Mitsubishi PLC with VFD Control: RS-485 Communication Setup

[Cengiz Özemli]

This guide describes the setup of RS-485 communication between a Mitsubishi FX3U PLC and an FR-D700 inverter using the FX3U-485-BD communication board. Speed control and system monitoring can be achieved with ladder logic.

In industry, motor control is often done via digital communication rather than separate digital and analog cables. The PLC and VFD are directly connected via a serial network method like RS-485. This simplifies wiring, eases diagnostics, and allows for motor speed control directly from the PLC.

### Communication Hardware and Connection

The example system uses the FX3U-485-BD expansion board of the FX3U PLC. This board enables serial communication with shielded twisted-pair cable and connects to field devices like the FR-D700. The VFD, with its internal RS-485 interface, supports Mitsubishi protocol and Modbus commands. For correct connection, it is necessary to use the SDA and SDB differential signal pairs and the SG ground reference.

### Technical Specifications

• PLC model: Mitsubishi FX3U
• Communication board: FX3U-485-BD
• Inverter model: FR-D700
• Communication standard: RS-485, 8-pin RJ45 connector (Note: RS-232/RS-485 is generally not 6-pin RJ12)

### VFD Parameter Configuration

Access to communication parameters is enabled by setting parameter Pr.160 on the inverter to 0. Pr.77 is then set to 2 to accept new settings. Subsequently, serial communication settings are configured to be compatible with the PLC. To apply these settings, the SET button must be pressed, and then the inverter must be powered off and on again. Initially, the "NET" indicator shows that the inverter is ready for communication.

### PLC Communication Settings

On the PLC side, the special D8120 register is used to define the serial format of the FX3U-485-BD board. This defines the communication channel and parameters such as speed/parity/bits. For example, the code H8097 is used to set 9600 bps, RS-485, 8 bits, even parity, and 1 stop bit.

### Communication Methods

• Traditional method: A MOV command is sent to the D8120 register with ladder logic.
• Modern method: Communication settings are made directly in the PLC parameters, without using ladder.

Only one of these methods should be chosen to prevent confusion. Additionally, the "Non-Procedural" protocol should be selected for inverter commands, and the full compatibility of PLC and inverter settings should be checked.

### Motor Speed Read/Write Logic

Frequency read/write operations in Mitsubishi PLCs:

• Follows a sequence with a time-based signal like M8012.
• Event-based synchronization like M8029 is preferred for multiple systems.

In the example above, M8012 is used to trigger the counter, and the control logic proceeds step by step.

### Inverter Commands

Three main functions:

• IVWR: Frequency write (e.g., frequency value is taken from D20 register)
• IVDR: Drive commands (forward, reverse, stop) managed from D10 control register
• IVRD: Frequency feedback read (data logging to D100 register)

The parameters and addresses for each command are set in detail.

### System Test and Troubleshooting

When communication is established, the SD and RD LEDs on the FX3U-485-BD board flash. If only SD is lit, no response is being received; this indicates reversed wiring, incorrect address, or setting issues.

Motor control commands are sent by triggering M10 and M11 for frequency and run commands. Smooth operation is confirmed by monitoring the frequency value in the D200 register and the RUN LED status.

### Conclusion

RS-485 implementations may vary among different manufacturers, but the basic principles are similar. The vast majority of communication problems arise from physical connection and protocol incompatibilities, not logic errors.

This guide comprehensively summarizes the setup of motor speed control and system monitoring by establishing RS-485 communication between a Mitsubishi FX3U PLC and an FR-D700 inverter.