Mitsubishi PLC and VFD RS-485 Communication Setup and Motor Control

[Erkan Teskancan]

This guide demonstrates how to establish RS-485 communication between a Mitsubishi FX3U PLC and an FR-D700 inverter using an FX3U-485-BD communication card. At the core, speed control and system monitoring are performed with ladder logic.

In industrial motor controls, instead of separate wiring for digital and analog signals, connecting the PLC directly to VFDs (Variable Frequency Drives) via serial networks like RS-485 is often preferred. This approach reduces complex wiring, provides ease of diagnosis, and offers the ability to control motor speed directly from the PLC program.

### Communication Hardware and Connection

In the example application, a connection is made between the FX3U PLC, equipped with an FX3U-485-BD expansion card, and the FR-D700 inverter via a shielded twisted-pair cable. The FX3U-485-BD card provides serial communication and must be used with appropriate hardware. The inverter supports Mitsubishi protocol and Modbus commands via its RS-485 interface.

• SDA and SDB differential signal pairs are used for RS-485 connection.
• The SG terminal is required for ground reference.
• The connector on the VFD side is an 8-pin RJ45 (not the 6-pin RJ12 common in RS-232/RS-485).

### FR-D700 and FX3U-485-BD Pin Mapping

Pin configuration and connections are detailed in the table. VFD parameter settings must be configured to match the serial communication protocol.

### VFD Parameter Settings

• Advanced access to communication parameters is enabled by setting Pr.160 to 0.
• Acceptance of new settings is ensured by setting Pr.77 to 2.
• Relevant parameter values are adjusted to harmonize serial communication settings between the VFD and PLC.
• After settings, the VFD is reset to activate the new settings.
• After correct setup, the VFD indicates readiness for communication with a "NET" display on the screen.

### PLC Side Communication Settings

• The serial port communication format for channel 1 of the FX3U-485-BD module is defined using special register D8120.
• This register sets baud rate, data format, and protocol settings. (Example: 9600 baud, RS-485, 8 data bits, even parity, 1 stop bit - 9600-8-E-1)
• Communication timeout can be written to register D8129 in ms.

### Communication Programming Approaches

• Traditional method: In the ladder program, code H8097 is assigned to register D8120 with a MOV command.
• Modern alternative: Communication settings can be made directly via PLC parameters from parameter navigation, without using ladder logic.

### Communication Control Logic

• The PLC triggers a counter using the M8012 time-based clock pulse and controls the servo drive step by step.
• Asynchronous inverter commands (IVWR, IVDR, IVRD) send frequency and operation commands, and feedback is received.

### Command Functions

• IVWR: To set the frequency value.
• IVDR: To manage drive operation commands (forward, reverse, stop).
• IVRD: Reads output frequency information.

### Application and Test

• Register D20 is used for frequency setting (e.g., a value of 2000 corresponds to 20.00 Hz for the motor).
• Registers D10 hold movement commands (2: forward, 0: stop).
• The SD and RD LEDs on the FX3U-485-BD indicate data transmission/reception status.
• If only the SD LED is lit, it means no response has been received from the device. In this case, connections, address, and parameters should be checked.
• Motor commands are given, and frequency output is checked with D200 registers.

### General Assessment

Although there are variations between manufacturers in RS-485 implementation, the basic principles are similar. The vast majority of communication problems arise from hardware connection and protocol incompatibility, not from the logic program.

All images shared with the author's permission.