OPC Technology

Contents

• History of OPC
• Why do we need OPC? its Benefits
• OPC Concepts & Terminology
• COM Interfaces & Component Categories
• OPC Compliance
• OPC Specifications
• Resources

Schematic of OPC Server.

Figure 1.

Introduction

COM/OLE applications are widely used in Industrial Automation and Process Control domains. Since not many programmers are aware of these technologies, I thought to share my experience in OPC and most developers would love to expand their knowledge about COM/OLE application areas.

This is my first ever article on Code Project, so if you feel anything needs to be improved, drop me a line. Your suggestions and comments are most welcome.

The goal of article: Give simple & understandable overview of OPC. More detailed information and specifications can be downloaded from OPC Foundation and websites mentioned in Resources section.

So first, about the above diagram...

Theory about Figure 1:
On left-bottom is a hardware device. It could be a PLC (programmable logic controller), a temperature sensor, roller belt controller, or just about any industrial device. Central part is the OPC server. Right side is the client that wants to operate the device. The OPC Server gives out standard COM interfaces so that any client aware of these OPC Interfaces can interface with server and operate the device - without programming device driver for that hardware. Hardware can be as varied as OPC server can handle and clients can still read and write from this hardware.

Now back to our OPC course. This article gives an overview of OPC and covers following topics. Source code is not available, since most of server code is specific to hardware. And client would still need a server to connect to. But yes, if you want to experiment with OPC, check out the Resources section below.

History of OPC

In 1996, a group of companies Rockwell Software, Intellution, et. al. came together to form a consortium that defined a standard for process control. SCADA and similar standardization efforts were being made at that time in process control and automation industry. This group created an initial draft that was based on Microsoft's Object Linking and Embedding (OLE). It was called OLE for Process Control (OPC). OPC Foundation publishes specifications and manages everything that goes into OPC technology.

Today more than 300 companies are part of OPC Foundation. Microsoft, GE, Siemens, Rockwell, ABB, Ford, Honeywell and more are members that actively contribute and consume OPC technology.

In 2005, OPC Foundation released OPC Unified Architecture draft. There are predecessor drafts released before - OPC HDA, OPC DX, OPC AE, OPC Commands, OPC Security. We will briefly study them in this article.

By now, you must have a question in your mind. Let me answer it below...

Why do we need OPC? its Benefits

I was also a first timer to OPC (coming from varied experience in DevTools, eLearning, CAD/PLM, Multimedia), and had no clue as to why do we need it. I found that learning it through a case study was very helpful, so I am going to do the same for you. To understand the need for standardization in process control industry, let me give you an actual case study performed by a giant organization in Petrochemicals business.

Refer to figure 2. It shows the devices and software application that were in action on this particular petrochemical plant.

Figure 2.

PLC, Vibration sensor, and Calculation engine work on different communication protocols - TSAA, Modbus, and DDE. So the software applications that need data from these devices also need to be aware of TSAA, Modbus, and DDE protocol. Additionally each device needs to serve all the three applications. The process historian, GUI application providing human machine interface and the machine condition monitor all communicating with all devices results in a complexity. This deployment took the company around 10 days. And its cost was approximately US $ 50,000.

Now, someone like me :) came to them and said, "hey guys, why don't we think smart and implement as in Figure 3?". Observe the reduced complexity. Now all software applications talk only to the OPC Servers. And all devices also respond only to the OPC server. OPC Server provides the Black Box for software applications around hardware.

Figure 3.

The new implementation took just 2 days to deploy and cost incurred was approximately US $10,000. They could have eliminated 2 OPC servers as well and implemented device drivers right inside a single OPC server. This is a tradeoff and off the shelf OPC servers may have standard protocols support like DDE, Modbus and TSAA.

As you observed correctly, the benefits are:

• Reduced load on device.
• Scalability of system has increased.
• OPC server provides caching of data.
• Client applications need not know hardware protocol details.
• Increased life for device (it need not serve multiple clients anymore).
• Interoperability (Unix/Linux and Windows - both platforms are supported by OPC)
• Standardization

Now that you have some insight into OPC benefits and history, let's take one step ahead to understand OPC terminology and concepts.

