issue 968 for dev branch

try, try again.

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.

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 )
     {