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

@@ -376,6 +376,7 @@ const int sensitive_pins[] = SENSITIVE_PINS; // Sensitive pin list for M42
 
 //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;
 
@@ -645,6 +646,7 @@ void loop()
           else
           {
             SERIAL_PROTOCOLLNPGM(MSG_OK);
+						previous_millis_ok = millis();
           }
         }
         else
@@ -3161,6 +3163,7 @@ Sigma_Exit:
           SERIAL_PROTOCOL(": ");
           SERIAL_PROTOCOL(servos[servo_index].read());
           SERIAL_PROTOCOLLN("");
+					previous_millis_ok = millis();
         }
       }
       break;
@@ -3235,6 +3238,7 @@ Sigma_Exit:
         SERIAL_PROTOCOL(" d:");
         SERIAL_PROTOCOL(unscalePID_d(bedKd));
         SERIAL_PROTOCOLLN("");
+				previous_millis_ok = millis();
       }
       break;
     #endif //PIDTEMP
@@ -3525,6 +3529,7 @@ case 404:  //M404 Enter the nominal filament width (3mm, 1.75mm ) N<3.0> or disp
           SERIAL_ECHO_START;
           SERIAL_ECHOLNPGM(MSG_ZPROBE_ZOFFSET " " MSG_OK);
           SERIAL_PROTOCOLLN("");
+					previous_millis_ok = millis();
         }
         else
         {
@@ -3911,6 +3916,7 @@ case 404:  //M404 Enter the nominal filament width (3mm, 1.75mm ) N<3.0> or disp
   ClearToSend();
 }
 
+
 void FlushSerialRequestResend()
 {
   //char cmdbuffer[bufindr][100]="Resend:";
@@ -3928,6 +3934,7 @@ void ClearToSend()
     return;
   #endif //SDSUPPORT
   SERIAL_PROTOCOLLNPGM(MSG_OK);
+	previous_millis_ok = millis();
 }
 
 void get_coordinates()
@@ -4352,6 +4359,14 @@ void manage_inactivity(bool ignore_stepper_queue/*=false*/) //default argument s
   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 )
     {