OPC Concepts & Terminology

OPC specification defines a consistent terminology to identify various entities involved. The best analogy to OPC way of data representation is that of a file system hierarchy. OPC calls it as OPC Address Space. Within this address space every entity can be uniquely identified by its "full-path". For example, refer to figure 4.

Figure 4.

OPC Address Space is hierarchical. It starts with a root. Then there are logical folders and sub-folders. Leaf nodes are OPC Items or Tags. Generally your sensors and actuators form the leaf nodes. This is an important point to understand and we will spend some time on it. For example, you have a device that can sense temperature, Pressure, and Humidity. Then these attributes are generally mapped as leaf nodes under some folder-subfolder hierarchy. We will see what that hierarchy can be in a moment. If you have an actuator (say, Turn_ON_Boiler), then that also becomes a leaf node. You can write to Turn_ON_Boiler values of 1 or 0 for example, and necessary action will be taken - of course we just assumed that your hardware control mechanism behaves this way. You can read from Temperature to know sensed value.

Hope we are all clear till this point. Now let's understand this "logical" folder stuff. The logical folders and sub-folders are used to represent state of your Industrial Automation system. This configuration can actually map to some physical configuration or even geographical setup. Let's spend some time to understand this one as well. For example, in this diagram we have an ethernet communication channel to which a wireless meter is connected. And within wireless meter, there are two classes "Average" and "Channel1". These folders and sub-folders are very effective means of grouping your process control parameters and values.

This address space is "published" to all clients of OPC server. The OPC clients can be simple spreadsheets like Excel, process historians that keep record of data coming from various industrial plant entities. Every client can create a logical entity called as group inside OPC server. Each OPC Group can contain a set of OPC Items.

Synchronous & Asynchronous OPC Item reading
OPC specification supports synchronous as well as asynchronous reading and writing on hardware devices. Since not all hardware device operations are fast-enough, the asynchronous mechanism of reading and writing to a device comes very handy.

COM Interfaces & Component Categories

Well, so now suppose you wish to write your first OPC client. If you were developing a client that runs on local machine as that of server, and if you were aware of all OPC Server ProgIDs, then we would not have any worries. But since that is not possible, we need an enumeration mechanism. Following is an enumeration mechanism.

OPC Component Categories

Enumeration issue is addressed by requiring OPC servers to implement
component categories. For example, all servers that are OPC DA 1.0
(more about this in later sections) compliant implement COM category
with CatID {63D5F430-CFE4-11d1-B2C8-0060083BA1FB}. Similarly the
servers compliant with OPC Data Access 2.0 implement category with
CatID {63D5F432-CFE4-11d1-B2C8-0060083BA1FB}.

So OPC Foundation distributes a Windows Service called as OPCEnum.

Enumerating OPC Servers
OPCEnum.exe runs as a Windows Service and provides enumeration
functionality. Using OPCEnum, the client applications can query
available OPC Servers on local as well as remote networked machines.

Now let's look at server component interfaces.

Figure 5.

There are various COM interfaces that OPC Server must support according the OPC Specification compliance required. These basic interfaces have specific functionality.

IOPCBrowse This interface has methods that allow browsing OPC Address Space. Client can query level by level folders and subfolders till the leaf nodes. The hierarchy looks like shown in figure 4.

IOPCItemIO Interface is used for performing OPC Item read and write operations.

Now let's understand how OPC Client interacts with the OPC Server. Important steps involved are summarized in figure 6, below.

Figure 6.

I know you must still be wondering about this OPC Group term. Let's spend some time understanding it. Every client when opens a session with OPC Server, needs to create a logical group of items that it wants to read or write. Client can maintain multiple groups. Each group is given a unique name and has following attributes:

• Update rate [milliseconds] at which the values of items are read.
• Active or In-active Flag [Boolean] only if group is active that the items are processed.
• Deadband [%] variations inside this band are considered NULL.
• IO Mode [Synchronous/ASynchronous] all operations performed are either sync or async.
• Time Bias [time] local time band with respect to GMT.

The OPC Group component supports following interfaces.

Figure 7.

As you may have identified these IOPCSyncIO etc interfaces that are relevant for synchronous and asynchronous OPC Item read and write operations. OPC Group Components also support connection point container for events that are used for client notifications. At least you have some idea by now that OPC is just another COM application in action - just that it is a standard meaning all OPC Clients and OPC Servers are bound by design contract.

