Merge pull request #1311 from i-make-robots/Development

issue 968 for dev branch

Marlin/Marlin_main.cpp

 
 //Inactivity shutdown variables
 static unsigned long previous_millis_cmd = 0;
+static unsigned long previous_millis_ok = 0;
 static unsigned long max_inactive_time = 0;
 static unsigned long stepper_inactive_time = DEFAULT_STEPPER_DEACTIVE_TIME*1000l;
 
           else
           {
             SERIAL_PROTOCOLLNPGM(MSG_OK);
+						previous_millis_ok = millis();
           }
         }
         else
           }
         }
         else if (servo_index >= 0) {
-          SERIAL_PROTOCOL(MSG_OK);
+					SERIAL_PROTOCOL(MSG_OK);
           SERIAL_PROTOCOL(" Servo ");
           SERIAL_PROTOCOL(servo_index);
           SERIAL_PROTOCOL(": ");
           SERIAL_PROTOCOL(servos[servo_index].read());
           SERIAL_PROTOCOLLN("");
+					previous_millis_ok = millis();
         }
       }
       break;
         SERIAL_PROTOCOL(" d:");
         SERIAL_PROTOCOL(unscalePID_d(bedKd));
         SERIAL_PROTOCOLLN("");
+				previous_millis_ok = millis();
       }
       break;
     #endif //PIDTEMP
           SERIAL_ECHO_START;
           SERIAL_ECHOLNPGM(MSG_ZPROBE_ZOFFSET " " MSG_OK);
           SERIAL_PROTOCOLLN("");
+					previous_millis_ok = millis();
         }
         else
         {
   ClearToSend();
 }
 
+
 void FlushSerialRequestResend()
 {
   //char cmdbuffer[bufindr][100]="Resend:";
     return;
   #endif //SDSUPPORT
   SERIAL_PROTOCOLLNPGM(MSG_OK);
+	previous_millis_ok = millis();
 }
 
 void get_coordinates()
   if( (millis() - previous_millis_cmd) >  max_inactive_time )
     if(max_inactive_time)
       kill();
+
+	// If 'OK' is garbled on sending PC won't receive it.  Both machines will wait on each other forever.
+	// This resends OK if nothing is heard from PC for a while to avoid this bad case.
+  if( (millis() - previous_millis_ok) >  max_inactive_time/4 ) {
+		SERIAL_PROTOCOL(MSG_OK);
+		previous_millis_ok=millis();
+  }
+
   if(stepper_inactive_time)  {
     if( (millis() - previous_millis_cmd) >  stepper_inactive_time )
     {