Arduino Uno SoftwareSerial libraries does not work correctly

Question

Good afternoon. The SoftwareSerial library allows you to connect two devices and work with them in turn using listen. Each device on its pins transmits data correctly, but when I try to receive them in turn, the data is not received

Here is an example code:

#include <SoftwareSerial.h>
SoftwareSerial portOne(4, 7);
uint32_t byte_1 = 0;
uint32_t read_1[2] = {};

SoftwareSerial portTwo(2, 8);
uint32_t byte_2 = 0;
uint32_t read_2[2] = {};

void setup() {
Serial.begin(9600);
portOne.begin(9600);
portTwo.begin(9600);
}

void loop() {
one();
delay(100);
two();
delay(100);
}

void one(){
portOne.listen();
portOne.write(0x4a);
while (portOne.available() > 0) {
if (portOne.available()) { read_1[0] = (portOne.read()); }
if (portOne.available()) { read_1[1] = (portOne.read()); }
}

Serial.print("read_1[0]: "); Serial.println(read_1[0]);
Serial.print("read_1[1]: "); Serial.println(read_1[1]);
}

void two(){
portTwo.listen();
portTwo.write(0x4c);
while (portTwo.available() > 0) {
if (portTwo.available()) {read_2[0] = (portTwo.read());}
if (portTwo.available()) {read_2[1] = (portTwo.read());}
}

Serial.print("read_2[0]: "); Serial.println(read_2[0]);
Serial.print("read_2[1]: "); Serial.println(read_2[1]);;
}

`listen` clears the receive buffer so everything received in that 100 milliseconds delay — Juraj, Jul 19 '23 at 08:13
You have read the [limitation](https://docs.arduino.cc/learn/built-in-libraries/software-serial) for SofwareSerial? - It cannot transmit and receive data at the same time. - If using multiple software serial ports, only one can receive data at a time. — Peter Plesník, Jul 19 '23 at 08:39
@PeterPlesník they use `listen()` to switch. just they use it wrong — Juraj, Jul 21 '23 at 04:39

Peter Plesník · Answer 1 · 2023-07-21T07:47:10.530

As Juraj say SoftwareSerial can be used in that way, but in this code is used wrong. Let see what is wrong. Check comments in source:

#include <SoftwareSerial.h>
SoftwareSerial portOne(4, 7);
uint32_t byte_1 = 0;
uint32_t read_1[2] = {};

SoftwareSerial portTwo(2, 8);
uint32_t byte_2 = 0;
uint32_t read_2[2] = {};

void setup() {
Serial.begin(9600);
portOne.begin(9600);
portTwo.begin(9600);
}

void loop() {
one();
delay(100);
two();
delay(100);
}

void one(){
portOne.listen(); //OK disable portTwo and enable listening on portOne
portOne.write(0x4a);  //OK send some data 
while (portOne.available() > 0) { //wrong you can't wait received data in this way
                                  //condition is immediately evaluated as false
                                  // because for data arrive need wait at least 1ms  
if (portOne.available()) { read_1[0] = (portOne.read()); }
if (portOne.available()) { read_1[1] = (portOne.read()); }
}

Serial.print("read_1[0]: "); Serial.println(read_1[0]); //print some random data
Serial.print("read_1[1]: "); Serial.println(read_1[1]);
}

void two(){
portTwo.listen();
portTwo.write(0x4c);
while (portTwo.available() > 0) { //Wrong - same as previously
if (portTwo.available()) {read_2[0] = (portTwo.read());}
if (portTwo.available()) {read_2[1] = (portTwo.read());}
}

Serial.print("read_2[0]: "); Serial.println(read_2[0]);
Serial.print("read_2[1]: "); Serial.println(read_2[1]);;
}

How it repair? Check comments again.

#include <SoftwareSerial.h>
SoftwareSerial portOne(4, 7);
uint32_t byte_1 = 0;
uint32_t read_1[2] = {};

SoftwareSerial portTwo(2, 8);
uint32_t byte_2 = 0;
uint32_t read_2[2] = {};

void setup() {
Serial.begin(9600);
portOne.begin(9600);
portTwo.begin(9600);
}

void loop() {
one();
two();
}

void one(){
  portOne.listen();
  portOne.write(0x4a);
  delay(100);  //wait 100ms, data are received to buffer in background process using interrupts

  // now it's possible that buffer contain some data
  if (portOne.available() > 0) {
    read_1[0] = (portOne.read()); 
    Serial.print("read_1[0]: "); Serial.println(read_1[0]);
    if (portOne.available()) {
      read_1[1] = (portOne.read()); 
      Serial.print("read_1[1]: "); Serial.println(read_1[1]);
    }
  }

}

void two(){
  portTwo.listen();
  portTwo.write(0x4c);
  delay(100);  //wait 100ms, data are received to buffer in background process using interrupts

  // now it's possible that buffer contain some data
  if (portTwo.available() > 0) {
    read_2[0] = (portTwo.read());
    Serial.print("read_2[0]: "); Serial.println(read_2[0]);
    if (portTwo.available()) {
      read_2[1] = (portTwo.read());
      Serial.print("read_2[1]: "); Serial.println(read_2[1]);
    }
  }
}

Arduino Uno SoftwareSerial libraries does not work correctly

1 Answers1