Browse Paths In General

An alternative way (to using a node ID) of specifying a node in OPC address space is using a browse path. The browse path is a sequence of node names, starting from a known location (e.g. the root of the OPC address space). The browse path can also be expressed as a string, where the individual node names are separated by delimiters (e.g. “/”, a slash).

Browse paths can be absolute (starting from a specified node), or relative.

The advantage of the browse paths is that you can construct them yourself, with just knowledge of the node names, without knowing the full node IDs. In the end, each item must be addressed by its node ID, and QuickOPC will resolve the browse path to a node ID as necessary.

OPC Classic Browse Paths Specifics

In OPC Classic, absolute browse paths always start at the root of the OPC address space, because the root is the only “pre-defined” or well-known node.

An absolute browse path for OPC Classic is contained in a BrowsePath object. Browse paths are commonly specified by the BrowsePath property of the DANodeDescriptor or DAItemDescriptor object.

If a non-null ItemID is specified in the descriptor, QuickOPC will use this item Id and ignore the browse path. If ItemID is null, QuickOPC will attempt to resolve the browse path contained in the BrowsePath property of the descriptor. Either item id, or browse path (or both) must be specified.

Besides the ability to get around usage of full node IDs, the other reason to use browse paths is for OPC browsing with OPC Servers that only support OPC Data Access 1.0 specification. Such OPC servers cannot start the browsing at a node in the address given just by its Item ID; the node must always be reached by browsing from the root level. If the ID of such node it already known, QuickOPC takes care of supplying the proper browse path automatically, but this cannot be always done. If, however, a browse path is given (which can be done by using the DANodeElement that is the output of the browsing to construct the DANodeDescriptor that is the input of further browsing), the browsing can proceed normally.

Examples

// This example shows how to read a single item using a browse path, and display its value, timestamp and quality.

using System;
using OpcLabs.EasyOpc;
using OpcLabs.EasyOpc.DataAccess;
using OpcLabs.EasyOpc.OperationModel;

namespace DocExamples.DataAccess._EasyDAClient
{
    partial class ReadItem
    {
        public static void BrowsePath()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            DAVtq vtq;
            try
            {
                vtq = client.ReadItem(
                    new ServerDescriptor("", "OPCLabs.KitServer.2"),
                    new DAItemDescriptor(null, "/Simulation/Random"));
            }
            catch (OpcException opcException)
            {
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message);
                return;
            }

            Console.WriteLine("Vtq: {0}", vtq);
        }
    }
}

# This example shows how to read a single item using a browse path, and display its value, timestamp and quality.

# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc

# Import .NET namespaces.
from OpcLabs.BaseLib.Navigation import *
from OpcLabs.EasyOpc import *
from OpcLabs.EasyOpc.DataAccess import *
from OpcLabs.EasyOpc.OperationModel import *


# Instantiate the client object.
client = EasyDAClient()

# Perform the operation.
try:
    vtq = IEasyDAClientExtension.ReadItem(client,
                                          ServerDescriptor('', 'OPCLabs.KitServer.2'),
                                          DAItemDescriptor(None, BrowsePath('/Simulation/Random')))
except OpcException as opcException:
    print('*** Failure: ' + opcException.GetBaseException().Message)
    exit()

# Display results.
print('Vtq: ', vtq, sep='')

' This example shows how to read a single item using a browse path, and display its value, timestamp and quality.

Imports OpcLabs.EasyOpc
Imports OpcLabs.EasyOpc.DataAccess
Imports OpcLabs.EasyOpc.OperationModel

Namespace DataAccess._EasyDAClient
    Partial Friend Class ReadItem
        Public Shared Sub BrowsePath()
            Dim client = New EasyDAClient()

            Dim vtq As DAVtq
            Try
                vtq = client.ReadItem(New ServerDescriptor("", "OPCLabs.KitServer.2"), New DAItemDescriptor(Nothing, "/Simulation/Random"))
            Catch opcException As OpcException
                Console.WriteLine("*** Failure: {0}", opcException.GetBaseException().Message)
                Exit Sub
            End Try

            Console.WriteLine("Vtq: {0}", vtq)
        End Sub
    End Class
End Namespace