If you want to interface with telco, retro, or industrial equipment, you'll probably run into RS-232 interfaces. The Adafruit RS232 Full-Pinout Breakout with 8 Channels of UART to RS-232 Level Shifters is your friend in such cases. It gives you 5 input and 3 output channels of level shifting and takes care of the high/negative voltage generation all in a low-cost breakout board. We use the trusty MAX3243 from TI, a classic chip part of the MAX232 lineage, so you know it will work great for all your RS-232 needs, up to 250Kbps.

RS-232 is what we had before USB: a 9- or 25-pin D-Sub connector that allowed data plus flow control lines. Many folks may remember these interfaces were used for mice, modems, barcode scanners, teletypes, and more. We still find devices sold with RS-232 ports, although many folks use a USB to RS-232 adapter these days.

If you want to use a microcontroller or microcomputer to chat with an RS-232, then thankfully, all you need is a serial port / UART (something just about any microcontroller has) and a level shifter. The level shifter is required because while most UARTs are 0-3.3V or 0-5V logic level, RS-232 requires +-6 to +-10V, yep the signal voltage goes negative! That means a specialized shifter is required to generate extra high and low voltages and safely convert the logic levels.

Sure you could buy a raw MAX232 chip and wire up the necessary capacitors, but this board does it all for you and even comes with a DE-9 connector for plugging in directly into your 'client' device. It can run on 3.3V power and logic, which many older chips can't do. It also can do all 8 data pins, so you can use all of the flow control signals like RTS, CTS, DTR, DSR, DCD and RI.

We also include two separate lines from the MAX3243: 'Valid' and 'Off'. The Valid line output will have logic level high when the chip detects signal voltages from the device its plugged into. So you can use it as a 'connection made' signal. There's also the Off pin, which when set to logic high by the microcontroller will tri-state all the pins for power reduction.

This breakout comes fully assembled with a UART side for low-voltage power/logic level, and a DE-9F RS-232 side for high-voltage signals. We also include some header so you can solder to a breadboard in a few minutes.

Power Pins

• Vin - this is the power pin. To power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V microcontroller like Arduino, use 5V. The MAX3243 can generate the extra high and low voltages and also safely convert the logic levels needed for RS-232.
• GND - common ground for power and logic.

DE-9 Connector

At the edge of the board is the DE-9 connector. It is a 9-pin connector for interfacing directly with RS-232 equipment. Its pinout is as follows:

• Pin 1: DCD
• Pin 2: RXI
• Pin 3: TXO
• Pin 4: DTR
• Pin 5: GND
• Pin 6: DSR
• Pin 7: RTS
• Pin 8: CTS
• Pin 9: RING

UART Pins

• RX - The data input for receiving serial data from the RS-232 line (pin 2 on the DE-9 connector)
• TX - The data output for sending serial data to the RS-232 line (pin 3 on the DE-9 connector)

Flow Control Pins

• DCD - The data carrier detect pin. This output from the DE-9 connector (pin 1) is used to indicate whether a device is connected.
• DTR - The data terminal ready pin. This output from the DE-9 connector (pin 4) indicates that the device has data ready to transmit.
• DSR - The data set ready pin. This input to the DE-9 connector (pin 6) indicates that the connected microcontroller is ready to connect.
• RTS - The request to send pin. This output from the DE-9 connector (pin 7) indicates that the RS-232 device is ready to send data.
• CTS - The clear to send pin. This input to the DE-9 connector (pin 8) indicates that the RS-232 device can send data.
• RI - The ring indicator pin. This input to the DE-9 connector (pin 9) indicates that an incoming call signal has been received; usually with RS-232 modems.

Extra MAX3243 Pins

• VLD - The Valid pin is a line output that will have logic level high when the chip detects signal voltages from the RS-232 device its plugged into. You can use it as a 'connection made' signal.
• OFF - This pin can be pulled low by the microcontroller to tri-state all the pins for power reduction. If left open (unconnected), the on-board pull-up is tied to the pin.

RS-232 Wiring

Making a connection between the adapter and an RS-232 connection on a cable or other piece of equipment may involve more than a compatible cable. The RS-232 standard and various implementations can be tricky, as people found "back in the day".

Below is a mini-FAQ on wiring used for RS-232. Refer to Wikipedia or other sources for more detailed information.

It's easy to use the RS-232 Full Pinout Breakout with Python or CircuitPython, and the built-in UART module. This module allows you to easily write Python code to send and receive UART messages.

You can use this driver with any CircuitPython microcontroller board or with a computer that has GPIO, UART and Python thanks to Adafruit_Blinka, our CircuitPython-for-Python compatibility library. For Blinka, this example was tested with a Raspberry Pi. To use the example as-is with built-in UART, you'll need to follow these configuration steps outlined in the Blinka Learn Guide.

