Communication control between Mitsubishi PLCs and variable frequency drives (VFDs) represents a critical application scenario in industrial automation. The configuration of communication parameters directly impacts system stability and control precision. To achieve efficient and reliable communication, systematic setup must address three dimensions: hardware connection, protocol configuration, and parameter matching. Key implementation points are outlined below:
I. Basic Hardware Connection Configuration
1. Physical Interface Selection
Mitsubishi FX series PLCs typically connect to VFDs via RS485 interfaces (e.g., FX3U-485ADP module). Use shielded twisted-pair cable (AWG22-18 gauge) for wiring, with a 110Ω terminating resistor. For FR-A800 series inverters, connect the PU port's SDA/SDB terminals to the corresponding PLC communication module. Keep ground wire length under 20cm to minimize interference.
2. Electrical Parameter Matching
Communication speed is recommended at 19200bps (reducible to 9600bps for long-distance transmission). The FR-F800 VFD requires signal voltages above ±7V. When using MR-JE-C series servos, note that their RS422 interface requires a converter for compatibility with the PLC's RS485.
II. Core Communication Protocol Parameters
1. Protocol Type Selection
Mitsubishi proprietary protocols (e.g., MC protocol) offer the fastest response speed, while Modbus RTU protocol provides superior compatibility. When using FX5U PLCs, configure the "Communication Protocol Support Function" in GX Works3 and select "Serial Communication" for channel parameters.
2. Station Number Setting Rules
Each device within the same network must be assigned a unique station number (1-31). For the FR-D700 inverter, this is set via parameter Pr.117 and must exactly match the station number declaration in the PLC program. When cascading multiple inverters, it is recommended to number them sequentially based on their physical location.
III. Parameter Synchronization and Mapping
1. Basic Inverter Parameters
● Run Command Source (Pr.79): Set to 5 (Network Communication Control).
● Frequency Command Selection (Pr.338): Set to 0 (RS485 Communication Command).
● Communication Error Handling (Pr.121): Recommended to set to 1 (Free Run Stop).
2. PLC Special Register Configuration
● D8120: Set communication format (Example: 0C8E indicates 19200bps/8-bit/even parity/1 stop bit).
● D8121: Set local station number.
● M8161: Switch between 8-bit/16-bit data processing mode.
IV. Communication Testing and Diagnostics
1. Initialization Program
Communication initialization must be executed during the PLC's first scan cycle:
MOV H0C8E D8120 // Set communication format.
MOV K1 D8121 // Set local station address.
2. Fault Code Analysis
When the inverter displays "AL.E6" indicating communication timeout, check:
● Single-ended grounding of the cable shield.
● Consistency of parameter settings Pr.117-Pr.124.
● Status of the PLC's SD/RD indicator lights.
V. Advanced Application Techniques
1. Optimizing Multi-Station Polling
When using Mitsubishi PLC's RS instruction, set the polling interval to 300ms or higher. A typical program for writing frequency commands to FR-A700 series inverters is:
RS D100 K4 D200 K10 //D100 sends initial 4 words, D200 receives 10 words.
2. Communication Watchdog Configuration
Set the communication detection time in inverter parameter Pr.549 (recommended 2-5 seconds). Combine this with PLC M8029 (command completion flag) to establish a dual-protection mechanism.
In actual projects, there was a case where a packaging line experienced invalid frequency commands due to incorrect setting of parameter Pr.338. The online monitoring function in GX Works2 allows real-time observation of communication data streams. It is recommended to use serial port monitoring tools (e.g., COM Monitor) during system debugging to capture raw packets, which is highly effective for troubleshooting protocol-level issues.
Note: Communication parameters vary across different inverter series (e.g., FR-E800 vs. FR-F800). Always consult the corresponding model's technical manual before implementation. For new bus control systems like CC-Link IE Field, dedicated communication modules (e.g., FX5-CCLGN-MS) must be used with network topology parameters configured.