Fix printer resets in farm mode if command "PRUSA SN" is received and 32U2 processor is not responding.

Firmware/Marlin_main.cpp

@@ -2508,14 +2508,25 @@ void gcode_M701()
 }
 /**
  * @brief Get serial number from 32U2 processor
+ *
+ * Typical format of S/N is:CZPX0917X003XC13518
+ *
+ * Command operates only in farm mode, if not in farm mode, "Not in farm mode." is written to MYSERIAL.
+ *
+ * Send command ;S to serial port 0 to retrieve serial number stored in 32U2 processor,
+ * reply is transmitted to serial port 1 character by character.
+ * Operation takes typically 23 ms. If the retransmit is not finished until 100 ms,
+ * it is interrupted, so less, or no characters are retransmitted, only newline character is send
+ * in any case.
  */
 static void gcode_PRUSA_SN()
 {
     if (farm_mode) {
         selectedSerialPort = 0;
         MSerial.write(";S");
-        // S/N is:CZPX0917X003XC13518 
         int numbersRead = 0;
+        Timer timeout;
+        timeout.start();
 
         while (numbersRead < 19) {
             while (MSerial.available() > 0) {
@@ -2525,15 +2536,18 @@ static void gcode_PRUSA_SN()
                 numbersRead++;
                 selectedSerialPort = 0;
             }
+            if (timeout.expired(100)) break;
         }
         selectedSerialPort = 1;
         MSerial.write('\n');
-        /*for (int b = 0; b < 3; b++) { 
+#if 0
+        for (int b = 0; b < 3; b++) {
             tone(BEEPER, 110);
             delay(50);
             noTone(BEEPER);
             delay(50);
-        }*/ 
+        }
+#endif
     } else {
         MYSERIAL.println("Not in farm mode.");
     }