panasonic FP7 CPU Unit User Manual

MODBUS TCP Master/Slave Communication

6.3 MODBUS TCP Master Communication (RECV)

6.3.1 Read Data from an External Device

 Instructions
In master communication, PLC has the sending right, and executes communication by
sending commands to devices that support MODBUS, and receiving responses. Messages in
accordance with the protocol are automatically generated by PLC. In the user program,
reading can be done simply by specifying the station no. and memory address and executing
RECV commands.
PLC
DT100 100
DT101 0
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Note: The above figure indicates the data part of Modbus function. In communication via LAN, a header including size information of the sent data is added.
 Sample program
• Send MODBUS commands (03) from the LAN port of the CPU unit, read data from the data
area of an external device 40001 - 40002 (MODBUS address 0000H - 0001H), and write the
content into PLC's data register DT100 - DT101.
• Confirm that Connection 1 is established in the master mode (X90), and that the sending
process is not in progress for the same port (Y90), and start up the SEND command.
• In the UNITSEL command, specify the slot No. (LAN port: U100) and the connection No. (U1).
• In the RECV command, specify and execute the types of MODBUS commands, as well as
the partner station no. (H0301), initial address (H0), No. of data (U2), and initial address on
the PLC side to save data (DT100). For the address of the partner device, please check
operating instructions, etc. of the relevant device.
6-4
Command message
(Hex) 01 03 00 00 00 02
(Hex) 01 03 04 00 64 00 00
Response message
Specify and read address based on RECV command
External device
supporting Modbus-TCP
40001 100
40002 0
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