How to Implement OpenPLC on STM32 with W5500 Ethernet and Modbus TCP

Step 1: Understanding the Hardware

This discussion explores implementing OpenPLC Runtime on an STM32 microcontroller with Ethernet connectivity for Modbus TCP communication.

The developer considered using either:

• WIZnet W5500
• WIZnet W5100
• ENC28J60

However, the OpenPLC project maintainer confirmed that the current runtime already supports W5100 and W5500, while ENC28J60 would require a custom Ethernet driver.

Typical hardware includes:

• STM32 microcontroller
• W5500 Ethernet controller
• SPI interface
• OpenPLC Runtime
• Modbus TCP communication

The W5500 provides:

• Hardwired TCP/IP stack
• Integrated Ethernet MAC + PHY
• 8 independent hardware sockets
• 32 KB internal TX/RX buffer
• SPI interface up to 80 MHz

Using the W5500 allows developers to add industrial Ethernet networking without implementing a software TCP/IP stack.

Step 2: Native W5500 Support in OpenPLC

One of the most valuable insights from this discussion is that OpenPLC Runtime already includes support for WIZnet Ethernet hardware.

According to the OpenPLC maintainer:

• Modbus TCP can be enabled directly from the upload dialog.
• The runtime supports W5100 and W5500.
• Ethernet communication uses the Arduino Ethernet libraries.
• Hardware wiring follows the same approach as a standard Arduino Ethernet Shield. 

This means developers can focus on PLC logic rather than low-level Ethernet driver development.

Step 3: System Architecture

The communication architecture is straightforward:

OpenPLC Runtime
        │
   Arduino Ethernet API
        │
        ▼
      W5500
        │ SPI
        ▼
      STM32 MCU
        │
    PLC Logic
        │
        ▼
  Modbus TCP Clients

The W5500 handles:

• TCP/IP processing
• Socket management
• ARP
• ICMP
• UDP
• TCP communication

Meanwhile, the STM32 executes ladder logic and application-specific PLC tasks.

This separation simplifies firmware development and helps maintain deterministic PLC execution.

Step 4: Firmware Integration

OpenPLC leverages the Arduino Ethernet API, so developers do not need to create a custom Ethernet stack for the W5500.

Conceptual initialization:

// Conceptual example based on WIZnet ioLibrary
// Not project-specific source code

uint8_t tx_size[] = {2,2,2,2,2,2,2,2};
uint8_t rx_size[] = {2,2,2,2,2,2,2,2};

void ethernet_init(void)
{
    wizchip_init(tx_size, rx_size);

    wiz_NetInfo netinfo = {
        .ip = {192,168,1,100},
        .sn = {255,255,255,0},
        .gw = {192,168,1,1}
    };

    wizchip_setnetinfo(&netinfo);
}

Conceptual integration example based on WIZnet ioLibrary (not project-specific).

The OpenPLC maintainer also notes that developers can reference the Arduino examples included with the OpenPLC editor when adapting the runtime for supported hardware.

Step 5: Industrial Applications

Combining STM32, W5500, and OpenPLC creates a compact and cost-effective industrial controller suitable for:

• PLC-based machine control
• Factory automation
• Modbus TCP remote I/O
• Industrial gateways
• Building automation
• Process control
• Educational PLC platforms

Because the W5500 offloads Ethernet communication in hardware, the STM32 can dedicate more processing time to real-time PLC execution.

FAQ

Q1: Why use W5500 with OpenPLC?

A: W5500 is officially supported by the OpenPLC runtime and provides a hardwired TCP/IP stack. This reduces software complexity and allows developers to implement Modbus TCP without writing a custom Ethernet driver.

Q2: Does OpenPLC support ENC28J60?

A: Not natively. According to the OpenPLC maintainer, developers would need to implement their own driver for ENC28J60, whereas W5100 and W5500 are already supported by the runtime.

Q3: How is the W5500 connected to STM32?

A: The W5500 connects via SPI and is accessed using the Arduino Ethernet API. OpenPLC uses this API, so wiring follows the same approach as an Arduino Ethernet Shield.

Q4: What advantages does W5500 provide for PLC applications?

A: The W5500 offloads TCP/IP processing, manages eight hardware sockets, and integrates the Ethernet MAC and PHY. This reduces MCU workload and improves communication reliability in industrial Ethernet applications.

Q5: What applications are suitable for this architecture?

A: Typical applications include industrial control systems, Modbus TCP gateways, educational PLC projects, machine automation, distributed I/O controllers, and Ethernet-enabled process monitoring.