Merge pull request #1750 from shauew/patch-2

Update portserial_m.c

components/net/freemodbus/port/portserial_m.c

@@ -130,12 +130,16 @@ void vMBMasterPortSerialEnable(BOOL xRxEnable, BOOL xTxEnable)
         /* enable RX interrupt */
         serial->ops->control(serial, RT_DEVICE_CTRL_SET_INT, (void *)RT_DEVICE_FLAG_INT_RX);
         /* switch 485 to receive mode */
+#if defined(RT_MODBUS_MASTER_USE_CONTROL_PIN)
         rt_pin_write(MODBUS_MASTER_RT_CONTROL_PIN_INDEX, PIN_LOW);
+#endif
     }
     else
     {
         /* switch 485 to transmit mode */
+#if defined(RT_MODBUS_MASTER_USE_CONTROL_PIN)
         rt_pin_write(MODBUS_MASTER_RT_CONTROL_PIN_INDEX, PIN_HIGH);
+#endif
         /* disable RX interrupt */
         serial->ops->control(serial, RT_DEVICE_CTRL_CLR_INT, (void *)RT_DEVICE_FLAG_INT_RX);
     }