Wiring & Connections

To facilitate communication between the Arduino (TTL logic) and the NPK Sensor (RS485 protocol), a MAX485 TTL-to-RS485 Converter is required. Follow the wiring guide below to ensure stable data transmission.

1. Arduino to MAX485 Module

The MAX485 module acts as the bridge. Most implementations use a digital pin to toggle between "Read" and "Write" modes.

| MAX485 Pin | Arduino Pin | Description | | :--- | :--- | :--- | | VCC | 5V | Power supply for the converter | | GND | GND | Common ground | | RO | RX (e.g., D10*) | Receiver Output (Connect to Arduino RX) | | DI | TX (e.g., D11*) | Driver Input (Connect to Arduino TX) | | DE | D2 | Driver Enable (High to Transmit) | | RE | D2 | Receiver Enable (Low to Receive) |

Note: DE and RE pins are typically jumpered together and connected to a single digital pin (default: D2) to control the communication direction. If you are using SoftwareSerial, ensure your RX/TX pins match your code configuration.

2. MAX485 to NPK Sensor

The RS485 bus uses differential signaling via two wires (A and B).

| MAX485 Pin | Sensor Wire | Signal | | :--- | :--- | :--- | | A | A / RS485+ | Non-inverting signal | | B | B / RS485- | Inverting signal |

3. Powering the Sensor

Industrial NPK sensors usually require a higher voltage than the Arduino can provide. Do not power the sensor directly from the Arduino 5V/3.3V pins unless specified by the manufacturer.

Sensor VCC (Brown/Red): Connect to External DC Power (typically 9V - 24V).
Sensor GND (Black/Blue): Connect to External Power Ground and Arduino Ground (Common Ground).

4. Connection Diagram (Logic)

[ NPK SENSOR ]          [ MAX485 MODULE ]          [ ARDUINO ]
   VCC (9-24V) <----------- External PSU (+)
   GND <------------------- External PSU (-) <------> GND
   Line A <---------------> A
   Line B <---------------> B
                            RO <--------------------> RX (D10)
                            DI <--------------------> TX (D11)
                            DE + RE <---------------> Control (D2)
                            VCC <-------------------> 5V

5. Configuration Check

Before uploading the sketch, verify the following in your code:

// Example pin definition in the sketch
#define RE_PIN 2
#define DE_PIN 2

// If using SoftwareSerial
SoftwareSerial modbusSerial(10, 11); // RX, TX

⚠️ Critical Considerations

Common Ground: Ensure the Ground (GND) of the external power supply is connected to the Arduino GND. Without a shared reference, the RS485 signals may be corrupted.
Termination Resistor: For very long cable runs (over 100 meters), add a 120-ohm resistor between lines A and B at the furthest end of the bus to prevent signal reflection.
Wire Polarity: If you receive no data or timeout errors, try swapping wires A and B; some manufacturers label them differently.