OPC Compliance

OPC Foundation provides compliance test applications. Using these suites one can execute certain number of test cases on OPC Server and determine their compliance to particular OPC Specifications. There are several OPC specifications available (explained in next section). A typical compliance report looks like as shown in figures 8 & 9.

Figure 8.

Figure 9.

OPC Compliance Test Suite also supports testing for performance whereby the OPC Server in question is loaded with multiple items read and write requests. These determine time required per item operation and also passed and failed test cases. Once compliance is achieved, the report can then be uploaded to OPC Foundation web site for public reference. So that customers buying OPC Server from a particular vendor can make sure that the server is compliant to a particular specification and may meet their integration need with other software packages used on the plant.

OPC Specifications

OPC Foundation is responsible for maintaining and releasing new specifications. It is a body made up of industry experts, companies that produce OPC products and organizations that consume OPC products. So far OPC Foundation has released following specifications.

• OPC-DA (Data Access) Provides access to real-time data. We can query most recent values of temperature, pressure, density, acceleration, and other types of process control data from OPC-DA server.
• OPC-HDA (Historical Data Access) Used to retrieve historical process data for analysis. This data is typically stored in archives, databases or remote telemetry systems.
• OPC-AE (Alarms & Events) OPC AE servers are used to exchange and acknowledge process alarms and events.
• OPC-DX (Data eXchange) Defines how one OPC server exchanges data with other OPC servers.
• OPC-XML (XML Data Access) Defines schema and data representation format based upon XML standard. Makes it possible to share & manipulate process control data across all operating systems – Windows, Unix, Solaris, etc.
• There were several efforts put onto - OPC Security, OPC Batch, OPC Commands, OPC for ERP.

Most recently OPC Foundation has released a draft for OPC Unified Architecture that attempts to provide even more integration of process control data and software with other industrial software systems like ERP.

Resources

There are plenty of resources available on the web about OPC.

• OPC Foundation's Website : http://www.OPCFoundation.Org
• OPC Unified Architecture : http://www.opcconnect.com/ua.php
• OPC Connect Website : http://www.opcconnect.com
• Control.com website also has some useful discussions on OPC: http://www.control.com

Hope you enjoyed knowing a bit of OPC in this article. If you have any questions or suggestions, please post them here or drop me a line. srt@Suchit-Tiwari.Org or Suchit.Tiwari@Ge.com Your suggestions and comments are most welcome.

