Sending >255 bytes array to arduino

Question

I am trying to send a 50x50 bitmap bytearray from my python client to arduino where it should be displayed on a screen.

I need to receive 350 bytes to have the data to show the bitmap but as soon as i initialise the array with > 255 elements it does weird things (i guess because of the memory):

When initialising the array with size 254 it does the expected:

Transmission of 350 completed in 2.805466s (arduino) Received: 253 elements

with size 350 I only get

"Received: 93 elements"

How can i split the bytes i receive with the available memory and then build the bitmap accordingly or what is a better way to do it? I would like to have it transmitted via serial rather than building the qrcode in arduino (as i couldn't get the libraries required working in my setup).

My python client does basically this:

for element in qrCodeBytes:
    countElementsSent+=1
    ser.write(element)
    print("Sending " + str(element))
ser.write("$".encode()) #stop char

My arduino code is based on the popular Serial input basics guide https://forum.arduino.cc/index.php?topic=288234.0

 const byte numChars = 254;
 byte receivedChars[numChars]; // an array to store the received data

boolean newData = false;

void setup() {
 Serial.begin(114000);
 Serial.setTimeout(10000);
 Serial.println("<Arduino is ready>");
 display.begin(SSD1306_SWITCHCAPVCC, 0x3C);  // initialize with the I2C addr 0x3D (for the 128x64) 
 display.clearDisplay();
 display.display();
}

void loop() {
 recvWithEndMarker();
 showNewData();
}

void recvWithEndMarker() {
 static byte ndx = 0;
 char endMarker = '$';
 char rc;

 // if (Serial.available() > 0) {
   while (Serial.available() > 0 && newData == false) {
     rc = Serial.read();
     if (rc != endMarker) {
       receivedChars[ndx] = rc;
       ndx++;
       if (ndx >= numChars) {
         ndx = numChars - 1;
       }
     }
     else {
     receivedChars[ndx] = '$'; // terminate the string
     Serial.println("Received: " + String(ndx) + " elements");
     ndx = 0;
     newData = true;
     }
   }
}

void showNewData() {
 if (newData == true) {
 Serial.println("This just in ... ");
 display.clearDisplay();
 drawBitMap(receivedChars,50,50,0);
 display.display();
 newData = false;
 }
}

void drawBitMap(unsigned char *img, int width, int height, int position){
  if (position ==0){
   display.drawBitmap(128/2-(width/2), 64/2-(height/2), img, width, height, 1);
  } else if (position == 1){
   display.drawBitmap(128-62,1, img, width, height, 1);
  } else if (position == 2){
   display.drawBitmap(0,5, img, width, height, 1);
  }
}

Answer 1

You use type byte for the variable ndx. Type byte is 8 bits, so the max value is 255. The variable ndx is incremented and after 255 it continues counting from 0.