Q. How to solve the Modbus TCP communication issue when PLC-S Ethernet module (CM3-SP01EET / Modbus TCP Master) and third party devices (Modbus TCP Slave) are turned on power together and will not communicate?
A. Use the Yx4 contact (Modbus Master Restart Request Flag).
For example, on a PLC-S using the Ethernet card (CM3-SP01EET) as a Modbus TCP Master, it is connected to 5 variable frequency drives (VFD)
as Modbus TCP Slaves. Power is turned off on all devices. When the power is turned back on the PLC-S and the VFD’s at the same time, SP01EET cannot communicate with the VFD’s unless the Run switch on the PLC-S is turned to Stop and then back to Run. To fix the problem temporarily, a user put a delay timer on the power to the PLC so that SP01EET is turned on power later than the VFD’s.
In this case, use the Yx4 contact which is a flag to restart the Modbus Master communication.
If your SP01EET module is located in Slot #1 next to CPU (Slot 0), then this flag will be Y14. Turn this Y14 ON and OFF for 1 second in the scan program, then it will restart Modbus TCP Master Communication even after turning on and off the all devices at the same time. The ‘Modbus Master Restart Request flag’ has been added in the firmware V4.03 and above for SP01EET.