In industrial automation control systems, the most common setup involves a PLC (Programmable Logic Controller) combined with a frequency inverter. Various methods exist for PLC-controlled inverters, but one widely adopted approach is RS-485 communication due to its strong anti-interference capability, high transmission speed, long distance, and low cost. However, implementing RS-485 communication requires addressing several technical challenges such as data encoding, checksums, framing, transmitting data, parity checks, timeout handling, and error retransmission. This often results in complex programming, requiring dozens of ladder logic instructions, which can be time-consuming and difficult for designers.
This article introduces a straightforward method using Mitsubishi FX series PLCs to control inverters through extended memory communication. By installing an RS-485 communication board or module on the PLC, or by using an affordable function expansion storage box, only four simple ladder instructions are needed to read, write, monitor, and control parameters of up to 8 inverters over distances of 50m or 500m. This method is easy to implement and ideal for engineers looking for an efficient solution without complicated programming.
The system includes an FX2N series PLC (V3.00 or higher), an FX2N-485-BD communication board, or an FX0N-485ADP module paired with an FX2N-CNV-BD board, along with the FX2N-ROM-E1 function expansion storage box. It also supports multiple Mitsubishi inverters (S500, E500, F500, F700, A500, V500 series) connected via RJ45 cables and terminating resistors. Optionally, a human-machine interface like a touch screen can be added for enhanced control.
Communication settings on the inverters must be configured properly, including station number, baud rate, stop bits, and parity. The PLC uses a master-slave communication mode, where it sends commands to the inverters, which respond accordingly. Simple instruction examples include monitoring frequency, controlling motor direction, reading and writing parameters, and adjusting operational settings.
Compared to other methods like switch signal control, analog signal control, RS-485 non-protocol communication, Modbus-RTU, and fieldbus communication, the extended memory communication method offers lower cost, ease of use, and reliable performance. While it has limitations—such as being restricted to inverters and limited numbers—it provides an effective and efficient solution for small-scale automation systems.
This method is particularly useful in applications like paper production lines, corrugated machinery, plastic film lines, and metal wire drawing machines. It simplifies complex RS-485 communication tasks, reducing programming workload and improving engineering efficiency. Despite its limitations, this approach remains a valuable and practical option for many industrial applications.
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