← Modbus RTU Chiller Control with an M-Duino PLC

Modbus RTU Chiller Control with an M-Duino PLC — full example

Control thermoelectric chillers over Modbus RTU with an M-Duino PLC. Write start/stop, cold/heat enable and a x10 temperature setpoint on RS485. Full code.

Complete, runnable program for the M-Duino (chiller-control-rtu.ino): wiring header, requirements and integration notes included.

Read-only preview.

/*
 * COMPLETE EXAMPLE — Thermoelectric chiller control over Modbus RTU
 *
 * Device:  M-Duino (Industrial Shields, Arduino PLC with RS485)
 * Based on: bioreactor control project, ModbusTCPSlaveRTUMaster.ino
 *
 * Bus: Modbus RTU over RS485, 9600 8N1. Two chillers at addr 1 and 2.
 *
 * Chiller RTU registers (write via function 0x06):
 *   0x06  start/stop          (1 = run, 0 = stop)
 *   0x08  enable cooling       (1 = active)
 *   0x09  enable heating       (1 = active)
 *   0x0A  temperature setpoint x10  (e.g. 18.5 C -> 185)
 *
 * Datasheet note: after each command write, leave a margin of about
 * 500 ms before the next command so the chiller consolidates it.
 *
 * Integration: in the plant these commands arrive from the "Gateway
 * Modbus TCP <-> RTU" module; here we demonstrate direct control of one chiller.
 */

#include 
#include 

ModbusRTUMaster master(RS485);

const uint8_t CHILLER[2] = {1, 2};   // RTU addresses of the two chillers

// Command registers
const uint16_t REG_RUN  = 0x06;
const uint16_t REG_COLD = 0x08;
const uint16_t REG_HEAT = 0x09;
const uint16_t REG_SETP = 0x0A;

void setup() {
  Serial.begin(115200);
  master.begin(9600);
  master.setTimeout(300);

  // Example startup: chiller 1 in cooling mode at 18.5 C
  startChiller(CHILLER[0], 1);
  setMode(CHILLER[0], true, false);     // cooling ON, heating OFF
  setSetpoint(CHILLER[0], 185);         // 18.5 C x10
}

void loop() {
  // Periodic setpoint maintenance (defensive refresh every 10 s)
  static uint32_t t = 0;
  if (millis() - t > 10000) {
    t = millis();
    setSetpoint(CHILLER[0], 185);
    Serial.println("Chiller 1 setpoint refreshed to 18.5 C");
  }
}

// Writes the run/stop register and respects the datasheet margin
uint8_t startChiller(uint8_t addr, uint8_t status) {
  if (!master.writeSingleRegister(addr, REG_RUN, status)) return 1;
  if (master.isWaitingResponse()) {
    ModbusResponse r = master.available();
    if (r && !r.hasError()) {
      delay(500);   // datasheet requirement
      return 0;
    }
  }
  delay(500);
  return 1;
}

// Enables cooling/heating in a mutually exclusive way
uint8_t setMode(uint8_t addr, bool cold, bool heat) {
  uint8_t err = 0;
  err += writeReg(addr, REG_COLD, cold ? 1 : 0);
  err += writeReg(addr, REG_HEAT, heat ? 1 : 0);
  return err;
}

// Temperature setpoint in tenths of a degree (18.5 C -> 185)
uint8_t setSetpoint(uint8_t addr, uint16_t tempx10) {
  return writeReg(addr, REG_SETP, tempx10);
}

// Generic write with response confirmation
uint8_t writeReg(uint8_t addr, uint16_t reg, uint16_t val) {
  if (!master.writeSingleRegister(addr, reg, val)) return 1;
  if (master.isWaitingResponse()) {
    ModbusResponse r = master.available();
    if (r && !r.hasError()) {
      delay(500);   // margin between consecutive commands
      return 0;
    }
  }
  delay(500);
  return 1;
}