unsigned long times = 0;
#define REPORT (5*1000L) // report every 5 seconds
unsigned long report = REPORT; 
unsigned long txchars = 0;
#define BAUD 9600 // 2400 is lowest supported, apparently

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600); // debug
  Serial3.begin(BAUD,SERIAL_8N1); // test channel
  Serial.print("Begin ");
  Serial.print(BAUD);
  Serial.println();
}

void loop() {
  // count how much the CPU gets done
  ++times;
  
  // keep serial transmitting at all times
  if (Serial3.availableForWrite() > 2) {
    Serial3.write('\x55');
    ++txchars;
  }

  if (millis() >= report) {
    Serial.print("Time:");
    Serial.print(report/1000L);
    Serial.print("s Tx:");
    Serial.print(txchars);
    Serial.print(" TxExpected:"); // compensate for buffersize
    Serial.print((report/1000L)*(BAUD/10) + (64-2));
    Serial.print(" Times:");
    Serial.println(times); 
    report += REPORT;
  }
}