OPC reading and writing not working withing thread

[Jeff Peery]

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# myOPC.py                                                                       #
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''' autor : Katarzyna Gebal
            Narrow Gate Logic
            www.nglogic.com'''
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''' OPC Client project file'''    
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import win32com.client
import win32api
import pythoncom
import time
import sys

""" OPC FUNCTIONS """

# get items from OPC server
def getOPCItems(server):
    # Get OPC servers and set the selected value
    serverList = getServerList()
    
    if serverList == None:
        Error('No servers available')
    elif server not in serverList:
        Error('Requested server not available')
    else:
        # connect to server
        (opcServer,group) = connectServer(self.main.deviceList[self.deviceText].server)
                                        
        # get the items available in OPC server
        try:
            itemList = getOPCItemsList(opcServer)    
        except Exception:    
            Error('Error obtaining server items') 

        # disconnect
        try:
            disconnectServer(opcServer)
        except:
            self.Error('Error disconnecting server')
        
        return itemList
            
       
# The function gets access to the the OPC automation object.
# computerName parameter is given when we want to make remote connection by using DCOM. 
# The function returns OPCServer object.
def connectCOMOPC(computerName=None):
     if computerName == None:
          opcServer = win32com.client.Dispatch('OPC.Automation.1')
     else:
          # can connect to remote computer
          opcServer = win32com.client.DispatchEx('OPC.Automation.1',computerName,clsctx=pythoncom.CLSCTX_ALL)
     return opcServer

     
# The function returns the tuple of names (ProgIDs) of the registered OPC Servers.
# This names are used when we connect to the server.
# computerName parameter is given when we want to make remote connection by using DCOM.
def getServerList(computerName=None):
    try:
        return connectCOMOPC(computerName).GetOPCServers()
    except Exception:
        return None
  

# The function connects to an opc server.
# It creates one private OPCGroup. This group will be used to to access items.   
# It returns the tuple of OPCServer object and the OPCGroup object.
def connectServer(serverName,computerName=None):
    try:
        opcServer = connectCOMOPC(computerName)
        opcServer.Connect(serverName)
        groups = opcServer.OPCGroups
        group = groups.Add() 
        group.IsActive = 1   
        group.IsSubscribed = 1
        return (opcServer,group)
    except Exception:
        return (None,None)
        

# The function disconects from the OPC Server.
def disconnectServer(opcServer):
    opcServer.OPCGroups.RemoveAll() # cleaning up
    opcServer.Disconnect()
    opcServer = None
    

# The function traverses recursively the hierarchical structure of item's namespaces.
# It appends all found items to the list. 
def opcAppendLeafs(browser,list):
    browser.ShowLeafs()
    for leaf in browser:
        list.append(browser.GetItemID(leaf))
    browser.ShowBranches()
    for branch in browser:
        browser.MoveDown(branch)
        opcAppendLeafs(browser,list)
        browser.MoveUp()
        

# Function tries to get the items that exist in the opcServer. 
# It uses OPCbrowser object.
# Sometimes it is not possible to get all existing item names. 
# The ability of browsing depends on the server configuration.
def getOPCItemsList(opcServer):
    try:
        list = []
        browser = opcServer.CreateBrowser()
        browser.AccessRights = 3
        browser.MoveToRoot()
        opcAppendLeafs(browser,list)
        return list
    except Exception:
        return None


# Adds item to the private group group
def addItem(itemID,group):
    try:  
        return group.OPCItems.AddItem(itemID,1)
    except Exception:
        return None


# Function reads value quality and timestamp of the item  
# 0x1 means that values are read from the OPCcache
# The second possible option (0x2) is reading directly from the device.
# Function returns the tuple of the value, quality and the time stamp
def readItemValue(opcItem):
    return opcItem.Read(0x1)
        

# Function writes value to the opc server
# An error appears if the conversion of value type to the required 
# item type is not possible.
def writeItemValue(opcItem,value):
    opcItem.Write(value)