How to Use MAX485: Examples, Pinouts, and Specs

The MAX485 is a low-power, half-duplex RS-485 transceiver manufactured by Arduino. It is designed for reliable, long-distance data transmission in noisy environments. The MAX485 supports data rates of up to 2.5 Mbps and is ideal for multipoint communication on a single twisted pair of wires. Its low power consumption and robust design make it a popular choice for industrial automation, building management systems, and other applications requiring reliable serial communication.

• Industrial automation and control systems
• Building management systems (e.g., HVAC, lighting control)
• Long-distance serial communication
• RS-485-based sensor networks
• Point-to-point and multipoint communication systems

The MAX485 transceiver is designed to meet the electrical specifications of RS-485 and RS-422 communication standards. Below are its key technical details:

• Supply Voltage (Vcc): 4.75V to 5.25V
• Data Rate: Up to 2.5 Mbps
• Operating Temperature Range: -40°C to +85°C
• Driver Output Voltage: -7V to +12V
• Receiver Input Voltage Range: -7V to +12V
• Low Power Consumption: 300 µA (typical)
• Driver/Receiver Enable Pins: Independent control for driver and receiver
• ESD Protection: ±15 kV (Human Body Model)

The MAX485 is an 8-pin IC with the following pinout:

The MAX485 is straightforward to use in RS-485 communication systems. Below are the steps and considerations for integrating it into a circuit.

1. Power Supply:

   • Connect the Vcc pin to a 5V power source.
   • Connect the GND pin to the system ground.

2. Bus Connections:

   • Connect the A and B pins to the RS-485 bus. Use a twisted pair of wires for optimal performance.
   • Terminate the bus with a 120-ohm resistor at both ends to prevent signal reflections.

3. Driver and Receiver Control:

   • Use the DE pin to enable the driver. Set DE high to transmit data.
   • Use the RE̅ pin to enable the receiver. Set RE̅ low to receive data.
   • For half-duplex communication, toggle DE and RE̅ as needed to switch between transmit and receive modes.

4. Data Transmission:

   • Send data to the DI pin for transmission on the RS-485 bus.
   • Received data will be output on the RO pin.

• Use proper termination resistors (120 ohms) at both ends of the RS-485 bus.
• Keep the twisted pair wires as short as possible to minimize signal degradation.
• Avoid running RS-485 cables near high-power or noisy equipment to reduce interference.
• Ensure that all devices on the RS-485 bus share a common ground.

Below is an example of how to connect the MAX485 to an Arduino UNO for RS-485 communication:

• MAX485 Pin 8 (Vcc): Connect to Arduino 5V.
• MAX485 Pin 5 (GND): Connect to Arduino GND.
• MAX485 Pin 4 (DI): Connect to Arduino digital pin 3 (TX).
• MAX485 Pin 1 (RO): Connect to Arduino digital pin 2 (RX).
• MAX485 Pin 3 (DE): Connect to Arduino digital pin 7.
• MAX485 Pin 2 (RE̅): Connect to Arduino digital pin 7 (same as DE for half-duplex).
• MAX485 Pins 6 (A) and 7 (B): Connect to the RS-485 bus.

// RS-485 Communication Example with MAX485 and Arduino UNO

#define DE_PIN 7  // Driver Enable pin
#define RE_PIN 7  // Receiver Enable pin (shared with DE for half-duplex)
#define TX_PIN 3  // Arduino TX pin connected to DI
#define RX_PIN 2  // Arduino RX pin connected to RO

void setup() {
  pinMode(DE_PIN, OUTPUT);
  pinMode(RE_PIN, OUTPUT);

  // Start Serial communication
  Serial.begin(9600);

  // Set MAX485 to receive mode initially
  digitalWrite(DE_PIN, LOW);
  digitalWrite(RE_PIN, LOW);
}

void loop() {
  // Example: Sending data
  digitalWrite(DE_PIN, HIGH);  // Enable driver
  digitalWrite(RE_PIN, HIGH); // Disable receiver
  Serial.write("Hello, RS-485!"); // Send data
  delay(100); // Wait for data to be sent
  digitalWrite(DE_PIN, LOW);  // Disable driver
  digitalWrite(RE_PIN, LOW); // Enable receiver

  // Example: Receiving data
  if (Serial.available()) {
    char received = Serial.read(); // Read received data
    Serial.print("Received: ");
    Serial.println(received);
  }

  delay(1000); // Wait before next iteration
}

1. No Data Transmission:

   • Ensure the DE pin is set high during transmission.
   • Verify that the DI pin is receiving data from the microcontroller.

2. No Data Reception:

   • Ensure the RE̅ pin is set low during reception.
   • Check the RO pin for output data.

3. Signal Reflections or Noise:

   • Verify that 120-ohm termination resistors are installed at both ends of the RS-485 bus.
   • Use twisted pair cables and avoid running them near noisy equipment.

4. Communication Errors:

   • Check the baud rate settings on all devices to ensure they match.
   • Verify that all devices share a common ground.

Q: Can the MAX485 be used for full-duplex communication?
A: No, the MAX485 is a half-duplex transceiver. For full-duplex communication, consider using a full-duplex RS-485 transceiver like the MAX488.

Q: What is the maximum cable length for RS-485 communication?
A: The maximum cable length depends on the data rate. For example, at 100 kbps, the maximum length is approximately 1200 meters.

Q: Can I connect multiple MAX485 devices on the same bus?
A: Yes, the MAX485 supports multipoint communication with up to 32 devices on the same RS-485 bus.