1. Introduction

OPC Data Access (DA) only deals with real-time data, using the follwoing attributes:
a value, the quality of the value and a timestamp.
These three attributes have to be returned to an OPC client (ALERTs OPC DA connector) making a request (advice or polled) to an OPC DA Server, see schema below.

According to the picture above, ALERT acts as an OPC DA client to manage the date of an OPC server.
You will find the settings for the type of request in the OPC DA data server properties later in the OPC DA Client settings part.

Remark: The OPC DA specification is using the Microsoft COM technology. That means, if the OPC server is located on a remote machine in a TCP/IP network, the complex configuration of the DCOM settings is required. In this case we suggest to use the ALERT OPC DA Tunneler or another tunneler software of a third party vendor.

The OPC DA client and server is a fix part of ALERT, there is no additonal selection during the installation process.

2. Installation

3. Configuration

1. Add a data server, see here: Adding a data server
2. Press the Source... button (1) to get the OPC source selection dialog box. Here pic the OPC DA radio button and click at the ... button (2) to get the list of available OPC DA servers.  Select the needed OPC DA server (3) and press the OK buttons (4) to confirm the selection.
   
3. The following entry will appear in the tree view at the left hand site, if the OPC DA data server was sucessfully embedded:
   
4. Do a right click at the data server you wish to import tags from and select Add a tag...
5. Click at the ... button at the data path selection (1) to get all the OPC DA elements of the server. At this point select the item to import (2) and press OK (3) to confirm.

   

6. The tag parameter definition dialog box will open, please configure it according to your needs.

4. OPC DA Client settings

The recommended advice request is set by default. In case to use the polled request, please set the check box of the periodic OPC Requests.

The OPC Data Access tab is only available if the OPC server provides "Data Access".

Data subscription method

Selection of the OPC protocol version:

• Auto (OPC 2.0 first, then OPC 1.0 if failed): preferential selection OPC 2.0
• OPC 1.0 only: OPC 1.0 forced

Polling cycle (periodic requests)

Define the OPC polling mode:

• Periodic OPC Requests
  
  Data polling is normally done by the subscription of the data to supervise. The OPC server automatically returns the value of the subscribed data at each change of value and optimizes the exchanges between the OPC client and server.
  If the subscription processing is not supported by the OPC server or to ensure the data acquisition, check this option to perform periodic read requests, according to a period defined below.
  
  
• No subscription (OPC Advise)

When "Periodic OPC requests" is selected, this option can be set to suppress the subscription of the data to be read periodically. Don't enable it, if the subscription is required to trigger the internal acquisition cycle of the OPC server.

• Polling period for requests
  
  Sets the periodic period for reading requests in seconds (5 seconds by default) when the "Periodic OPC Requests" is checked.

5. Excursus: ALERT OPC DA Server

ALERT also provides an OPC DA Server, i.a. to supervise the communication and the data server connections or to switch the users on call status, etc..