diff --git a/@MyNewpUsbComm/MyNewpUsbComm.m b/@MyNewpUsbComm/MyNewpUsbComm.m
index 7e2ec6b..f515a9b 100644
--- a/@MyNewpUsbComm/MyNewpUsbComm.m
+++ b/@MyNewpUsbComm/MyNewpUsbComm.m
@@ -1,84 +1,104 @@
 classdef MyNewpUsbComm < MySingleton
     
     properties (GetAccess = public, SetAccess = private)
         
         % Driver in use
         isbusy = false
     end
     
     properties (Access = public)
         
         % An instance of Newport.USBComm.USB class 
         Usb
     end
     
     methods(Access = private)
         
         % The constructor of a singleton class should only be invoked from
         % the instance method.
         function this = MyNewpUsbComm()
             disp(['Creating a new instance of ' class(this)])
             loadLib(this);
         end
     end
     
     methods(Access = public)
         
         % Load dll
         function loadLib(this)
             dll_path = which('UsbDllWrap.dll');
             if isempty(dll_path)
                 error(['UsbDllWrap.dll is not found. This library ',...
                     'is a part of Newport USB driver and needs ',...
                     'to be present on Matlab path.'])
             end
             NetAsm = NET.addAssembly(dll_path);
             
             % Create an instance of Newport.USBComm.USB class
             Type = GetType(NetAsm.AssemblyHandle,'Newport.USBComm.USB');
             this.Usb = System.Activator.CreateInstance(Type);
         end
         
         function str = query(this, addr, cmd)
             
             % Check if the driver is already being used by another process.
             % A race condition with various strange consequences is 
             % potentially possible if it is.
             assert(~this.isbusy, 'Newport Usb Comm is already in use.')
             
             this.isbusy = true;
             
             % Send query using QueryData buffer. A new buffer needs to be
             % created every time to ensure the absence of interference 
             % between different queries.
             QueryData = System.Text.StringBuilder(64);
             
             stat = Query(this.Usb, addr, cmd, QueryData);
             
-            if stat == 0
-                str = char(ToString(QueryData));
-            else
-                str = '';
-                warning('Query to Newport usb driver was unsuccessful.');
+            if stat ~= 0
+                warning(['Query to Newport usb driver was unsuccessful.'...
+                    errorCodeToMessage(this, stat)]);
             end
             
+            str = char(ToString(QueryData));
+            
             this.isbusy = false;
         end
     end
+    
+    methods (Access = private)
+        
+        % Convert the code returned by read/write/query functions to
+        % a message
+        function str = errorCodeToMessage(~, stat)
+            switch stat
+                case 0
+                    
+                    % No error
+                    str = ''; 
+                case -2
+                    str = 'Error: Device timeout';
+                case 1
+                    str = 'Error: Duplicate USB address';
+                otherwise
+                    str = sprintf('Error Code = %i', stat);
+            end
+        end
+    end
    
     methods(Static)
         
         % Concrete implementation of the singleton constructor.
         function this = instance()
             persistent UniqueInstance
 
             if isempty(UniqueInstance) || ~isvalid(UniqueInstance)
                 this = MyNewpUsbComm();
                 UniqueInstance = this;
             else
                 this = UniqueInstance;
             end
         end
     end
 end