HardwareSerial RS485Serial(1);
ModbusMaster node;

// -----------------------------
// RS485 Pin Assignments (Corrected)
// -----------------------------
const int RS485_RX = 6; // UART1 RX
const int RS485_TX = 5; // UART1 TX
const int RS485_EN = D2;

// Sensor/Modbus parameters (from datasheet)
#define MODBUS_SLAVE_ID 0x0d
#define NUMBER_OF_REGISTERS_TO_READ 0x03
#define REG_LUX_HIGH 0x0000
#define REG_LUX_LOW 0x0001
#define REG_STATUS 0x0002

// Forward declarations for ModbusMaster callbacks
void preTransmission();
void postTransmission();


void setup() {
  Serial.begin(9600);
  delay(5000);

  Serial.println("ModbusMaster: Seeed SKU 314990740 starting");

  // Wait for the hardware serial to be ready
  while (!Serial)
    ;
  Serial.println("Serial done");

  pinMode(RS485_EN, OUTPUT);
  digitalWrite(RS485_EN, LOW);  // Start in RX mode

  // Datasheet: 9600 baud, 8N1
  RS485Serial.begin(9600, SERIAL_8N1, RS485_RX, RS485_TX);
  while (!RS485Serial)
    ;
  Serial.println("RS485 done");

  // Tie ModbusMaster to the UART we just configured
  node.begin(MODBUS_SLAVE_ID, RS485Serial);

   // Register callbacks for half-duplex direction control
  node.preTransmission(preTransmission);
  node.postTransmission(postTransmission);
}

// Toggle DE/RE around TX per ModbusMaster design
void preTransmission() {
  digitalWrite(RS485_EN, HIGH);  // enable driver (TX)
  delayMicroseconds(250);        // transceiver turn-around margin
}

void postTransmission() {
  delayMicroseconds(250);       // ensure last bit left the wire
  digitalWrite(RS485_EN, LOW);  // back to receive
}

void loop() {

  uint8_t result = node.readInputRegisters(REG_LUX_HIGH, NUMBER_OF_REGISTERS_TO_READ);

  if (result == node.ku8MBSuccess) {
    uint16_t luxHigh = node.getResponseBuffer(REG_LUX_HIGH) ;
    
    uint16_t luxLow = node.getResponseBuffer(REG_LUX_LOW);

    uint16_t status = node.getResponseBuffer(REG_STATUS);
    
    uint32_t lux = ((uint32_t)luxHigh << 16) | luxLow;

    Serial.printf("illuminance: %u lux Status:%02X \n", lux, status);
  }
  else
  {
    Serial.printf("Modbus error: %d\n", result);      
  }

  delay(60000);
}