пример работает

libraries/SoftwareSerial/examples/SoftwareSerialExample/SoftwareSerialExample.ino

@@ -28,10 +28,7 @@ SoftwareSerial mySerial(2, 6); // RX, TX
 void setup() {
   // Open serial communications and wait for port to open:
   Serial.begin(115200);
-  while (!Serial) {
+  while (!Serial) ; // wait for serial port to connect. Needed for native USB port only
-    ; // wait for serial port to connect. Needed for native USB port only
-  }
-
   Serial.println("Goodnight moon!");
 
   // set the data rate for the SoftwareSerial port
@@ -40,10 +37,11 @@ void setup() {
 }
 
 void loop() { // run over and over
-  if (mySerial.available()) {
+  while (mySerial.available()) 
     Serial.write(mySerial.read());
-  }
+  
-  if (Serial.available()) {
+  while (Serial.available()) 
     mySerial.write(Serial.read());
-  }
+  
+  delay(50);
 }

libraries/SoftwareSerial/src/SoftwareSerial.cpp

@@ -145,8 +145,7 @@ void SoftwareSerial::handle_interrupt() // .ram_text
   }
 }
 
-extern "C" void softSerial_interrupt_handler(void) __attribute__((noinline, section(".ram_text")));
+extern "C" void __attribute__((noinline, section(".ram_text"))) softSerial_interrupt_handler(void)
-extern "C" void softSerial_interrupt_handler(void)
 {
   SoftwareSerial::handle_interrupt();
 }