public int SubscribeItem(ITag tag, Action<ITag, TagValueChangedEventArgs> onChangeAction)
        {
            int requestedUpdateRate = 1000;
 
            //The method returns an integer handle that uniquely identifies the item subscription.
            var result = _easyDaClient.SubscribeItem(
                _plcParamereters.MachineName,
                _plcParamereters.ServerClass,
                tag.Name,
                requestedUpdateRate,
                TagValueChangedCallback
                );
 
            _callbacks.Add(result, new TagValueChangedCallback { Tag = tag, TagValueChangedAction = onChangeAction });
 
            return result;
        }
 
        private void TagValueChangedCallback(object sender, EasyDAItemChangedEventArgs e)
        {
            Debug.WriteLine(e.ToString());
 
            TagValueChangedCallback tagValueChangedCallback = null;
 
            try
            {
                var subscribedItem = e.Handle;
 
                tagValueChangedCallback = _callbacks[subscribedItem];
            }
            catch (KeyNotFoundException)
            {
            }
 
            if (tagValueChangedCallback == null)
                throw new Exception("OpcClientCallback not defined");
 
            if (e.Vtq == null || e.Vtq.Value == null)
            {
                var message = string.Format("Tag value invalid: tag:{0};", tagValueChangedCallback.Tag.Name);
                Debug.WriteLine(message);
            }
 
            var tagValueChangedEventArgs = new TagValueChangedEventArgs
            {
                Value = e.Vtq == null ? null : e.Vtq.Value,
                Quality = e.Vtq == null ? string.Empty : e.Vtq.Quality.ToString(),
                QualityIsGood = e.Vtq != null && e.Vtq.Quality.IsGood(),
                Timestamp = e.Vtq == null ? DateTime.MinValue : e.Vtq.Timestamp,
                ErrorCode = e.ErrorCode,
                ErrorMessage = e.ErrorMessage,
                Exception = e.Exception
            };
 
            // update the tag value
            if (e.Vtq != null && e.Vtq.Value != null)
                tagValueChangedCallback.Tag.Value = e.Vtq.Value;
 
            // invoke the callback delegate
            tagValueChangedCallback.TagValueChangedAction(tagValueChangedCallback.Tag, tagValueChangedEventArgs);
        }
 

//Master Start Constructor
 
        public MasterStart(
            ILogWriter logWriter,
            IEdhrQueryResultHandler edhrQueryResultHandler,
            IPlcClient plcClient,
            ITagNameGenerator tagNameGenerator
            )
            : base(
                logWriter,
                edhrQueryResultHandler,
                plcClient,
                tagNameGenerator
                )
        {
 
 
 
            _tagEventTriggerFlag = new Tag<bool>(TagNameGenerator.Generate("lsNextLotCheck"));
 
            _tagErrorNum = new Tag<int>(TagNameGenerator.Generate("lsErrorNum"));
            _tagErrorDescription = new Tag<string>(TagNameGenerator.Generate("lsErrorDescription"));
 
            _tagMasterNo = new Tag<string>(TagNameGenerator.Generate("lsLotNoNext"));
 
 
 
 
            _masterStartTags = new List<ITag>
            {
 
                _tagMasterNo,
 
            };
 
 
 
            // subscribe for tag change events
            PlcClient.SubscribeItem(_tagEventTriggerFlag, MasterStartFlag_OnChange);
        }
 
 
 
//Master Start on Change
        private void MasterStartFlag_OnChange(ITag tag, TagValueChangedEventArgs args)
        {
 
 
            const string eventName = "LotStart";
 
            try
            {
 
                // read PLC tags
                PlcClient.ReadMultipleItemValues(_masterStartTags);
 
 
                if (!_tagEventTriggerFlag.Value)
                {
                    return;
                }
 
 
 
                WriteTolog("Starting Event with Value " + _tagMasterNo.Value + " - " + _tagEventTriggerFlag.Value, eventName, true);
 
		// Do database work....
 
 
            }
            catch (Exception ex)
            {
                WriteTolog(ex, eventName);
                _tagEventTriggerFlag.Value = false;
                _tagErrorDescription.Value = ex.Message;
                _tagErrorNum.Value = ProductionEventConstants.EDHR_ERROR; //Set to 2 to flag 
 
                try
                {
                    // Write tags to PLC
                    PlcClient.WriteMultipleItemValues(new List<ITag> { _tagEventTriggerFlag, _tagErrorDescription, _tagErrorNum });
                }
                catch (Exception)
                {
                    // ignored
                }
            }
        }