Arduino MKR Wifi 1010 Serial Communication with Arduino Mega

I am writing a serial communication program between a mkr wifi 1010 and a mega in which the mkr sends either a H or L char to the mega, and the mega responds by reading the char and setting it's onboard LED to either high or low depending on the char. When I use delay(#) on the mkr the code works just fine and the mega's LED blinks on and off.

However when I create delay-less code the mega's led will not blink. It either stays low or stays high, most of the time stays low. I have checked the mkr code by reading the serial port and it indeed sends 72(H) and 76(L) alternately at the correct interval in both versions of the code.

Wiring diagram:

MKR          Mega
gnd    ->     gnd
tx     ->     rx
rx     ->     tx

I have level shifted the mega's tx pin down to 3.3v before the mkr's rx pin.

mkr delay code:

void setup()
{
  uint32_t baudRate = 9600;
  Serial1.begin(baudRate);
}

void loop()
{
  Serial1.print('H');
  delay(500);
  Serial1.print('L');
  delay(500);
}

mkr delay-less code:

unsigned long curT = 0;
unsigned long prevT = 0;
const int interval = 500;
int byteToSend = 'L';
void setup()
{
  uint32_t baudRate = 9600;
  Serial1.begin(baudRate);
}

void loop()
{
  curT = millis();
  
  if (curT - prevT > interval)
  {
    if (byteToSend == 'L')
    {
      byteToSend = 'H';
    }
    else
    {
      byteToSend = 'L';
    }
    
    Serial1.print(byteToSend);
    
    prevT = curT;
  }
}

mega code:

const int ledPin = 13; // the pin that the LED is attached to
int incomingByte;      // a variable to read incoming serial data into

void setup() {
  // initialize serial communication:
  Serial.begin(9600);
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);
}

void loop() {
  // see if there's incoming serial data:
  if (Serial.available() > 0)
  {
    // read the oldest byte in the serial buffer:
    incomingByte = Serial.read();
    // if it's a capital H (ASCII 72), turn on the LED:
    if (incomingByte == 'H')
    {
      digitalWrite(ledPin, HIGH);
    }
    // if it's an L (ASCII 76) turn off the LED:
    if (incomingByte == 'L')
    {
      digitalWrite(ledPin, LOW);
    }
  }
}

As far as I know these 2 different mkr programs do exactly the same thing and send serial data in the exact same way, it's just that one is blocking and one is non-blocking. Why is it that the delay-less code does not blink?

In the second sketch you used the wrong data type. If you change

int byteToSend = 'L';

to

char byteToSend = 'L';

your code should work as expected.

Thank you that fixed the issue.