Add get_response_pdu_size to WriteMultipleRegistersRequest (#267)

Author: yy123xiang

pymodbus/register_write_message.py

@@ -192,6 +192,13 @@ def execute(self, context):
         context.setValues(self.function_code, self.address, self.values)
         return WriteMultipleRegistersResponse(self.address, self.count)
 
+    def get_response_pdu_size(self):
+        """
+        Func_code (1 byte) + Register Address(2 byte) + Quantity of Registers (2 bytes)
+        :return: 
+        """
+        return 1 + 2 + 2
+
     def __str__(self):
         ''' Returns a string representation of the instance