diff --git a/FreeModbus/port/port.c b/FreeModbus/port/port.c
index fb11c0f..952999a 100644
--- a/FreeModbus/port/port.c
+++ b/FreeModbus/port/port.c
@@ -43,6 +43,14 @@ void vMBDelay(ULONG nCount)
   for(; nCount > 0;nCount--);
 }
 
+#if MB_MASTER_RTU_ENABLED > 0 || MB_MASTER_ASCII_ENABLED > 0
+/**
+ *
+ * Just use it for modbus master.
+ * @note There functions will block modbus master poll while execute OS waiting.
+ * So,for real-time of system.Do not execute too much waiting process.
+ *
+ */
 void vMBMasterErrorCBRespondTimeout(UCHAR ucDestAddress, const UCHAR* pucPDUData,
 		USHORT ucPDULength) {
 
@@ -57,3 +65,4 @@ void vMBMasterErrorCBExecuteFunction(UCHAR ucDestAddress, const UCHAR* pucPDUDat
 		USHORT ucPDULength) {
 
 }
+#endif