How to use a MAX232 with Arduino based PLC

December 11, 2018 by
How to use a MAX232 with Arduino based PLC
Alejandro Jabalquinto


This post it is showed how to connect a Max3232 to an M-Duino PLC family.                                      M-Duino PLC family

Max3232 has two lines of RS-232 communication and their respective two channels of TTL and also power supply pins (3-5,5V-GND). 


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How to connect M-Duino PLC with the Max232?

M-Duino has two available serial TTL, RX1/TX1 (or Serial1) and RX3/TX3 (or serial3). Connecting RX1/TX1 with T1IN/R1OUT and RX3/TX3 with T2IN/R2OUT the MAX3232 is ready to transmit and receive the data (don’t forget to connect the power supply). Next, it is shown an electric connections diagram:

 electric connections diagram

Once the connections are set it’s possible to proceed to upload a sketch to test if the MAX232 is working as expected. On this sketch, it’s just used one of them because the other it’s just a copy. Actually, the MAX232 is doing all the hard work, for the user just have to code as a normal TTL serial port using serial1 or serial3 as is shown below.


Next, it is shown the test sketch:

#define TTL Serial1 // Serial1 or Serial3

void setup() {
  Serial.println("ttl started");


void loop() {
  if (Serial.available()) {
    byte in =;

  if (TTL.available()) {
    byte in =;


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How to use a MAX232 with Arduino based PLC
Alejandro Jabalquinto December 11, 2018
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