6.3 MODBUS RTU Master Communication (SEND)6-56.3 MODBUS RTU Master Communication (SEND) ProcedurePLC has the transmission right in master communication, and master communication isperformed by sending commands to, and receiving responses from, devices that supportMODBUS-RTU. Data can be written by specifying unit numbers and memory addresses andexecuting F145 (SEND) instruction in a user program, because PLC automatically createsmessages according to the protocol.External devices thatsupport MODBUS-RTU(Hex) 01 10 03 E8 00 02 04 00 64 00 00(CRC)(Hex) 01 10 00 00 00 02 (CRC)Command messagesResponse messagesDT100 100DT101 0・・・・・・ ・・・・・・・・・・・・・・ ・・・・・・・・1000 1001001 0・・・・・・ ・・・・・・・・・・・・・・ ・・・・・・・・PLCPerform writing by specifying Unit number andaddress using the SEND instruction Sample program• Send a command from COM1 port of the unit, and write the content of the data registersDT100 to DT101 of PLC into the data areas 1000 to 1001 (H03E8 to H03E9) of an externaldevice (unit no. 1).• Confirm that transmission is not in progress (SEND / RECV executable flag R913C: ON) forthe same port, and start up SEND instruction.• F145 (SEND) instruction is executed by specifying the port No., MODBUS command to beused, the starting address of PLC memory area that stores data to be sent, unit number of adestination unit, and the number of send data. Check addresses of destination devices in theinstruction manuals of devices.③② ④H1601 ⑦⑥⑤K2K1000DT100H1601F145 SEND①R0 R913C(Note): The slave side may not be able to respond, when the scan time of PLC as a master unit is short and intervalbetween transmission sessions is short. In such cases, insert a timer instruction into the execution conditions forinstructions, and make the sending interval longer.
