Adafruit RS232 Pal

A pal for your RS-232 devices

by Liz Clark

published July 12, 2024, last edited January 22, 2025

posted in Components Adafruit Products Arduino Compatibles CircuitPython Breakout Boards

Intermediate

Product guide

Arduino

Using the RS232 PAL with Arduino involves wiring up the breakout to your Arduino-compatible microcontroller with an external RS-232 device and running the provided example code.

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Wiring

Wire as shown for a 5V board like an Uno. If you are using a 3V board, like an Adafruit Feather, wire the board's 3V pin to the breakout VIN.

Here is an Adafruit Metro wired up to the breakout with an external RS-232 connection:

Metro 5V to breakout Vin (red wire)
Metro GND to breakout GND (black wire)
Metro pin 2 to breakout low voltage R1 (green wire)
Metro pin 3 to breakout low voltage T1 (blue wire)
Breakout high voltage R1 to RS-232 RXD [pin 2 on DE-9] (white wire)
Breakout high voltage T1 to RS-232 TXD [pin 3 on DE-9] (yellow wire)
Breakout GND to RS-232 GND [pin 5 on DE-9] (black wire)

Here is an Adafruit Metro wired up using a solderless breadboard:

Metro 5V to breakout Vin (red wire)
Metro GND to breakout GND (black wire)
Metro pin 2 to breakout low voltage R1 (green wire)
Metro pin 3 to breakout low voltage T1 (blue wire)
Breakout high voltage R1 to RS-232 RXD [pin 2 on DE-9] (white wire)
Breakout high voltage T1 to RS-232 TXD [pin 3 on DE-9] (yellow wire)
Breakout GND to RS-232 GND [pin 5 on DE-9] (black wire)

There are no additional libraries needed for this example.

Example Code

You can change the baud rate for your RS-232 device at the top of the code by editing the baud define:

#define baud 38400

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// SPDX-FileCopyrightText: 2024 Liz Clark for Adafruit Industries
//
// SPDX-License-Identifier: MIT

#include <SoftwareSerial.h>

// update this for your RS-232 device baud rate
#define baud 38400
// define RX and TX pins for the software serial port
#define RS232_RX_PIN 2
#define RS232_TX_PIN 3

SoftwareSerial rs232Serial(RS232_RX_PIN, RS232_TX_PIN);

void setup() {
  Serial.begin(115200);
  while ( !Serial ) delay(10);

  rs232Serial.begin(baud);

  Serial.println("Enter commands to send to the RS-232 device.");
  Serial.println();
}

void loop() {

  if (Serial.available() > 0) {
    String userInput = Serial.readStringUntil('\n');
    userInput.trim();  // remove any trailing newlines or spaces
    if (userInput.length() > 0) {
      // send the command with a telnet newline (CR + LF)
      rs232Serial.print(userInput + "\r\n");
      Serial.print("Sent: ");
      Serial.println(userInput);
    }
  }

  // check for incoming data from RS-232 device
  while (rs232Serial.available() > 0) {
    char response = rs232Serial.read();
    // print the incoming data
    Serial.print(response);
  }

  delay(50);
}

// SPDX-FileCopyrightText: 2024 Liz Clark for Adafruit Industries
//
// SPDX-License-Identifier: MIT

#include <SoftwareSerial.h>

// update this for your RS-232 device baud rate
#define baud 38400
// define RX and TX pins for the software serial port
#define RS232_RX_PIN 2
#define RS232_TX_PIN 3

SoftwareSerial rs232Serial(RS232_RX_PIN, RS232_TX_PIN);

void setup() {
  Serial.begin(115200);
  while ( !Serial ) delay(10);

  rs232Serial.begin(baud);

  Serial.println("Enter commands to send to the RS-232 device.");
  Serial.println();
}

void loop() {

  if (Serial.available() > 0) {
    String userInput = Serial.readStringUntil('\n');
    userInput.trim();  // remove any trailing newlines or spaces
    if (userInput.length() > 0) {
      // send the command with a telnet newline (CR + LF)
      rs232Serial.print(userInput + "\r\n");
      Serial.print("Sent: ");
      Serial.println(userInput);
    }
  }

  // check for incoming data from RS-232 device
  while (rs232Serial.available() > 0) {
    char response = rs232Serial.read();
    // print the incoming data
    Serial.print(response);
  }

  delay(50);
}

View on GitHub

Upload the sketch to your board and open up the Serial Monitor (Tools -> Serial Monitor) at 115200 baud. You can send commands to your RS-232 device via the Serial Monitor. If any messages are received from the RS-232 device, they will print to the Serial Monitor.

The commands that you send will vary by the RS-232 device that you connect to the RS232 Pal. The output above was sent to a StarTech HDMI switcher. The AVI=n commands switch the selected HDMI port and the VS command gives a current status of the device.

Page last edited January 22, 2025