Implementing an OPC DA client from scratch - opc

Question

I would like to implement my own OPC DA client (versions 2.02, 2.05a, 3.00) from scratch but without using any third-party. Also I would like to make use of OPCEnum.exe service to get a list of installed OPC servers. Is there any kind of document that explains detailed and step by step the process to implement an OPC client?

Answer 1

I have a c# implementation but actually it's hard to fit it in here. I'll try to summarize the steps required.
Mostly you need to have OpcRcw.Comn.dll and OpcRcw.Da.dll from the OPC Core Components Redistributable package downloable for free from Opcfoundation.org. Once installed, the files are located in C:\Windows\assembly\GAC_MSIL. Create a reference in your project.
About coding, this is what you should do (there are three objects you want to implement, Server, Group and Item):
Let's start with server:
Type typeofOPCserver = Type.GetTypeFromProgID(serverName, computerName, true);
m_opcServer = (IOPCServer)Activator.CreateInstance(typeofOPCserver);
m_opcCommon = (IOPCCommon)m_opcServer;
IConnectionPointContainer icpc = (IConnectionPointContainer)m_opcServer;
Guid sinkGUID = typeof(IOPCShutdown).GUID;
icpc.FindConnectionPoint(ref sinkGUID, out m_OPCCP);
m_OPCCP.Advise(this, out m_cookie_CP);
I've striped a LOT of checking to fit it in here, take it as a sample...
Then you need a method on server to add groups:
// Parameter as following:
// [in] active, so do OnDataChange callback
// [in] Request this Update Rate from Server
// [in] Client Handle, not necessary in this sample
// [in] No time interval to system UTC time
// [in] No Deadband, so all data changes are reported
// [in] Server uses english language to for text values
// [out] Server handle to identify this group in later calls
// [out] The answer from Server to the requested Update Rate
// [in] requested interface type of the group object
// [out] pointer to the requested interface
m_opcServer.AddGroup(m_groupName, Convert.ToInt32(m_isActive), m_reqUpdateRate, m_clientHandle, pTimeBias, pDeadband, m_LocaleID, out m_serverHandle, out m_revUpdateRate, ref iid, out objGroup);
// Get our reference from the created group
m_OPCGroupStateMgt = (IOPCGroupStateMgt)objGroup;
Finally you need to create items:
m_OPCItem = (IOPCItemMgt)m_OPCGroupStateMgt;
m_OPCItem.AddItems(itemList.Length, GetAllItemDefs(itemList), out ppResults, out ppErrors);
Where itemlist is an array of OPCITEMDEF[]. I build the above using GetAllItemDefs from a structure of mine.
private static OPCITEMDEF[] GetAllItemDefs(params OpcItem[] opcItemList)
{
OPCITEMDEF[] opcItemDefs = new OPCITEMDEF[opcItemList.Length];
for (int i = 0; i < opcItemList.Length; i++)
{
OpcItem opcItem = opcItemList[i];
opcItemDefs[i].szAccessPath = "";
opcItemDefs[i].bActive = Convert.ToInt32(opcItem.IsActive);
opcItemDefs[i].vtRequestedDataType = Convert.ToInt16(opcItem.ItemType, CultureInfo.InvariantCulture);
opcItemDefs[i].dwBlobSize = 0;
opcItemDefs[i].pBlob = IntPtr.Zero;
opcItemDefs[i].hClient = opcItem.ClientHandle;
opcItemDefs[i].szItemID = opcItem.Id;
}
return opcItemDefs;
}
Finally, about enumerating Servers, I use this two functions:
/// <summary>
/// Enumerates hosts that may be accessed for server discovery.
/// </summary>
[SecurityPermission(SecurityAction.LinkDemand, UnmanagedCode = true)]
public string[] EnumerateHosts()
{
IntPtr pInfo;
int entriesRead = 0;
int totalEntries = 0;
int result = NetServerEnum(
IntPtr.Zero,
LEVEL_SERVER_INFO_100,
out pInfo,
MAX_PREFERRED_LENGTH,
out entriesRead,
out totalEntries,
SV_TYPE_WORKSTATION | SV_TYPE_SERVER,
IntPtr.Zero,
IntPtr.Zero);
if (result != 0)
throw new ApplicationException("NetApi Error = " + String.Format("0x{0,0:X}", result));
string[] computers = new string[entriesRead];
IntPtr pos = pInfo;
for (int ii = 0; ii < entriesRead; ii++)
{
SERVER_INFO_100 info = (SERVER_INFO_100)Marshal.PtrToStructure(pos, typeof(SERVER_INFO_100));
computers[ii] = info.sv100_name;
pos = (IntPtr)(pos.ToInt32() + Marshal.SizeOf(typeof(SERVER_INFO_100)));
}
NetApiBufferFree(pInfo);
return computers;
}
/// <summary>
/// Returns a list of servers that support the specified specification on the specified host.
/// </summary>
[SecurityPermission(SecurityAction.LinkDemand, UnmanagedCode = true)]
public string[] GetAvailableServers(Specification specification)
{
lock (this)
{
// connect to the server.
ArrayList servers = new ArrayList();
MULTI_QI[] results = new MULTI_QI[1];
GCHandle hIID = GCHandle.Alloc(IID_IUnknown, GCHandleType.Pinned);
results[0].iid = hIID.AddrOfPinnedObject();
results[0].pItf = null;
results[0].hr = 0;
try
{
// create an instance.
Guid srvid = CLSID;
CoCreateInstanceEx(srvid, null, CLSCTX.CLSCTX_LOCAL_SERVER, IntPtr.Zero, 1, results);
m_server = (IOPCServerList2)results[0].pItf;
// convert the interface version to a guid.
Guid catid = new Guid(specification.ID);
// get list of servers in the specified specification.
IOPCEnumGUID enumerator = null;
m_server.EnumClassesOfCategories(1, new Guid[] { catid }, 0, null, out enumerator);
// read clsids.
Guid[] clsids = ReadClasses(enumerator);
// release enumerator
if (enumerator != null && enumerator.GetType().IsCOMObject)
Marshal.ReleaseComObject(enumerator);
// fetch class descriptions.
foreach (Guid clsid in clsids)
{
try
{
string url = CreateUrl(specification, clsid);
servers.Add(url);
}
catch (Exception) { }
}
}
catch
{
}
finally
{
if (hIID.IsAllocated) hIID.Free();
if (m_server != null && m_server.GetType().IsCOMObject)
Marshal.ReleaseComObject(m_server);
}
return (string[])servers.ToArray(typeof(string));
}
}
I know I've striped a lot but maybe it can still help you ;)
Please mark the answer as correct if you think I've been clear ;)
Kind Regards,
D.