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Exploring OPC servers sreejesh_s_warrier 0:41 16 Oct '08   Hi, I want to explore all the OPC servers installed localhost/remot machine. Please assist me which all APIs are required. Or please let me know about sample source code links. Thanks Sreejesh. Sign In·View Thread·PermaLink 1.00/5 (1 vote) Regarding OPCHDA client! jitendra.mitra 3:38 29 Jul '08   Hi Suchit, Its a great article u have written for all the guys who r new to OPC technology. Hats of for that.. I m currently developing an OPCHDA client for one of my application. i have written two application one in UNICODE env and other in ASCII (MBCS) env. the broiler code that i wrote in UNICODE application runs fine for connecting to a HDA server, gets Item Handles successfully and also reads HDA tags value successfully (sync read). In my ASCII application, connection is made successful to HDA server. The COM call for GetItemHandle (OPCHDA specification 1.2) is also successful. But after that when i make sync ReadRaw call, it always fails. The error code tells that there is an INVALID arg passed. here is the snapshot of the code: // Variables DWORD dwTransactionID; DWORD dwCancelID; DWORD dwCount; HRESULT* pErrorsAsyncRead; OPCHDA_TIME* phtStartTime; OPCHDA_TIME* phtEndTime; DWORD dwNumValues; bool bBounds; DWORD dwNumItems; OPCHANDLE *phServer; OPCHDA_ITEM *pItemValues; HRESULT* pErrors; HRESULT hr; HRESULT* pErrorsRead; OPCHANDLE hClient; HRESULT hrRead; // Initializations dwCount = 1; dwNumValues = 0; hClient = 0x001; bBounds = FALSE; dwNumItems = 1; dwNumValues = 1; int iLen; // Allocating and assigning the time stamps // for which the historical data is to be requested phtStartTime = new OPCHDA_TIME[dwCount]; phtEndTime = new OPCHDA_TIME[dwCount]; phtStartTime->bString = TRUE; // WCHARFromCHAR function converts a char string into wchar string WCHAR* cStrtTm = WCHARFromCHAR(oTagDetails.cStartTime); // wstrGlobalDupl function does mem allocation using CoTaskMemAlloc for wide string phtStartTime->szTime = wstrGlobalDupl(cStrtTm); phtStartTime->ftTime.dwHighDateTime = 0; phtStartTime->ftTime.dwLowDateTime = 0; phtEndTime->bString = TRUE; WCHAR* cStopTm = WCHARFromCHAR(oTagDetails.cStopTime); phtEndTime->szTime = wstrGlobalDupl(cStopTm); ::GetSystemTimeAsFileTime(&(phtStartTime->ftTime)); Sleep(10000); ::GetSystemTimeAsFileTime(&(phtEndTime->ftTime)); LPWSTR pszItemID; WCHAR* cStopTm = WCHARFromCHAR(oTagDetails.cTagName); pszItemID = wstrGlobalDupl(cStopTm); // Get the server handle for this item hr = m_pAtlHDAServerPtr->GetItemHandles(dwCount, &pszItemID, &hClient, &phServer, &pErrors); // Making an synchronous advise call to the OPC HDA server hrRead = pOPCHDASyncRead->ReadRaw(phtStartTime, phtEndTime, dwNumValues, bBounds, dwNumItems, phServer, &pItemValues, &pErrorsRead); Can u suggest me or refer any HDA client side code that can help me overcoming this hurdel. Thanks in advance Regards, Jitendra Mitra Sign In·View Thread·PermaLink 1.00/5 (1 vote) Re: Regarding OPCHDA client! jitendra.mitra 0:24 30 Jul '08   hi suchit..i got the solution.. Thanks, Jeet Sign In·View Thread·PermaLink 1.00/5 (1 vote) OPC without COM dhina@opc 20:14 10 Jul '08   Feasibility studies for OPC Server implementation in Linux, reveals EntireX DCOM and OPC UA as possible solutions.(Lets keep apart UA). As I hope all the SDKs available provides APIs(DLLs), that internally takes care of COM and OPC implementations. So they makes sense for COM based OPC servers development. Now, for the sake for OPC server development in linux,does it makes sense to isolate OPC requirements(specifications) and COM dependency. To put in simple terms, Shall I implement the interfaces that OPC demands seperately and then replace COM by some other communication methodology. And then claim it as OPC compatible. If so, how to go about OPC requirements implementation, are there any SDKs? Sign In·View Thread·PermaLink OPC server architecture. dhina@opc 21:46 9 Jul '08   How should be a OPC server architectured? As per my unserstanding, OPC server has two basic entities viz., Device driver: Which speaks to device, using a standard protocol like Modbus or TCP/IP or even proprietary. OPC Server: Which picks the data from driver and makes it available for the client. If the above said is true, 1. How should OPC server be interfaced to driver? (Are the commercially available SDKs meant for this?) 2.Does OPC classify devices into types? (Electrical, Automation, etc., or something like that) Sign In·View Thread·PermaLink 2.00/5 (1 vote) Re: OPC server architecture. .Suchit 3:57 10 Jul '08   1. How should OPC server be interfaced to driver? (Are the commercially available SDKs meant for this?) This is pretty much specific to a particular OPC Sever implementation. If you have the code for OPC Server, you will have to implement calls to specific device drivers in the places where initialization happens, reading/writing happens, and where disconnection happens. I doubt if there are any ready-made tools for interfacing any generic driver to any OPC server. 