Mitsubishi A-Series PLC Multi-Device Concurrent Communication Guide

Many manufacturing facilities still rely on legacy Mitsubishi A-Series PLCs, such as the A2USHCPU-S1, which have been running reliably for over a decade. These controllers were designed in an era when RS422 serial communication was the standard. Today, the need to integrate these PLCs into modern Ethernet-based networks for SCADA, HMI, and MES connectivity is critical. The challenge is that the built-in programming port only supports point-to-point RS422 at speeds up to 115.2 kbps. This creates a bottleneck when multiple devices need to access the PLC simultaneously. Upgrading to a newer PLC with native Ethernet is often costly, time-consuming, and risks production downtime. A more practical solution is to use an Ethernet converter module that bridges the RS422 port to TCP/IP, enabling concurrent multi-device communication without altering the existing PLC program or hardware.

This article explores the technical background, challenges, and a proven approach to achieve multi-device concurrent communication with Mitsubishi A-Series PLCs. We will cover the limitations of the original RS422 interface, the principles of Ethernet conversion, configuration steps, and the benefits for industrial automation systems.

Understanding the RS422 Limitation on Mitsubishi A-Series PLCs

The Mitsubishi A2USHCPU-S1 and similar CPUs feature a 25-pin D-sub programming port that uses RS422 signaling. This interface was originally intended for connecting a single programming device or a dedicated HMI. The electrical characteristics of RS422 support differential signaling, which provides good noise immunity over distances up to 1200 meters, but the protocol is inherently point-to-point or multi-drop with limited addressing. In practice, only one master device can communicate with the PLC at a time. If you need to connect both an HMI and a SCADA system, you would have to physically swap cables or use a manual switch, which is not feasible for continuous production monitoring.

The data rate of 115.2 kbps translates to roughly 11.5 KB/s under ideal conditions. When polling multiple data registers, the overhead of request/response frames reduces effective throughput. For example, reading a block of 100 registers (200 bytes) might take around 20-30 ms, but if you need to read thousands of registers for a SCADA system, the scan time can become several hundred milliseconds. This is inadequate for applications requiring real-time control or fast data logging.

The Role of Ethernet Converters in Legacy PLC Upgrades

An Ethernet converter, also known as a serial device server, encapsulates serial data into TCP/IP packets. For Mitsubishi A-Series PLCs, specialized converters are available that understand the MELSEC communication protocol and can handle multiple client connections. These devices typically connect directly to the PLC’s programming port via a dedicated cable and present an Ethernet port to the network. They operate in TCP Server mode, allowing multiple TCP clients (such as SCADA, HMI, and programming software) to connect simultaneously.

The converter manages the serial communication with the PLC transparently. When a client sends a request, the converter forwards it to the PLC and returns the response to the correct client. Advanced converters support up to 6 concurrent connections, effectively turning the single serial port into a multi-client Ethernet interface. This eliminates the need for manual switching and enables seamless integration with modern industrial networks.

Key Features of an Effective Ethernet Converter for Mitsubishi A-Series

When selecting an Ethernet converter for this application, consider the following features:

• Protocol Compatibility: Must support the Mitsubishi MELSEC protocol (usually via MC protocol or dedicated A-Series protocol). Some converters auto-detect the PLC type and serial parameters, simplifying setup.
• Multi-Client Support: Ability to handle at least 4-6 simultaneous TCP connections, allowing concurrent access from SCADA, HMI, and engineering stations.
• Industrial Design: Wide operating temperature range (-25°C to 75°C), DIN rail mounting, and robust isolation to withstand factory floor noise.
• Web-Based Configuration: Built-in web server for easy setup, diagnostics, and firmware updates without proprietary software.
• Modbus TCP Support: Optional Modbus TCP server functionality allows direct integration with MES or other systems that use Modbus, with automatic mapping to PLC data areas.

Step-by-Step Implementation Guide

The following steps outline a typical installation using a converter module (such as the YC8000-A, a representative model for this purpose):

1. Hardware Connection

Power off the PLC. Mount the converter on a DIN rail inside the control cabinet. Connect the converter’s COM1 port to the PLC’s programming port using a dedicated SC-09 compatible cable. Supply 24V DC power to the converter. Connect the converter’s Ethernet port to the factory network switch using a standard CAT5e/6 cable. Ensure the SCADA server, HMI, and other devices are on the same subnet or have proper routing.

