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On these case, we build system with PLC K120S from LS Industrial Systems as Master device and VSD (Variable Speed Drive) Altivar-31 from Schneider Electric as 1st Slave device and iG5A from LS Industrial Systems as 2nd Slave Device. All communication between Master and Slave device performed by Modbus RTU protocol.

PLC K120S with K7M-DR20U type is low price PLC with powerful built-in functions, like : PID function, Modbus RTU protocol, High Speed Counter (HSC), Pulse Output. Otherwise, the disadvantage of PLC K120S is could not processing floating data. All analog data will processed using integer format.

Goal target on this case is PLC K120S could set the reference frequency, acceleration, deceleration and monitor current value of output frequency, output current, output voltage and output power from each VSD.

There is a little trick when using modbus function on PLC K120S ladder programming. That is PLC K120S could not send/receive multiple parameter from/to slave device within same time. There will be occuring error communication. Error communication also occuring when PLC K120S request data to multiple slave devices within same time. We attach below the little trick to resolve that issue.

We also develop similar system using PLC Panasonic. Our system using PLC FPX Series from Panasonic as Master device and 2 units VSD iG5A from LS Industrial Systems as Slave device. There is no built-in RS-485 port on PLC FPX Series, so we need add-on communication cassette AFPX-COM3 to perform Modbus RTU communication. On these system, PLC FPX Series also needed similar ladder programming method like on PLC K120S.

We got last confirmation from the end-users that all above design are powerful system. Please fell free to develop your system by using our experiences.

Tags: LSIS (LG), Schneider Electric