How to Use AM26LS31CN RS422 Driver / transmitter: Examples, Pinouts, and Specs

The AM26LS31CN is a high-speed differential line driver manufactured by Texas Instruments. It is specifically designed for RS-422 applications, where it converts single-ended signals into differential signals. This conversion enhances noise immunity and allows for reliable data transmission over longer distances. The device is ideal for use in environments with high electrical noise or where long cable runs are required.

• RS-422 communication systems
• Industrial automation and control systems
• Data transmission in noisy environments
• Long-distance serial communication
• Motor control and encoder feedback systems

The following table outlines the key technical specifications of the AM26LS31CN:

The AM26LS31CN is housed in a 16-pin PDIP package. The pinout and descriptions are as follows:

1. Power Supply: Connect the Vcc pins (12 and 16) to a regulated 5V power supply. Connect the GND pins (4 and 8) to the ground of the circuit.
2. Input Signals: Provide single-ended input signals to the input pins (1A, 2A, 3A, 4A). Ensure the input voltage is within the range of 0V to Vcc.
3. Differential Outputs: The corresponding differential outputs (e.g., 1Y/1Z, 2Y/2Z) will generate the RS-422 compliant signals. Connect these outputs to the differential receiver of the RS-422 system.
4. Termination Resistors: For long-distance communication, use a termination resistor (typically 100Ω to 120Ω) across the differential output lines to minimize signal reflections.

• Ensure the power supply is stable and within the specified range (4.5V to 5.5V).
• Use decoupling capacitors (e.g., 0.1µF) close to the Vcc pins to filter out noise.
• Avoid leaving unused input pins floating; connect them to GND or Vcc as needed.
• For optimal performance, use twisted-pair cables for differential signal transmission.
• Ensure proper grounding to minimize noise and interference.

The AM26LS31CN can be used with an Arduino UNO to transmit data over an RS-422 interface. Below is an example of how to send data using the Arduino's digital pins:

1. Connect the Arduino's digital pin (e.g., D2) to the 1A input of the AM26LS31CN.
2. Connect the 1Y and 1Z outputs to the RS-422 receiver.
3. Power the AM26LS31CN with a 5V supply and connect the GND pins to the Arduino's ground.

// Example code to send data using the AM26LS31CN with an Arduino UNO

const int driverInputPin = 2; // Pin connected to 1A input of AM26LS31CN

void setup() {
  pinMode(driverInputPin, OUTPUT); // Set the pin as an output
}

void loop() {
  digitalWrite(driverInputPin, HIGH); // Send a HIGH signal
  delay(1000); // Wait for 1 second
  digitalWrite(driverInputPin, LOW);  // Send a LOW signal
  delay(1000); // Wait for 1 second
}

1. No Output Signal:

   • Verify that the power supply is connected and within the specified range.
   • Check that the input signal is properly connected and within the voltage range.
   • Ensure the ground connections are secure.

2. Distorted Output Signal:

   • Use a termination resistor across the differential output lines.
   • Check for noise or interference in the power supply.
   • Ensure the cable used for transmission is a twisted-pair cable.

3. Overheating:

   • Ensure the output current does not exceed the maximum rating of ±20mA.
   • Verify that the device is not short-circuited.

Q1: Can the AM26LS31CN be used for RS-485 communication?
A1: While the AM26LS31CN is designed for RS-422, it can be used in RS-485 systems with proper configuration. However, RS-485 typically requires bidirectional communication, which the AM26LS31CN does not support natively.

Q2: What is the maximum cable length supported?
A2: The maximum cable length depends on the data rate and cable quality. For lower data rates (e.g., 100 kbps), distances of up to 1,200 meters are achievable.

Q3: Can I leave unused outputs unconnected?
A3: Yes, unused outputs can be left unconnected, but ensure that the corresponding inputs are tied to a defined logic level (GND or Vcc).

Q4: Is the AM26LS31CN compatible with 3.3V systems?
A4: No, the AM26LS31CN requires a supply voltage of 4.5V to 5.5V. It is not compatible with 3.3V systems without level shifting.