2.Does OPC classify devices into types? (Electrical, Automation, etc., or something like that) No. OPC Does not have any implicit classification method. You can manage "Logical" classification through use of OPC Groups. You can have some predefined groups in your Address Space Hierarchy. And then populate your device reading parameters or the sensors information under appropriate branch. Sign In·View Thread·PermaLink Re: OPC server architecture. dhina@opc 19:47 10 Jul '08   Suchit, Thanks for your reply. I've few more questions which I shall start as new thread. Sign In·View Thread·PermaLink How the communication between Client And server takes place...? Codemastergabriel 22:53 29 Jun '08   Can any one please help me out to understand the concept of communication between OPC Client and Server as i need to develope an OPC Client in Java... and one more thing that can any one please help me out to find out some documents for developing OPC Client in Java Sign In·View Thread·PermaLink Re: How the communication between Client And server takes place...? .Suchit 0:07 30 Jun '08   OPC Server is a COM Server. So basically your Java application will need to understand COM - you can use JNI or related commercial products that will your Java application a COM awareness. And then see the figure above that documents the steps involved in building up Server-Client communication channel and reading writing items .. Sign In·View Thread·PermaLink 2.00/5 (1 vote) Problem with Connect to Server stancrm 6:30 23 Apr '08   Hello... I'm using OPC Core Components 3.0 and using OPC DA Sample Client. If I connect to sample server, it's ok. But if I connect to remote machine (CoDeSys.OPC.02), I get an error message : "...HRESULT : 0x80004002 (E_NOINTERFACE)...". I don't know what is that.. But something is weird. If I use another OPC Client Tool, it's ok, without error. I can read all OPC variable. Are there something wrong in OPC Core Components or OPC DA Sample Client ? Thanks... Sign In·View Thread·PermaLink Re: Problem with Connect to Server .Suchit 6:56 23 Apr '08   I suspect it to be DCOM issue. The OPC Connections over network happen over DCOM. DCOM needs to be configured specifically if you are using Windows XP SP 2. The Service Pack 2 of Windows XP, has to be enabled for a TCP/IP Port 165 for DCOM to work properly. Follow the guide lines mentioned by OPC Foundation to get this DCOM and OPC working properly. Let me know if the issue persists, even after you have made these changes. Sign In·View Thread·PermaLink Re: Problem with Connect to Server stancrm 21:32 23 Apr '08   Hello... Both PCs are Windows 2000 without firewall. The server is CoDeSys.OPC.02 and the client is OpcDaSampleClient. Both PCs have the same username and same password under same workgroup. If I try to connect to server using OpcClient.exe (windows app without .NET), it works. But if I try to connect using OpcDaSampleClient (.NET), it fails to connect. The error message is: Unable to cast COM object of type 'System.__ComObject' to interface type 'OpcRcw.Comn.IOPCServerList2'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{9DD0B56C-AD9E-43EE-8305-487F3188BF7A}' failed due to the following error: Schnittstelle nicht unterstützt (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)). Somewhere in google.., I just found, that cast from ComObject to IOPCServerList2 is working only in C++ but not in C#. Do you know something about this ? Do I need to have the same version of OPC Core Components in Server and Client ? The next thing I want to do is try to update the OPC Core Components. Thx... Sign In·View Thread·PermaLink Re: Problem with Connect to Server stancrm 1:24 24 Apr '08   Ok.. I just solve it. I install also the latest OPC Core Component in Server and Client. It works... Sign In·View Thread·PermaLink Getting list of tags in a server Mikil Bhatkar 23:25 13 Aug '07   i am currently using the IOPCBrowseServerAddressSpace::BrowseOPCItemIDs call to achieve this, is there any better way to do this... How is IOPCBrowse different from this? Thanks in advance. Mikil Sign In·View Thread·PermaLink 2.00/5 (1 vote) Re: Getting list of tags in a server .Suchit 1:22 28 Aug '07   IOPCBrowse interface provides improved methods for browsing the server address space and for obtaining the item properties. The methods in this interface are designed to mirror the corresponding methods in the XML-DA interface. IOPCBrowseServerAddressSpace interface is deprecated. Meaning, legacy usage might persist. However, if you are writing new applications, you should avoid using it. Prefer IOPCBrowse instead. Hope this helps. Sign In·View Thread·PermaLink 2.00/5 (1 vote) COM in Unix? is it possible? Rameswara 19:48 2 Jul '07   Hi, You have mentioned that OPC supports interoperablility between Unix/Linux and windows. How is it done? Can