🛠 Server Modbus TCP : exposer les relais sur Ethernet

Objectives

Turn the Opta into a Modbus TCP slave on its integrated 10/100BASE-T Ethernet port. All variants have this port (RMII, MDIX); the ArduinoModbus library provides ModbusTCPServer. You will expose the 4 relays as coils 0..3, controllable from any supervisor (SCADA, PLC, Python script).

Hardware

• Arduino Opta (any variant) powered by 24 VDC
• RJ45 Ethernet cable to your network (DHCP or static IP)
• A test client: QModMaster, ModScan, or your BMS gateway
• Arduino IDE + "Ethernet" and "ArduinoModbus" libraries

Complete code

// opta_modbus_tcp_server.ino — coils 0..3 = les 4 relais, port 502
#include <Ethernet.h>
#include <ArduinoModbus.h>

EthernetServer modbusServer(502);   // port standard Modbus TCP
ModbusTCPServer modbusTCPServer;

void setup {
  Serial.begin(9600);
  if (Ethernet.begin == 0) {      // DHCP ; sinon Ethernet.begin(ip)
    Serial.println("Echec DHCP Ethernet");
    while (1);
  }
  Serial.print("IP Opta : "); Serial.println(Ethernet.localIP);
  if (!modbusTCPServer.begin) { Serial.println("Defaillance Modbus TCP"); while (1); }
  modbusTCPServer.configureCoils(0x00, 4); // coils 0..3
  modbusServer.begin;
}

void loop {
  EthernetClient client = modbusServer.available;
  if (client) {
    modbusTCPServer.poll(client);
    while (modbusTCPServer.available) {
      uint16_t addr  = modbusTCPServer.readAddress;
      uint16_t value = modbusTCPServer.readValue;
      if (addr < 4) digitalWrite(RELAY1 + addr, value ? HIGH : LOW);
    }
    client.stop;
  }
}

Expected verification

The Opta's IP address is displayed on the serial port. From the Modbus client, connect to port 502, write to coil 0: relay 1 clicks. >

Common errors

• Confusing it with Modbus RTU: RTU goes through RS-485 (RS485/WiFi variants) with ArduinoRS485, not through Ethernet; the Opta does not expose CAN on its terminals.
• No network termination: port 502 must be reachable; corporate firewalls and industrial VLANs often block it — test first on a direct network.
• Silent DHCP failure: in a workshop without DHCP, switch to a static IP via Ethernet.begin(ip).
• Not reading back the state: this server writes coils to the relays but does not synchronize reading; add reverse register logic for a true slave mode.