Strange Communication Delay in Serial Bus, mitigated by adding Serial.print().

Hardware Arduino Due
7

Oh..

#define RTS 9

unsigned long cmillis, lastread, nextscan, roundrobintimer = 0;

setup partially:

  // Setup serial output
  Serial.begin(115200);

  // Setup rs485
  digitalWrite(RTS, LOW);
  pinMode(RTS, OUTPUT);
  digitalWrite(RTS, LOW);
  Serial1.begin(115200);
  Serial1.flush();

  setupCommands();

  setupGraphics();

  drawWindow();
  drawScreen();
  nextscan = millis() + 2000;
  lastread = millis();

loop:

void loop() {
  cmillis = millis();

//  if ( lastread != cmillis ) {
    while ( Serial1.available() != 0 )
      COM.receive(Serial1.read());
//    lastread = millis();
//  }

  if (( roundrobintimer != 0 ) && ( cmillis < roundrobintimer )) roundrobintimer = 0;
  if (( infoRoundRobin != 0 ) && ( cmillis < infoRoundRobin )) infoRoundRobin = 0;
  
  if (( infoRoundRobin == 0 ) && ( infoTimer != 0 ) && ( cmillis > infoTimer )) {
    drawBottomBar();
    infoTimer = 0;
  }

  if (( roundrobintimer == 0 ) && ( cmillis > nextscan )) {
    if (!loginInProgress) readFingerPrint();

    roundrobintimer = nextscan;
    nextscan = cmillis + 700;
    roundrobintimer = nextscan > roundrobintimer ? 0 : roundrobintimer;
  }
}