we implement COM interfaces in Unix? thanks ram Sign In·View Thread·PermaLink Re: COM in Unix? is it possible? .Suchit 21:03 2 Jul '07   Hi Ram, Your post actually has two distinct questions. OPC interoperability across platforms is in the form of OPC UA. OPC Unified Architecture is SOA based. That means the applications written across Windows and Unix can utilize OPC functoinalities through Web Services. Coming to COM on Unix, There are implementations of COM see for your reference "EntireX" under UNIX. This helps extend OPC on Unix. In either of approaches, considering future directions, the OPC UA is a better solution. Hope this answers what you are looking for. Suchit Recently posted by .Suchit : Microsoft SilverLight Sign In·View Thread·PermaLink Information about ServerVersion? Biasto 2:23 1 Feb '07   Hello, i've just a simple question for experts How can I get an information from the server about specifacion version it is implemented? Is there any informationstatus avaible by OPC Servers. I think, this should be useful, to create an OPC CLient, who can work with DA2.0 and DA3.0 Server! Thanks for the answers! Biasto Sign In·View Thread·PermaLink Re: Information about ServerVersion? Edner 1:45 6 Nov '07   Hello Biasto, I hope that should not too late to an answer... In fact when you ask to OPCEnum (OPC Foundation component) an OPC List Server you can do it by specification (or category how is called). When each OPC Server is registered it is mandatory to enter information about the specification (it is called Category registration CATID_OPCDAServer10 CATID_OPCDAServer20) it can deal. So when you run the client and ask to OPCEnum you can receive a list by category (it means by specification). If you ask for an interface that the OPC Server do not support... you will receive an E_NOINTERFACE answer. Best regards, Edner Sign In·View Thread·PermaLink DA Server 3.0 versus DA Server 2.0 Biasto 23:24 25 Jan '07   Hello, now I've just another question. I've implemented for my client application the IOPCBrowse-methods for getting Properties and Item-information from the Server. How does a 2.0 Server answer using this functions? Has there anyone an intension? Thank for all... Tobias Sign In·View Thread·PermaLink Re: DA Server 3.0 versus DA Server 2.0 .Suchit 23:38 26 Jan '07   When OPC Client queries IOPCBrowse interface on a OPC 2.0 DA server, then E_NOINTERFACE error code will be received. E_NOINTERFACE means "No Such Interface Is Implemented in COM Server". Since OPC DA 2.0 specification does not have IOPCBrowse, and hence this is the expected outcome. Sign In·View Thread·PermaLink 3.00/5 (2 votes) CoCreateInstanceEx doesn't work!!! Biasto 5:31 25 Jan '07   Hello, I've got the experiance to create a IEC-OPC-Interface for our process control tool. So for this, it's needable to implement some OPC-Functionallity into our IEC-TOOL. Therefore we used the Toolkit of Technosoftware, for creating the Client. But now, it's useful to implment also the Browse-Functionallity to our OPC-CLient. For this, i've tried to initiate the IOPCBrowse-interface, which is given in DA-Specification 3.0. But my VC++ 6.0 reports, he can't find the undeclared variable 'CoCreateInstanceEx'. Which header-files are missing, so this failure come up? Perhaps anybode can help me ? Thanks!!! Sign In·View Thread·PermaLink 2.00/5 (1 vote) Re: CoCreateInstanceEx doesn't work!!! Biasto 23:22 25 Jan '07   I've just find the fault: For implementation of method 'CoCreateInstanceEx' it's necessary to implement / define preprocessor-hint ' #define _WIN32_COM ' at the beginning of the code. Therefore it must be declared before the headerfile objbase.h is implemented. Sign In·View Thread·PermaLink OPC server not any more connected NorGUI 3:30 22 Jan '07   Hello, I ask you direclty a question because I do not know were found this information. I am using a connection to a OPC server that is started from a application x. When my application start I connect me to the OPC server that this application propose. It works fine but my problem begin when the application x close then the server is not any more available. How to check if the server is running and shoul I CoUninitialize() and release() after that; Thank you for your help. AutreChien Sign In·View Thread·PermaLink 2.00/5 (1 vote) Re: OPC server not any more connected .Suchit 18:04 22 Jan '07   Hi AutreChien, This is a generic problem of COM server life time validation. OPC Server being just another COM server, you can use the same technique as follows: Call AddRef() on your OPC server interface. You can also use your any existing method calls made on IOPCServer interface. If this method call succeeds, then server is alive. If the method call fails, then the server is probably not alive or there is some communication error. The return code HRESULT can be looked into for more details. Suchit Sign In·View Thread·PermaLink

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