You'll need an external RS-232 device to use this example with the breakout:

CircuitPython Microcontroller Wiring

First, plug in an RS-232 device to the DE-9 connector on the RS-232 breakout. Then, wire up the RS-232 breakout to your board. The following is the breakout wired to a Feather RP2040:

The following is the breakout wired to a Feather RP2040 using a solderless breadboard:

Python Computer Wiring

Since there are dozens of Linux computers/boards you can use, we will show wiring for Raspberry Pi. For other platforms, please visit the guide for CircuitPython on Linux to see whether your platform is supported.

Here's the Raspberry Pi wired to the RS-232 breakout. You'll plug in an RS-232 device to the DE-9 port on the breakout:

Here's the Raspberry Pi wired up using a solderless breadboard:

CircuitPython Usage

To use with CircuitPython, you'll need to update code.py with the example script.

Thankfully, we can do this in one go. In the example below, click the Download Project Bundle button below to download the necessary libraries and the code.py file in a zip file. Extract the contents of the zip file, and copy the code.py file to your CIRCUITPY drive.

There are no additional libraries needed for this example. All of the modules used are built-in.

Python Usage

Once you have the library pip3 installed on your computer, copy or download the following example to your computer, and run the following, replacing code.py with whatever you named the file:

python3 code.py

Example Code

If running CircuitPython: Once everything is saved to the CIRCUITPY drive, connect to the serial console to see the data printed out!

If running Python: The console output will appear wherever you are running Python.

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

import time
import board

# baud rate for your device
baud = 38400
# Initialize UART for the CH9328
# check for Raspberry Pi
# pylint: disable=simplifiable-condition
if "CE0" and "CE1" in dir(board):
    import serial

    uart = serial.Serial("/dev/ttyS0", baudrate=baud, timeout=3000)
# otherwise use busio
else:
    import busio

    uart = busio.UART(board.TX, board.RX, baudrate=baud)

print("Enter commands to send to the RS-232 device. Press Ctrl+C to exit.")
while True:
    user_input = input("Please enter your command: ").strip()
    if user_input:
        # send the command with a telnet newline (CR + LF)
        uart.write((user_input + "\r\n").encode('ascii'))

    # empty buffer to collect the incoming data
    response_buffer = bytearray()

    # check for data
    time.sleep(1)
    while uart.in_waiting:
        data = uart.read(uart.in_waiting)
        if data:
            response_buffer.extend(data)

    # decode and print
    if response_buffer:
        print(response_buffer.decode('ascii'), end='')
        print()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42

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

import time
import board

# baud rate for your device
baud = 38400
# Initialize UART for the CH9328
# check for Raspberry Pi
# pylint: disable=simplifiable-condition
if "CE0" and "CE1" in dir(board):
    import serial

    uart = serial.Serial("/dev/ttyS0", baudrate=baud, timeout=3000)
# otherwise use busio
else:
    import busio

    uart = busio.UART(board.TX, board.RX, baudrate=baud)

print("Enter commands to send to the RS-232 device. Press Ctrl+C to exit.")
while True:
    user_input = input("Please enter your command: ").strip()
    if user_input:
        # send the command with a telnet newline (CR + LF)
        uart.write((user_input + "\r\n").encode('ascii'))

    # empty buffer to collect the incoming data
    response_buffer = bytearray()

    # check for data
    time.sleep(1)
    while uart.in_waiting:
        data = uart.read(uart.in_waiting)
        if data:
            response_buffer.extend(data)

    # decode and print
    if response_buffer:
        print(response_buffer.decode('ascii'), end='')
        print()

The code begins by initializing the UART port. There is a check for a Raspberry Pi single board computer. If a Raspberry Pi is detected, then serial is used instead of busio for UART over GPIO. The baud rate for your RS-232 device is defined with baud. Edit this value to match your RS-232 device.

# baud rate for your device
baud = 38400

1
2

# baud rate for your device
baud = 38400

In the loop, the code waits for user input in the serial console. If any incoming data is received from the RS-232 device, it is printed to the serial console.

The commands that you send will vary by the RS-232 device that you connect to the RS-232 breakout. The output below 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.

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

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. 

First, plug in an RS-232 device to the DE-9 connector on the RS-232 breakout. Then, wire up the Adafruit Metro to the breakout:

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

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

// 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);
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

// 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);
}

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 RS-232 breakout. 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.

Files

• MAX3243 Datasheet
• EagleCAD PCB Files on GitHub
• 3D models on GitHub 
• Fritzing object in the Adafruit Fritzing Library

Schematic and Fab Print