exit status 1 'mySerial' does not name a type
simple mistake mySerial.begin(4800);
  mySerial.println("Hello, world"); lines was out of void {}

#include &lt;SoftwareSerial.h&gt;

SoftwareSerial mySerial(10, 11);

uint16_t buf[256];

void setup() {
  Serial.begin(9600);
  while(!Serial);
  for(int i = 0; i &lt; 256; i++)
    buf[i] = (uint16_t)i;
   mySerial.begin(4800);
  mySerial.println("Hello, world"); //printLn  L, not I 
}


void loop() {
  Serial.write((uint8_t*)buf, 512);
  if (Serial.available()) {
    mySerial.write(Serial.read());
  }
}