2. Network Configuration

Access the converter’s web interface by entering its default IP address (e.g., 192.168.1.100) in a browser. Set a new IP address that matches your network scheme. Configure the serial port parameters: baud rate (typically 115200), data bits (8), parity (odd/even as per PLC setting), and stop bits (1). Many converters auto-negotiate these settings for Mitsubishi PLCs. Set the operation mode to TCP Server and specify a port number (e.g., 5002). Enable any security features like password protection.

3. Client Setup

In your SCADA software (e.g., KingSCADA, WinCC, Ignition), configure a new Mitsubishi A-Series Ethernet driver. Enter the converter’s IP address and port number. The driver will communicate as if the PLC has a native Ethernet port. For HMIs like Weintek cMT series, select the Mitsubishi A-Series TCP/IP driver and input the same connection details. Multiple clients can use the same IP and port; the converter handles session multiplexing.

4. Testing and Verification

Power on the system. Verify that the converter’s link and activity LEDs indicate proper connection. Use a Modbus TCP client or the SCADA system to read known registers from the PLC. Check that both HMI and SCADA can read/write data simultaneously without conflicts. Monitor the converter’s diagnostic page for any communication errors or timeouts.

Performance Comparison: Before and After

Real-World Application in a Battery Manufacturing Plant

A lithium battery production facility had multiple A2USHCPU-S1 PLCs controlling various stages of the assembly line. The plant wanted to implement a centralized SCADA system for real-time monitoring and data logging, while keeping the existing HMI panels operational. The original RS422 ports could only connect to one device at a time. By installing Ethernet converters on each PLC, the plant achieved the following:

• SCADA system (KingSCADA) connected to all PLCs via Ethernet, polling data every 100 ms.
• Existing Weintek HMIs continued to operate without any reconfiguration, simply by changing the connection settings to the converter’s IP.
• MES integration was achieved through the converter’s Modbus TCP server, mapping production counts and status to the MES database.
• The entire upgrade was completed during a scheduled maintenance window of 2 hours for 10 PLCs, with no changes to PLC programs.

This approach saved an estimated 80% in costs compared to replacing the PLCs and avoided weeks of production downtime.

Technical Considerations for Reliable Operation

While Ethernet converters are robust, proper installation is key to avoiding communication issues:

• Grounding and Shielding: Ensure the serial cable is properly shielded and the converter is grounded to prevent noise interference. Use ferrite cores on Ethernet cables if necessary.
• Network Segmentation: Place the converters and PLCs on a dedicated control network VLAN to isolate them from office traffic and reduce latency.
• Connection Limits: Do not exceed the maximum number of concurrent connections specified by the converter. Excessive polling from multiple clients can overload the serial link, causing timeouts.
• Firmware Updates: Keep the converter firmware up to date to benefit from protocol improvements and security patches.

Extending Functionality with Modbus TCP and OPC

Many Ethernet converters offer built-in Modbus TCP server functionality. This allows any Modbus TCP client (such as a MES system, IoT gateway, or custom application) to read and write PLC data using standard Modbus registers. The converter automatically maps Modbus addresses to the PLC’s data registers (D), relays (M), and other devices. For example, Modbus holding register 40001 might map to D0 in the PLC. This feature eliminates the need for an OPC server in simple integrations, though OPC UA can also be used for more complex scenarios.

For advanced applications, the converter can be accessed via a programming library (e.g., using C# or Python sockets) to build custom data logging or control applications. This opens up possibilities for predictive maintenance, energy monitoring, and cloud connectivity.

Conclusion

Enabling concurrent multi-device communication for Mitsubishi A-Series PLCs is a cost-effective way to extend the life of legacy equipment while meeting modern connectivity demands. By using an Ethernet converter, manufacturers can integrate SCADA, HMI, and MES systems without replacing the PLC or modifying existing programs. The result is improved data visibility, reduced downtime, and a solid foundation for Industry 4.0 initiatives. When selecting a converter, prioritize protocol compatibility, multi-client support, and industrial-grade reliability to ensure a successful upgrade.