problem with serial

Using Arduino Programming Questions
1

Slave: Arduino uno
Master: Arduino mega

code for uno:

#include <Servo.h>
String state;
String s1open = "s1 open";
String s1close = "s1 close";
Servo servo1;

void setup() {

 Serial.begin(9600);

 servo1.attach(9);
}

void loop() {
 while(Serial.available()==0) {}
 
  state = Serial.readStringUntil('\n');

if(state == s1open){
  servo1.write(180);
}
if(state == s1close){
  servo1.write(0);
}
}

code for Mega:

String state;

void setup() {
Serial1.begin(9600);
Serial.begin(9600);
Serial.println("arduino is ready");

}

void loop() {

while(Serial.available()==0){}

state = Serial.readString();

Serial1.println(state);
Serial.println(state);

}

wireing: TX uno --> Rx1 mega
Rx uno --> Tx1 Mega
common ground
servo--> pin 9 uno

problem:
if i type in s1 close or s1 open in the serial or serial1 monitor of the Arduino mega, it doesnt work
if i type in s1 close or s1 open in the serial, but rewire the wires as:
Tx uno --> Rx0 mega
Rx uno --> Tx0 mega
it does work.

if i rewrite the code a little and i make the uno master and the mega slave it also works fine without rewireing. Can someone tell me where i went wrong making the uno slave and the mega master ? maybe a tip or a correction ?

2

Why don't you have readStringUntil() in the Mega?

I never use those functions anyway. Have a look at the examples in Serial Input Basics - simple reliable ways to receive data.

Also, while it is NOT the cause of your problem, it is not a good idea to use the String (capital S) class on an Arduino as it can cause memory corruption in the small memory on an Arduino. Just use cstrings - char arrays terminated with 0.

When you connect the Uno to Serial on the Mega it will receive the data direct from the Serial Monitor.

...R

3

When you connect the Uno to Serial on the Mega it will receive the data direct from the Serial Monitor.

so of i understand correctly, it is not possible to connenct the serial1/2/3 from the mega to the serial of the uno ?

4

i did some tests and found out that my uno is not recieving anything from my mega/serial 1

5

PlatoBE:
so of i understand correctly, it is not possible to connenct the serial1/2/3 from the mega to the serial of the uno ?

Sure you can. It's just that if you do that, you can't use the Uno serial to communicate with the USB host.

6

PlatoBE:
i did some tests and found out that my uno is not recieving anything from my mega/serial 1

...which could mean a million things.