I am trying to read data from an RS232 equipped sensor using a serial monitor connected to an Arduino Mega with RS-232 shield but I do not receive any data.

I am unable to detect the exact cause of the problem, as it only occurs when combining sensor and RS232shield/Arduino, while communication works as expected when testing them separately

What works:

  • Connecting the sensor to a computer with a RS232-USB converter cable.
  • Passing data from the computer through the USB-RS232 converter cable to RS232 shield and back through Arduino USB output.
  • Connecting the RS232-pin 2 to port 10 using {SoftwareSerial} library with inverted logic.

What does not work:

  • Passing data from the sensor to the computer via the MAX232 and Arduino.

The setup:

  • Arduino Mega 2560
  • Seeed RS232 shield (MAX232 based)
  • Sensor with RS232 output (57600 8N1)

I have connected the RS232 output of the sensor to the shield using an M-M gender changer. The shield and Arduino are connected with three wires: GND-GND , 5V-5V, 232_TX-RX1), and the Arduino connects to a Macbook via USB. The serial monitor on the computer is set to 56700 baud.

I use the following sketch to transfer data from the RS232 shield to Serial:

void setup()
// Open serial communications at 5700 baud

// Check whether Serial1 is active
if (Serial1.available())
     while (!Serial1) {
    Serial.println("Serial 1 is active");

void loop()
    if (Serial1.available()){

  • Does your sensor require hardware flow control signals that the shield doesn't provide?
    – Majenko
    Dec 2, 2019 at 11:50

1 Answer 1


I have solved the above and will share the answer as it may be of help to others:

As the Tx indicator LED on the RS232 shield would not light up when connected to the sensor, I checked the voltage difference between pin 2 (Tx) and pin 5 (GND) of the RS232 connector. This turned out to be only 5V, indicating that the protocol used is TTL-RS232 rather than 'true' RS232.

Due to the low voltage, I have connected the RS232 Tx and GND pins directly to the Arduino. With Tx connected to digital port 10, I used the SoftwareSerial library with logic inversion to read the data: SoftwareSerial mySerial(10, 11, 1).

This works!

  • this is not an answer to the question, because the answer uses information not mentioned in the question. what sensor is it?
    – Juraj
    Dec 2, 2019 at 14:25

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