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How to use a MAX232 with Arduino based PLC


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

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


Ethernet or 20 I/Os PLC:      Ethernet PLC       

Industrial Shields boards:   Industrial Shields Boards


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 showed a electric connections diagram:


Once the connections are set it’s possible to proceed to upload a sketch to test if the MAX232 are 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 showed bellow.


Next it is showed 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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